Chapter 1 Introduction
Chapter 1. 1 Overview
1
Chapter 1 Introduction
Chapter 1. 1 Overview
This thesis describes improvements to the Shape Deposition Process (SDM)
for the development of tool steels die inserts. SDM is a rapid prototyping process,
which is used at Stanford University and Carnegie Mellon University to develop
metallic parts by rapid fusion of powder metal by laser deposition or by plasma
melting of metal wire. The approach given here was to develop an understand the
microstructure produced by the laser deposition of tool steels. This understanding is
essential to building die inserts. Die inserts have stringent material requirements
because of the intense service conditions to cast aluminum parts. The presented
research developed three significant outcomes: 1. The microstructual and
morphological characterization of laser layered deposition of carbon steels. 2. A set of
design rules which one must consider when depositing carbon steels with laser based
layered manufacturing processes. 3. A methodology for using phase transformation
and other microstructual knowledge to design improved parts, Designing for
Microstructural Manipulation (DFM
2
) was also developed.
The benefits of characterizing carbon steel microstructure and cataloging the
morphological evolution are the application of this knowledge to reduce part
deflection and enhance part properties. Also, the carbon steel deposition methodology
will increase building throughput, reduce part failure, and extend the range of the
applications that can use laser layered manufacturing. The benefits to DFM
2
are the
ability to leverage the transient heating patterns of laser layered manufacturing to
manipulate phase percentage, grain size, and phase location to enhance wear
Chapter 1 Introduction
Chapter 1. 2 Thesis Outline
2
resistance, deformation, and strength. Conventional Shape Deposition Manufacturing
techniques do not leverage knowledge of the deposited microstructure to create
improved parts or improved designs.
Chapter 1. 2 Thesis Outline
Chapter 2 provides a detailed background material on rapid tooling processes,
the SDM process, and other laser deposition processes. Chapter 3 describes the
experimental procedures used to analyze microstructure and material properties.
Chapter 4 describes the initial investigation into using tool steels with the SDM
process to make small die casting inserts (less than 25 mm in thickness) and low aspect
ratios. Chapter 5 details the characterization of SDM deposited 400 series Martensitic
Stainless steels and other carbon steels. Chapter 6 discusses utilizing the
characterization to select die cast insert material and to reduce part deflection. Chapter
7 describes design rules for laser depositing carbon steels in a new process which
focuses on depostion microstructure, Designing for Microstructural Manipulation.
Chapter 8 provides a short conclusion to this thesis. Additional information is
included in the appendix.
Chapter 1. 3 Individual / Group Work Statement
Part of the work presented here represents group work. The Stanford Rapid
Prototyping Lab is an environment in which teamwork and collaboration of ideas is
encouraged for the purpose of enriching and progressing the pace of research. All of
the work presented was developed under this environment. The idea, testing and use
of the martensitic expansion to reduce CTE shrinkage deflection, and the development
of Designing for Microstructural Manipulation process are completely my own work.
The parametric modelling of this phenomena in laser deposited martensitic steels is
also my own work. Most of the analysis by X-ray diffraction, transmission electron
microscope, electron microbeam analysis, and pixel-phase color analysis are also my
own work. The development of “Blue,” the pixel color intensity matching program,
was designed by Rudolph Leitgeb. Many samples were etched, polished, and analyzed
by Monikka Mann, Tony Nguyen and Tonya Huntley. Ms. Mann helped to perfect the
Chapter 1 Introduction
Chapter 1. 3 Individual / Group Work Statement
3
electron backscattering probe technique for bulk carbon steel samples. Finite element
modelling was performed by Alexander Nickel. Many of the 316L SDM tooling
efforts were built by John Fessler, Alexander Nickel, and Xiochun Li. The laser SDM
process was perfected by John Fessler and Alexander Nickel. Any errors presented in
this work are my responsibility.
Chapter 2 Background
Chapter 2. 1 Tooling Industry
4
Chapter 2 Background
Chapter 2. 1 Tooling Industry
The prototype tooling industry is about 300 Billion dollar a year industry.
Prototype parts are often used by designers to test alternative designs. Prototype parts
can range from conceptual to functional. Barkan and Iansiti developed a detail study
of the levels a prototyping stages which can occur during the design process (2.1).
Simple models or mockups are conceptual parts which can be made from the simplest
materials and processes. They can be made of plastics, paper, etc. They can be formed
by gluing, simple machining, or blade shaping. Mock up parts do not have to
necessarily fit tight tolerances. The primary purpose for conceptual parts are “look
and feel” attributes. They are typically used in the early part of the design cycle.
Subsystem and mechanical prototypes have a wide range of model classes
which very in level of integration and tolerances. Parts in these two subclass range
from moderate to highly tight tolerances. They can also be semi-functional to
functional. These parts are used for design validation. Often these parts are used to
determine part fit within packaging requirements. These parts made be made from
traditional prototyping techniques or rapid prototyping techniques.
Show parts are subsystem prototypes which high accuracy but are used
primarily for display or review purposes. These parts are not typically made of
conventional engineering materials, but are often painted or decaled to have show
quality finishes. These parts are typically made very quickly from foams, ren board,
and epoxy plastics. They often require manual finishing to insure tight tolerances and
part dimensions.
Chapter 2 Background
Chapter 2. 1 Tooling Industry
5
Breadboards are subsystem prototypes which are very simple with low
tolerances but exhibit part function. These types of are typically used for testing
concept function or local part/subsystem function. The ability to make changes to
these systems rapidly is a prime feature of breadboards. Tooling to build such
prototypes is relatively simple, typically glue, solder, simple linkages, etc.
Mechanical prototypes are functional prototypes which typically support all
the functions of the final model but do not necessarily represent the final size or shape
of the actual part. These prototypes may include several breadboard prototypes,
Tooling for these types of prototypes, typically consists of off the shelf technology.
For examples, standard housing may be used for prototype printers. Non standard
metal and plastic parts may be machined from simple stock materials.
Engineering prototypes are classes of models which are functional prototypes
like the mechanical prototypes but are typically made out the same engineering
material as the final part. The size or footprint of the prototypes match the final
design. These prototypes are used as final design checks or limited field testing. Low
volume prototype casting processes are used for exotic parts as opposed to machining
them out of stock. Metal parts may be sand cast or gravity poured. Even though parts
may be made from the design intent engineering material, quality may still differ from
the production run parts.
Production prototypes are prototype parts which are made from the same
engineering material that the final part will be made and manufactured from similar
processes. The purpose of these parts is to test the manufacturing process and
production part quality. These prototypes will have the material characteristics of
production intent manufacturing. Therefore, early cycle time scenarios and
production volumes can be forecasted from these prototypes. Also, reliability or
failure studies on the actual part can be run with prototypes. Unlike many engineering
prototypes, the parts could be included within validation cycles. Tooling for these
prototypes are very expense and usually require long lead times because actual tooling
inserts are required. As the Figure 2.1 below shows, changes to prototype design at
this point is very expense.
Chapter 2 Background
Chapter 2. 2 Conventional Tooling Process
6
Figure 2.1 Adapted from Barkan and Iansiti (1993)
Chapter 2. 2 Conventional Tooling Process
To build prototype parts many conventional tooling processes are used. There
are two approaches which are usually taken to make prototypes parts: part simulation
and process simulation. Part simulation typically involves using simple processes like
material removal to just get prototype parts made. Similar stock material may be
machined to get a model. Other part simulations includes the models, mock-ups, and
low level simulations. Prototypes built with process simulation are cast, drawn forged,
stamped, etc. to build parts which will be similar to the production intent pieces.
Process simulations include the mechanical, engineering, and production prototypes.
The two types of tooling processes are common to both part and process simulations
are material-removal processes and casting. Joining processes are also common but
will not be described explicitly.
Chapter 2. 2.1 Material-Removal Processes
Traditional tooling processes for building parts typically encompasses material
Models, Mock-Ups
ComputerSimulation
Subsystem
Mechanical
Engineering
Production
1K
10K
100K
Timing
Chapter 2 Background
Chapter 2. 2 Conventional Tooling Process
7
removal process. These processes include cutting, abrading, burning, and eroding.
Cutting processing involve single or multiple point cutting tools such as milling or
drilling bits. Abrading processes involve grinding, polishing or sanding. Burning and
eroding processes involve utilizing electricity, chemicals, heat, or hydrodynamics to
shape or remove material.
Milling and drilling processes remove material by using a single point or multi-
point tool to shear material (chips) away from the workpiece, the material being
formed into the part. The workpiece is typically fixtured so that the cutting tool can
remove material by rotating the tool while feeding the workpiece toward the cutter’s
tool face. High tolerances and sharp corners can be achieved by these processes.
These processes are typically coupled with other conventional tooling processes.
Similar processes include lathing, turning, planing and reaming.
Grinding, polishing and sanding processes are very similar to milling and
drilling operations in that it is a chip removal process with the cutting tool being the
individual abrasive grain. More chip deformation occurs with highly abrasive
processes because of the highly negative rake angles of the grains. When grinding,
high temperatures can be reached at the surface. These raised temperatures can cause
tempering, burning or heat-checking at the surface of the workpiece. Heat checking is
cracking at the surface which leads to low toughness and low fatigue and corrosion
resistance. The temperature gradients within the workpiece. Similar processes
include lapping,
Electrical discharge machining is a material removal process which erodes
metals by spark discharges. A shaping tool called the electrode delivers DC power to
the workpiece. The workpiece is submerged in dielectric fluid. When the voltage
potential difference between the electrode and workpiece is sufficiently high, a
transient spark discharges through the fluid, removing a small amount of the
workpiece. Even though EDM has a localized effects on the workpiece, the first 500
µ
m of a tool steel workpiece may have undergone significant phase changes. The
average rate of removal is 10
-6
to 10
-4
mm
3
with discharges repeating between 50kHz
to 500kHz.
Electrochemical machining is another material removal process which unlike
electroplating deposits or build up material, erodes material. The workpiece is
submerged in an electrolytic fluid which is a current carrier. As the electrolyte moves
over the workpiece, metal ions are washed away. This constant flowing of electrolyte
keeps the ions from plating on to the tool. The tool serves as a cathode and the
Chapter 2 Background
Chapter 2. 2 Conventional Tooling Process
8
workpiece serves as a anode. This process does have a tendency of eroding sharp
corners, developing uneven flat sections, loosing tight tolerances. Another similar
material removal process is electrochemical grinding.
Thermally assisted machining or hot machining uses a heat input to lower local
yield strength to allow for easier or more efficient machining requiring lower cutting
forces. The heat input can be a torch, electron beam, laser, or plasma arc. Because
high temperatures are involved and uniform workpiece temperatures are hard to
achieve, the microstructure of the full workpiece may be affected. If the high energy
beams sources and machining conditions are well regulated, only local microstructure
will be affected.
Hydrodynamic machining or abrasive water jet machining is a material
removal process which uses a jet of water to remove material. The water pressure can
be as high as 1600 MPa. Up to a depth of 7.5 m/min of material can be removed. This
process also has a localized temperature and deformation on the workpiece.
These processes are needed because they typically can provide higher
dimensional accuracy and smoothness of surface finish than casting, forming, or other
shaping processes. Also, they can produce features with sharp corners or flatness
which cannot be formed by other shaping processes.
Material removal process typically have a localized influence on the workpiece
or part. Phase transformation, plastic deformation, and surface residual stress resulting
from removal processes, typically occur very close to the cutting surface and not
within the bulk influence. The chips or material removed absorb most of the heat.
Chapter 2 Background
Chapter 2. 3 Casting
9
Figure 2.2 Percentage of heat generated which is absorbed by workpiece, tool and chip as a function of removal speed. (Adapted from Manufacturing Engineering and Technol-ogy,2.3)
Chapter 2. 3 Casting
Casting is one of the oldest methods of manufacturing dating back to 4000
B.C. Casting is not limited to metals, but can also be used with plastics, glasses, and
ceramics. Casting is most often used because it can produce very complex shapes.
These shapes may also have cast features like internal cavities and hollow sections.
Large parts can be produced by casting. Many hard to work with materials can be
shaped much more easily with casting than other processes.
Metal casting processes are of most interest to the topic presented and will be
discussed in detail in this section. Casting processes for other materials will not be
discussed. Metal casting processes can be divided into two categories expendable and
permanent.
Chapter 2. 3.1 Sand Casting
One of the earliest forms of casting metal is sand casting. Sand casting
consists of using a pattern shaped like the final cast shape to make an imprint in sand.
This imprint or cavity will be filled with molten metal. The sand will also having
gating or flow systems for the metal to enter the cavity of the sand mold. Many large
parts are cast with this method like engine blocks and pump housings.
Energy
(%)
Workpiece
Tool
Chip
Removal Speed
Chapter 2 Background
Chapter 2. 3 Casting
10
The sand used for these molds are typically have a silica base making them
have high resistance to temperature. When the casting cools, they shrink, and the sand
mold collapses around the part. If the sand did not collapse hot tears or cracks would
form in the casting. The sand is molded into the cope (the top of the mold and the drag
(the bottom of the mold. Cores which are used to represent interior surfaces like
hollows or cylinders can also be made out of sand. They are held in place in the sand
mold to cast these features.
All types of metals can be sand cast. Shape complexity of sand cast parts can
be quite complex. There is no limit on part size, but small size parts are very hard to
cast because of the difficult of maintaining gating and regulating metal flow into these
cavities. The accuracy of tolerances of sand cast parts is lowest when compared to all
other methods of casting. Typical surface finishes range from 5-25
µ
m. Lastly, one
sand mold is usually made for one part. As an expendable mold, the sand mold is
destroyed after casting. The sand material is typically reusable. The grain
development in sand casting has dendrite grains and must be heat treated.
Figure 2.3 Sand Casting Mold System
Chapter 2. 3.2 Shell Casting
Shell-mold casting was developed in the 1940’s. A fine sand is coated and
fired upon a mounted pattern made of ferrous or aluminum material. The fired fine
sand is now a highly accurate mold. It is removed from the metal pattern. The shell is
removed and often supported by sand, and gated. The shell is from 5-10 mm thick.
Cope
Drag
Mold Cavity
Vent Pour basin (cup)
Flask
Sand
Parting Line
GateChoke
Blind Riser
Runner
Open riserCore(sand)
Chapter 2 Background
Chapter 2. 3 Casting
11
High precision gears and other precision small parts. With proper gating, multiple
parts can be cast at one time. Most metals can be cast by shell casting.
Although, the these molds have high tolerances, the fine grain sand does not
permit much venting. Trapped gasses can cause improper filling of the mold. These
molds are also susceptible to porosity and tears. Casting weight should not exceed a
few hundred kilograms. Shape complexity is limited. Achievable surface finish is in
the range of 1-3
µ
m.
Chapter 2. 3.3 Lost Foam Casting
An aluminum or metal die is formed to replicate a mold for a casting.
Polystyrene beads are placed in the mold and heated. The polystyrene expands to fill
the mold. The polystyrene casting is then placed in a sand filled container with gating.
The inflowing metal evaporates the foam. The dissolving foam causes the metal to
solidify faster than in sand casting leading to directional solidification of the metal.
Because of the sand, the vaporizing polystyrene vents easily.
Unlike the other processes no parting lines, cores, or riser systems are needed.
The process for the most part is inexpensive with the polystyrene, sand and containing
units being relatively inexpensive. Only the aluminum shaping die can be costly.
Therefore, low volume runs can be expensive.
Lost foam castings have no size limit. Casting from this process can have
surface finishes from 5- 20
µ
m. Dimensional accuracy from this process are better
than sand castings.
Figure 2.4 Lost foam casting of a (A.) water pump housing and a (B) bearing plate. (Cour-tesy of Diversa Cast Tech.)
Chapter 2. 3.4 Plaster Mold Castings
A plaster made of gypsum or calcium sulfate with talc and silica flour is poured
A.
B.
Chapter 2 Background
Chapter 2. 3 Casting
12
over a pattern and set. The pattern is then removed and baked at 120C. Molten metal
is poured into the mold. The maximum metal temperature is 1200C so only aluminum,
magnesium and other non-ferrous metals can be cast. Patterns for this process have to
be of fine quality. Wood patterns cannot be used because the liquid plaster will cause
swelling to pattern ruining the mold. Also the plaster does not permit much venting of
gases. Thus, the part must be poured in vacuum or under pressure.
The high strength of these molds allow them to maintain good dimensional
accuracy much higher than sand or lost foam casting. Surface finishes from this
process are 1-2
µ
m.These molds have very low thermal conductivity. Therefore the
casting cools more slowly yielding a much more uniform grain structure and less
warpage. The maximum size limit is about 50 kg. This process is best suited for low
production runs because of the expense of producing patterns. Also, the time to make
these molds as well as the entire molding process is quite lengthy.
Figure 2.5 A. Plaster Molds drying in an oven. B. The drag portion of a plaster mold for an air compressor housing. C. Aluminum 356 air compressor housings from a plaster mold.
A.
C.
B.
Chapter 2 Background
Chapter 2. 3 Casting
13
Chapter 2. 3.5 Ceramic-Mold Casting
Ceramic Mold Casting is very similar to Plaster Castings except that refractory
high-temperature materials like zircon or aluminum oxide instead of gypsum or
calcium sulfate. This is a cope and drag technique. A ceramic slurry covers a pattern
which has been put in a flask, holding container. Once the slurry is set, the pattern is
removed. The slurry is then dried and burnt to remove volatile matter. It is then baked.
Often, to improve strength of mold, fireclay is added to the backings of the mold. This
additional processing is called the Shaw process.
These high temperature molds can be used with all metal including ferrous
alloys. Parts cast in these molds can weigh as much as 700 kg. Castings can have
surface finishes of 1-2 mm. Dimensional accuracy is also very high. This process is
very expensive.
Typical parts made with this method are impellers, cutters, dies for metal working, or
molds for plastic parts.
Figure 2.6 A Ceramic Mold made with the Shaw Process
Chapter 2. 3.6 Investment Casting
Investment casting or the lost wax process is an old process dating back to
4000 B.C. First a metal die is made which is used to cast a pattern of the intended
part. Wax and plastic is injected into the die. Several wax pattern are created. Special
care in handling patterns must be done in order keep them from breaking or distorting.
The patterns are then attached to a pattern assembly or tree. The tree will help develop
Cope
Drag
Parting Line
Fireclay Backup
Ceramic Facing
Chapter 2 Background
Chapter 2. 3 Casting
14
gating in the future investment molds. The tree is then invested with refactory
materials by dipping the tree into a ceramic slurry. This dipping is repeated over and
over to build up thickness of the coating. This mold is now dried in air and then
heated to about 100C in an inverted position to melt out the wax. This may take up to
12 hours. Four additional hours are spent firing the mold to 650 -1050 C to drive off
any remaining water.
Figure 2.7 Steps 1-3 of the investment casting process
Figure 2.8 Steps 4-7 of the investment casting process
The mold can now be filled with molten metal. Once the metal has solidified,
the mold can be broken up to remove the castings. Highly accurate and complex
castings can be made. Surface finishes will range from 1-3 mm. Parts cast in this
method should be under 100 kg. The development of patterns and the use of labor
throughout the process can be very expensive. Investment castings are most cost
effective at high production volumes.
Injecting Wax or Plastic
Pattern Tree
SlurryCoating
Pattern Melt out Pouring Shakeout
Detachingof Castings
Chapter 2 Background
Chapter 2. 3 Casting
15
Chapter 2. 3.7 Hard Mold Casting
For hard mold casting, a metal mold made from cast iron, steel, bronze,
graphite, or refractory metal alloys. The mold is machined with gating. Internal
cavities are maintained by cores made from metal, plaster or sand and placed in the
mold prior to casting. Various parts of the mold which are sensitive to high wear can
have inserts.
To increase the life of the mold, the surfaces of the mold is coated with a
ceramic slurry like sodium silicate. These coatings serve as a parting agents or
thermal barriers to control the rate of cooling of the casting. Ejector pins may also be
placed in these molds. The mold halves are clamped together and then heated to 150-
200C to aid metal flow through the mold and reduce thermal damage to the mold. The
molten metal is then poured through the gating system to fill the mold. Once the
casting solidifies, the mold is opened and the casting is removed. The mold may be
water cooled or cooled by fins.
All metals can be cast in this method, but high metaling point metals like steels
need dies built or heat resistant materials. Surface finishes range from 2-3
µ
m.
Maximum part weight made in this method is about 300 kg. Achievable shape
complexity is from moderate to low. Typical parts made in this method are
kitchenware, connecting rods and gear blanks. Accuracy of castings is very high.
Minimum part thicknesses allowable is 2 mm. Machinery costs and labor can make
this process quite expensive. This process is most economical for high volume runs.
Chapter 2. 3.8 Low Pressure Casting
Low pressure casting or pressure pouring is a process similar to hard mold
casting. The process involve mold haves which are clamped together and filled by
molten metal forced upward by gas pressure. The mold may be made from graphite or
metal. The pressure is maintained until the metal has completely solidified in the
mold. The metal may also be driven upward to fill the mold by a vacuum. The
vacuum helps remove dissolved gases lowering porosity.
Very high quality casting are made with this process. Surface finishes range
from 1-3 mm. All metals can be cast in this process. Castings have very high
accuracy, but moderate to low complexity. The process is very expense because of
equipment. Typical parts made by this process are railroad wheels.
Chapter 2 Background
Chapter 2. 3 Casting
16
Figure 2.9 Low Pressure Casting Systems
Chapter 2. 3.9 Die Casting
Die casting was developed in early 1900’s. Molten metal is forced into a die
cavity by pressure ranging from .7 -700 MPa. In order to achieve, to die cast a special
machine is required. Two basic types of die cast systems are available: hot-chamber
and cold chamber systems.
The hot chamber process involves using a piston which traps a specific volume
of molten metal and injects it into a die cavity. The shot chamber or piston path way is
heated. The die cavity is formed by two die halves called an ejector die and cover die.
The ejector die is movable. The die halves clamp together to receive the shot of
metal. The cavity is kept under pressure until the metal solidifies. The dies are cooled
by circulating water or oil through various cooling channels in the die blocks. This
cooling aids in improving die life and in rapid cooling of metals. Usually cycle times
can reach up to 900 shots per hour. Zinc, tin and other low metaling point alloys are
cast using the hot chamber process.
Air Tight Chamber
Ladle
Refractory Tube
Air Pressure
Casting
Mold
Molten Metal
Chapter 2 Background
Chapter 2. 3 Casting
17
Figure 2.10 Hot Chamber Die Casting System
The cold chamber process involves molten metal which is placed in an
injection cylinder. The injection cylinder or shot chamber is not heated. The metal is
then forced into a two part die cavity. The cavity is very similar to the ones used in the
hot chamber process. The metal injection pressures average about 20-150 MPa.
Aluminum, magnesium and copper are commonly cast in this process. Other high
melting temperature metals can also be cast in this manner.
Figure 2.11 Cold Chamber Die Casting System
Dies are typically made from H13 or other hot working steels. Wear on dies
increases with the temperature of the molten metal. Surface cracking from repeated
heating and cooling of dies, heat checking, is a major result of die wear. Conventional
die cast dies can last for more than 500,000 shots before significant wear occurs.
Die cast parts have high accuracy and can maintain tight tolerances. Bearing
surfaces can be produced by die casting. Surface finishes are between 1-2 mm.
Maximum casting weigh from less than .05 kg to 50 kg. The minimum thickness of
Molten Metal
Furnace
HydraulicShot Cylinder
Nozzle
Cover DieEjector Die
Pot
Die Cavity
Ladle
Plunger Rod
Hydraulic Cylinder
Stationary PlatenEjectorPlaten(moves)
Shot Sleeve
Cavity
Ejector Die Half
EjectorBox
Chapter 2 Background
Chapter 2. 3 Casting
18
sections can be a thin as .5 mm. Volume production for die casting is typically high.
The expense of making die inserts, getting needed equipment and the time to produce
die inserts makes only large volume runs economical.
Chapter 2. 3.10 Centrifugal Casting
Centrifugal casting process has been used since the 1800’s. The process uses
spins a mold about an axis of rotation. Molds are typically made of graphite, steel or
iron. All types of metals can be cast in this method. Often the inside of the mold
cavity is coated with a refactory lining to reduce mold wear. Typical parts cast in this
method are pipes, gun barrels and street posts.
Centrifugal cast parts have high degree of accuracy and very little porosity.
The surface finish will range from 2-10
µ
m. Maximum part weight is above 5000 kg.
The cost of the mold and equipment is quite expensive so only large volume runs are
economically feasible.
Figure 2.12 Centrifugal Casting
Chapter 2. 3.11 Crystal Growing Casting
Crystal Growing casting processes originate from the 1960. The has a
corkscrew gate into the cavity chamber. The corkscrew constriction is designed so that
only favorably oriented grains can grow. The die cavity is contained on a moving
platform. The die cavity is heated by heat baffles which radiate heat. As the
constriction only permits a single favorably oriented crystal to grow.
Rollers
Spout
DriveShaft
Free RollerDriveRoller
Mold
Molten MetalMold
Chapter 2 Background
Chapter 2. 4 The Use of Conventional Tooling to Make Prototype Parts
19
As the platform lowers, the radiant heat of the baffles causes the single to grow
and fill the die cavity. All other grains are stopped at the walls of the constriction.
When the mold is complete, a single crystal casting in the shape of the die cavity
The parts have a high degree accuracy, good surface finish. Equipment is and
the die cavity is very expensive. Part strength is higher than conventional castings
because it is a single crystal.
Figure 2.13 Single Crystal Casting
Chapter 2. 4 The Use of Conventional Tooling to Make Prototype Parts
To produce simple models or mockups which to progress the design process,
stock, material can be machined to produce prototypes. When material removal
processes are used to make prototypes, the grain structure of the metal prototypes
resembles that of the stock material. If the stock material is rolled, then the prototype
will have a rolled grain structure. Parts made in this manner typically will not have the
strength or yield characteristics of the production part. For example, an aluminum
valve cover machined from wrought stock will not have the structure of a die cast
valve cover.
Prototype parts for shell or investment casting can be cheaply done with sand
castings. The grain structures will be very similar. Validation or other lifecycle testing
could occur on these prototypes because of the similarity of the grain structure. It is
more difficult to develop prototyping techniques which will yield similar grain
structure and size that the conventional metal mold process produce. However, to
simulate die cast prototypes, special heat treatment must be used to gain
microstructure similar from any other type of casting or stock machined material.
Growing Single Crystal
Constriction to orient and grow 1 preferred grain
Radiant Heat
Chill Plate
Chapter 2 Background
Chapter 2. 4 The Use of Conventional Tooling to Make Prototype Parts
20
Producing prototype die cast parts is a tradeoff between time, money, and material
properties. The heat treatment of stock materials will be costly and time consuming.
Using sand casting technology to produce one die cast prototype would be relatively
inexpensive. However, the additional processes needed to attain similar microstructures
and material properties will consume design lead time. Figure compares the average yield
strength of die cast parts to cast
Figure 2.14 The Cost to Produce 1 Part versus the as cast or produced grain size
Figure 2.15 Comparison of Yield Strength for various types of Castings .
Cost to Produce 1 Part -Without Heat Treatment
Incr
easi
ng G
rain
Siz
e
Sand
Cutting EDM Milling ECM Drilling Hydro
ThermalLost Foam
Ceramic
ShellInv Casting
S. Crystal
Permanent Mold
Die Casting
Centrifugal
0 5 0 100 150 200
Sand Castings
Plaster CastingsGrain Refined
Chilled-cast
Plaster CastingGrain-Refined
Die Castings
Yield Strength (MPa)
Chapter 2 Background
Chapter 2. 5 The Need for Rapid Tooling.
21
Chapter 2. 5 The Need for Rapid Tooling.
As the preceding Figure 2.1 shows, the ability to make design changes is most
cost effective during the early stages of the design cycle. Today, concurrent or
simultaneous engineering that have multi-discipline design teams can now design
manufacturing processes along with new parts (2.2). The ability to test part features
and function in engineering materials and at the same time closely prototype the
manufacturing processes can occur under these new design paradigms with rapid
tooling.
Conventional tooling methods are too costly and require too much lead time to
practically use them to construct prototype parts. A typical die cast insert can take up
to 4 months to build and heat treat. Testing preliminary designs with conventional
tooling methods would eventually reduce the number of prototypes possible during
limited design times. An alternative method is required to produce functional
prototyping tooling, rapid prototyping.
Rapid tooling is the needed alternative process. Rapid tooling is the use of
solid froufrou processes to rapidly construct die insert or other forming tools to build
actual parts in simulated manufacturing rigs. Rapid tooling methods are typically
faster and less expensive than conventional tooling methods.
Chapter 2. 6 Solid Freeform Fabrication
Solid freeform fabrication is a process of building three dimensional objects in
a layered fashion. The three dimensional object is built in 2 dimensional layers
typically in an automated fashion. Therefore, complex parts can be built quite when
resolved in to two dimensional structures. Objects which cannot be built with
conventional manufacturing paradigms like high speed milling can not make
conformal cooling passages. Solid freeform fabrication techniques can build cooling
passages which follow intricate part surfaces because of the 2D layering approach.
For solid freeform fabrication a computerized model typically represented by
CAD (computer aided modelling) is then divided into layers by a hierarchical
algorithm. This algorithm divides the part by prioritizing layer order with shaping and
depositing precedences. The part is then built in the z direction, layer by layer with a
rapid prototyping process. The Figure 2.16 models the process.
Chapter 2 Background
Chapter 2. 7 Commercial Rapid Prototyping Processes used in Rapid Tool-ing
22
Figure 2.16 A CAD model is divided into layers and then programed and built by a rapid proto-typing process.
Chapter 2. 7 Commercial Rapid Prototyping Processes used in Rapid Tooling
Several commercial process have been used in rapid tooling.
Stereolithography, selective laser sintering, and laminate object manufacturing are all
primarily polymeric in nature and have been either used as a tool to create an insert or
have been used directly as the tooling insert.
Chapter 2. 7.1 Stereolithography
Stereolithography is a polymer based process which uses a laser or ultaviolet
light to cure an epoxy or plastic resin. This process was commercialized by 3D
Systems in and Beta tested by General Motors in . The part substrate or starting point
is a movable elevator platform which rests on the surface a large vat of curable resin.
The laser draws a pattern in the epoxy resin which solidifies and acts as bonded
supports on the platform. The supports are between 5 mm -10 MS thick which is about
20 to 100 layers. With each drawn layer the elevator submerges the part to rewet the
surface.
Once the support layers are drawn, the first layer of the part is then scanned.
The scanning process begins with the laser curing the outline of a 2D cross section of
motion control trajectories
data exchange
format
ComputerizedSolid Model
Physical Object
CAD System
Automated Process Planner
Automated Fabrication MachineRapid Prototyping Process
Chapter 2 Background
Chapter 2. 7 Commercial Rapid Prototyping Processes used in Rapid Tool-ing
23
the part. The inside of the cross sections are then rastered or weaved scanned. The
platform and scanned layers are then rewet by resin with the submerging of the part
beneath the liquid resin surface. After a 30 second (or shorter) wait time, the part is
raised to just below the surface the distance of one layer. Next the laser begins
scanning of the next layer. This process continues until the part is completed.
Once the part is completed, it is drained and then removed from the vat. The part is
then rinsed in a solvent to remove any uncured resin from the part surface. It is then
detached from the platform and placed in an ultraviolet curing oven for a post cure.
This process insures part strength and rigidity.
Figure 2.17 Stereolithography
Figure 2.18 SLA Part building on platform
UV LightSource
UV Curable Liquid
Liquid Surface
Platform
FormedPart
Vat
Chapter 2 Background
Chapter 2. 7 Commercial Rapid Prototyping Processes used in Rapid Tool-ing
24
Chapter 2. 7.2 Stereolithography Rapid Tooling
This process can be used for undercut and novel overhanging features which
cannot be easily produced with conventional machining. Thus, this process has been
used to advance rapid tooling in several ways: as EDM electrodes, the negative blank
to form the tool or as the positive insert. Researches have begun using
Stereolithography patterns to make investment cast die inserts.
Electrical discharge machining (EDM) is a tooling process which expends high
amounts to shape metal by burning or vaporization. Tool steel die inserts often used
EDM to build complex shapes and contoured surfaces. By plating Stereolithography
electrodes with copper, these electrodes have been used successfully to rough, semi-
rough and finish metal parts (2.4).
Prototype vacuum casting molds have been made with stereolithograhy (2.5).
These type of molds are ideal for stereolithography because pressure requirements and
casting temperatures are low. This process typically uses pressure to have the material
flow to all parts of the mold cavity. Rapid pressure changes typically do not occur.
Polyurethane plastic parts have been made with these stereolithography rapid tooled
molds.
In addition to vacuum casting stereolithography rapid prototyping technology
was used to create injection molding inserts, These insert unlike the vacuum casting
require much higher cycle pressures. Thermoplastic parts have been built with these
tooling inserts. Thermo-plastic specimens made using epoxy inserts and steel inserts
were compared in tensile strength, impact strength, and bifringence stress. The epoxy
tooled specimens had properties within 5 -10% of the parts molded with steel inserts.
These epoxy tooled parts had greater tensile strength, lower impact strength and lower
birefringence stress levels than there steel tooled parts (2.6).
Polyurethane and thermoplastic materials seem to perform well in
stereolithography tooling. Experiments have shown that as many as 500 parts have
been shot from a single epoxy die (2.7) . However, more demanding plastics like ABS,
polycarbonate and glass filled nylons have not fared as well because of the higher
melting temperatures have caused warping or galling of the parts. Thin walled features
like ribs and bosses are very vulnerable. One research investigation using vapor
deposited metal coating of nickel, copper, and zirconium nitride upon
stereolithography epoxy insert set. Regardless of the coating wear damage was
evident fairly quickly. The maximum number of parts, 14 parts were produced with
the copper coated tool before tool failure (2.8).
Chapter 2 Background
Chapter 2. 7 Commercial Rapid Prototyping Processes used in Rapid Tool-ing
25
Figure 2.19 A, Vacuum Casting Using a SLA Models B. Injection Molds made with SLA Technology
The ability to use stereolithograpy inserts seems to be a tradeoff analysis
between timing, tolerances, temperature(2.9), material, and volume.
Stereolithography tooling can produce prototype plastic parts quickly, giving the
designers the ability to get near-production parts quickly. However, the designer
ability to sufficient cool the part will dictate the materials and volume which can be
successfully produced via these inserts. Although more research is occurring,
currently, the best option to produce tightly tolerances parts in materials like ABS or
nylon still require metal inserts.
Chapter 2. 7.3 Selective Laser Sintering
Selective laser sintering is a SFF process which sinters powered material in a
layer by layer automated fashion to produce a part. Metal and plastic powders can be
used in this process. The powder is coated with a polymer binder and laser intensity is
used to melt the binder and fuse the powder together. The process pioneered by
University of Texas was commercialized by DTM Corporation.
The commercial process has a powder bed with a roller which spreads a thin
layer of material on to a platform. The laser then scans the outline and rasters the
B.
A.
Chapter 2 Background
Chapter 2. 7 Commercial Rapid Prototyping Processes used in Rapid Tool-ing
26
interior of a 2 D layer. Once the layer is completed the platform indexes downward.
The roller then once again spreads a thin and even layered of material across the top of
the last layer. The laser then draws the next layer. This process of lowering,
spreading, and scanning continues until the part is completed. The part must be
removed from the platform and surrounding powder bed and shaken or air blown to
remove excess powder.
Unlike the stereolithography process the build chamber is heated to just below
the glass-transition temperature or melting point of the material or binder. This reduces
the amount of laser power energy needed to consolidate the part. It also reduced part
stress because local part temperature is only raised slightly above the bulk part. The
chamber is also filled with nitrogen to make sure that chemical reactions do not occur.
Typically this process builds parts from polymer materials. ABS, nylon, glass-
filled nylon, and polycarbonate plastics have been used in this process. Metals, like
low carbon steel have also been used. However, these parts have to undergo additional
sintering and infiltration processes. Metal powder used in this process are simply not
heated enough to attain fusion. Only the polymer powder coating the powders is
melted. Additional heat treatment is needed to sinter the metal material and vaporize
out polymer binder. Once this done, the process must be infiltrated to fill pores. The
circular powder when lased simply cannot fuse or shrink together enough to eliminate
pores because of the spherical nature of the powders. Copper infiltration is used to fill
pores and produce a dense part. The final metal part is a hybrid or composite with
reasonable strength and thermal conductivity (2.10, 2.11).
Figure 2.20 SLS Parts Being shaken out of Powder Support Bed.
Chapter 2 Background
Chapter 2. 7 Commercial Rapid Prototyping Processes used in Rapid Tool-ing
27
Chapter 2. 7.4 SLS Rapid Tooling
Most rapid tooling activity have developed from the non-metal SLS process.
Part masters in wax have been used to make rapid investment casting molds for
General Motors. The SLS master is dipped into a ceramic slurry and coated. The
slurry coated part is then heated in a furnace, fusing and solidify the ceramic slurry
while burning or vaporizing the original SLS master. The surviving slurry is now a
perfect pattern to produce cast metal
Injection molding inserts have been built using the metal SLS process (2.12).
Cavity inserts and mold components have been successfully made with SLS for low
part volume injection molding. Fixturing methods have to be considered when using
these inserts. Good contact must be established so that these infiltrated molds will
dissipate heat well and reduce residual stress from thermal cycling. Surface finish in
infiltrated dies needs to be improved. Currently a lot of post processing is required
finish the cavities to acceptable levels. Over fifty polypropylene injection molded
parts have been in one of these infiltrated die sets (2.13).
Die casting molds have also been built for magnesium applications. Aluminum
die casting, because of metal reactivity can not be used in copper composite inserts.
Accuracy problem have limited the development of tooling insert using this approach.
Varying part geometries and wall thicknesses of die inserts lends themselves to non-
uniform shrinkage during the sintering process. Tooling for injection molding or die
casting need to have accurate tolerancing or simpler modelling techniques could be
used to build the prototypes. Sandcasting for metals and direct SLA parts for plastics
can built parts with higher accuracies than using SLS tooled inserts.
Chapter 2 Background
Chapter 2. 7 Commercial Rapid Prototyping Processes used in Rapid Tool-ing
28
Figure 2.21 Rapid Steel Infiltrated SLS Injection Molding Inserts
Chapter 2. 7.5 Laminate Object Manufacturing
Laminate object manufacturing (LOM) is the process of building prototypes by
assembling 2 dimensional sections of adhesive material together. The material is
typically long sheets of adhesive backed paper. During preprocessing, the part is
divided into 2-D layers which are the thickness of the paper. The part is built upon a
wooden substrate which is screwed on to a steel plate. The plate is supported in the
LOM machine for stability. A few layers are adhered to the substrate by the machine
advancing the paper on to the board. A heated roller then presses the paper on to board
melting the adhesive to it. A laser then scans the outline of the paper and hatch a
pattern over the interior of the outline. The paper is then advanced forward. The
outline acts a separation so that only the unused paper, everything in the exterior of the
outline advances. This is repeated for a few layers to aid with part removal.
Now 2D part cross sections are scanned by the laser over the substrate. The
exterior of the cross section is hatched by additional laser scans. This continues until
the part is completed. When the part is completed, it surrounded by excess paper
composite which is similar to a type of sacrificial material. It was needed to support
the part during the building cycle. It is removed in a process called decubing. The
hatching process allows the sacrificial material to be removed as cubes. This process
can be very time consuming. When the part is finally decubed and removed from the
Chapter 2 Background
Chapter 2. 8 Adapted Rapid Tooling Processes
29
substrate, it must be sealed to prevent swelling from humidity and moisture. Epoxy or
wood sealants are typically applied to the LOM parts.
Figure 2.22 Laminate Object Manufacturing
Chapter 2. 7.6 LOM Tooling
LOM paper material parts are most often used with investment or sand
casting. This type of tooling is often used to make cores or patterns. These paper parts
look like wood parts which is similar to traditional pattern parts. LOM is excellent for
tooling applications, because it can build very large tooling beds. Where the SLA and
SLS have size constraints, LOM has a build size of 50.8 cm x 50.8 cm x 50.8 cm.
Also, the paper parts are combustible and are quite suitable for investment casting
because they can be burned out during the firing of the investment shell. LOM inserts
have been used in blow molding, hydroforming, and injection molding. LOM inserts
have been used successfully to die cast magnesium inserts (2.15).
Chapter 2. 8 Adapted Rapid Tooling Processes
Many rapid tooling processes have arisen from the combination of traditional
prototyping methods and commercial rapid prototyping processes. The most advanced
of these are the Nickel Transfer Molding and Keltool Process.
Laser
Roll of Paperor Material
Part in CubePaper Support
Used Paper Roll
Chapter 2 Background
Chapter 2. 8 Adapted Rapid Tooling Processes
30
Chapter 2. 8.1 Nickel Transfer MoldingNickel Transfer Molding is a process which has been commercialized by
CEMCOM corporations. From the CAD file of the part an SLA master is developed to
simulate the die insert set top and bottom faces needed to build the part. The SLA is
then placed in a nickel plating bath and plated. Several millimeters are plated on to the
model. The plated model is then suspended into a supporting frame box. The box is
then filled with a ceramic slurry which solidifies around the model. The SLA model
itself establishes the parting line. Once the slurry has hardened the two halves formed
by the SLA part’s parting line. The newly formed die set is removed from the SLA
model. The interiors of each halve are polished. The completed inserts can now be
used in standard injection molding frames.
Figure 2.23 Nickel Plating Transfer Process: (Courtesy of CEMCOM)
Although these molds wear faster than soft tooling P20 molds, die sets made
in this manner have made over 45,000 glass filled nylon parts.
Part is translated into anSLA part which representnegatives of mold cavities.
SLA part is supported in Nickel plating bath.
When plating has finished,The SLA part is fixtured ina supporting frame. The SLA nickel plated part forma parting line within support
The cavities are filled on both sides with a ceramicslurry.
Chapter 2 Background
Chapter 2. 8 Adapted Rapid Tooling Processes 31
Figure 2.24 NPTP Demolding Process: (Courtesy of CEMCOM)
Figure 2.25 NPTP: Ejector Pins are Placed in Die Cavities
Chapter 2. 8.2 KeltoolKeltool is a process commercialized by 3D Systems, which also uses a SLA
model as a master. Keltool is a sintering technology which creates die inserts from
powdered material. The cavity and core of the tool to create the prototype part are
designed from the 3D CAD file. These core and cavity is then built by
stereolithography. These SLA parts are typically highly detailed. These SLA parts are
now called the master patterns.
Next these SLA masters are used with Room Temperature vulcanized silicone
rubber molding process. Molds of these cavities and cores are created by suspending
them in a frame and filling the frame silicone rubber. The SLA masters are removed
from the newly formed silicone molds. These molds are then filled with a mixture of
Once the slurry has hardened, the two halvesare demolded, leaving2 die inserts.
Ejector pins and other die cavity utilities are drilled orinserted. A complete dieset is now ready for injectionmolding.
Chapter 2 Background
Chapter 2. 8 Adapted Rapid Tooling Processes 32
A6 tool steel powder, tungsten carbide powder, and epoxy binder. When the epoxy
binder mixture has cured within the mold, a green part is made. The green part is then
de-molded and sintered. The part which is the insert is sintered in a hydrogen-
reduction furnace. During the sintering process, the binder material is burned off
leaving a brown part which is a composite of A6 steel and tungsten carbide. The
composite insert now has voids from the burned out binder. The insert is then
infiltrated with copper to make the part fully dense. The final insert is 70% steel and
tungsten carbide and 30% copper. The insert is heat treatable and can achieve
hardness of 40-44 Rc.
Figure 2.26 The Keltool Process (Courtesy of 3D Systems)
Chapter 2. 8.3 Metal Spray ToolingEarly in the 1900’s, Dr. M.U Schoop found that by pouring molten metal into a
high pressure gas stream found that the metal would particulate into drops and deposit
in coatings. Schoop found similar results by passing metallic powder through flame.
Both of theses experiments led to the development of equipment to spray metal in wire
form. Common object sprayed in the 1930’s were dental light bulbs, refrigeration cold
plates, turbine wheels, and brake drums with soft metals like zinc, lead, or copper.
Most metal spraying had been accomplished by electric arc or flame from
Keltool Insert
SLA Cavity & CoreModels
Injection MoldedParts
Chapter 2 Background
Chapter 2. 9 Other Rapid Tooling Processes 33
oxyacetylene torches. By the early 1960’s, a similar process is plasma spraying of
coatings began depositing hard metal or ceramic coatings. Thermal barriers and
oxidation or corrosion resistant coatings are the most common uses of metal spraying.
These coatings can also be designed to reduce wear and friction. Surfacing of die
inserts can be accomplished with spray processes.
In addition to now surfacing pumps with wear resistant coatings, heat and
corrosion resistant coating on electrical boards, massive depositions of spraying have
been attempted.
Chapter 2. 9 Other Rapid Tooling Processes
In addition to Stanford University, other academic or national lab researchers
have been developing processes to produce tooling inserts. The most advance of these
processes are the 3D Printing Process and LENS.
Chapter 2. 9.1 3D PrintingThe 3D Printing process was developed by Massachusetts Institute of
Technology in 19 . The process is very similar to inkjet printing technology. To build
a metal tooling insert, liquid binder is selectively secreted on powdered material. On a
layer by layer basis, the binder is applied in a process very similar to the way ink is
ejected on to the paper from an inkjet printer. The liquid hardens binding the powder
to the part. When the part is completed, it now a matrix of metal and binder. The part
is then sintered removing the binder. The part is then infiltrated with a metal or epoxy
to make a dense part (2.16).
Molding inserts have been built with this process. A ceramic mold made from
alumina powder and colloidal silica binder was built using the 3D Printing process.
The mold was made up of 100 powder binder layers. The mold was used to create a
bras casting in a gravity poured process. For the gravity poured process, the brass is
heated to melting temperature and poured in to the mold set under atmospheric
pressure (2.17). This tooling process has been licensed by Extrude-Hone company
Chapter 2. 9.2 LENS ProcessLaser Assisted Net Shaping process is a direct metal fabrication process which
can produce fully dense parts which can be used as die inserts, patterns, or metal
casting. A ND:YAG lase is used to melt metallic powder or powder mixtures. This is
an additive process building the part layer by layer.
Chapter 2 Background
Chapter 2. 10 Shape Deposition Manufacturing 34
The LENS process uses a computer model to develop a laser deposition path
which represent the part in 2D cross-sections. The sections are deposited successively
in Z direction. The laser paths are changed with each layer. Alternative layers are
deposited at 90-degree angles to the previous layer. The part is built on a moving
platform which can move in X-Y translation. The powder injection nozzle moves
upward to compensate for the building of part height in the z direction.
The process has been used to make fully functional metal parts and metal
molds for injection molding. The LENS process has even been used to repair injection
molding molds. However surface finish and dimensional accuracy are a problem for
this technology. A LENS mold may require manual processing to produce good
surface finished and dimensional accuracy. Optomec is attempting to commercialize
the process.
Figure 2.27 LENS Process (Courtesy of Optomec)
Chapter 2. 10 Shape Deposition Manufacturing
Shape Deposition Manufacturing is an SFF process which was started at
Carnegie Mellon University and developed by Stanford University to produce laser
Powder DeliveryLaser Beam
X Translation
Y Translation
Chapter 2 Background
Chapter 2. 10 Shape Deposition Manufacturing 35
deposited structures. Unlike other SFF processes like stereolithography and laser
sintering, Shape Deposition Manufacturing is a layered manufacturing process which
builds fully dense metal parts by incremental deposition and CNC shaping of material
layers (2.18, 2.19). First, a computer aided design model of a part is sliced into layers.
The layers are in the z-direction and derived by custom planning software. Next a layer
is deposited. The layer is deposited as near-net shape. This near-net shaped layer is
then milled to final dimensions by a 5-axis CNC mill. Support material is then
deposited around the layer to protect the features of this layer and provide a base for
overhanging features in following layers. The next layer of the part material is then
deposited, and the process continues.
Figure 2.28 The Shape Deposition Manufacturing
The combination of layered manufacturing and sacrificial support material
enables the production of complex features such as undercuts or conformal cooling
channels (2.20). Also this technique lends itself to the production of multi-material
structures. For instance, an insert can be produced which is primarily a hard ferrous
alloy with copper deposits for enhanced thermal conductivity. Using the multi-
material strategy, sensors can also be embedded in the die during the build sequence to
develop “smart dies.”
A laser / powder deposition system was used to deposit material for testing and
for construction of a test die (2.21). The system uses a 2.4 kW Neodymium YAG Laser
to fuse metallic powders into fully dense material. The laser is delivered by fiber
optics to an end effector mounted on a four-degrees-of-freedom robotic arm. The end
RemoveDeposit Part withSupport Material
RemoveSupportMaterial
ShapeDeposit MetalWith Laser
Depositand ShapeSupportMaterial
RemoveSupportMaterial
Chapter 2 Background
Chapter 2. 11 SDM Tooling Efforts 36
effector focuses the light on the substrate, creating a melt pool. Metal powder is
added to the melt pool via a powder feed tube and a bead of deposited metal is created
as the robot transverses the substrate. This technique, which is similar to laser
cladding, has been very effective in forming fully dense metal layers. Nitrogen gas
shrouds the deposition to help prevent oxides from forming during the deposition
process.
Figure 2.29 Powder placement during the SDM Process
Chapter 2. 11 SDM Tooling Efforts
Several tooling inserts have been built using the SDM process. The GM
injection molding insert, the Alcoa tool injection molding insert were built for industry
partners of the Stanford Rapid Prototyping Lab.
substrate
deposited layer
laser
direction of travel
powder
Chapter 2 Background
Chapter 2. 11 SDM Tooling Efforts 37
Figure 2.30 Equipment Setup for the SDM process
Chapter 2. 11.1 GM Injection Molding ToolThe SDM process was used to produce an injection mold for an electronics
compartment cover with snap fit tabs. The part was split into three main elements a
support level, cooling channel level, and a feature level. The support level is simply
the base of the injection molding insert. This level has no distinct features and merely
needed to allow the insert to fit into the molding base. The cooling channel level
contains the concentric cooling channel design needed to control temperature within
the insert and reduce warpage of the insert. The feature level is the top part of each
insert which will serve to mold and eject the part.
By segmenting the insert design in this manner, one can plan the deposition
and shaping of the part to enhance build time and reduce tool wear. The support level
of the insert 15 mm thick plate of 316L stainless steel. As described above a substrate
is deposited upon to build parts with the SDM method. Thus the support level and part
substrate are combined. Next, 316L stainless steel powder is then deposited and fused
to form the channel level of the part. After, a thickness of 10 mm is deposited, cooling
end effector
nitrogen shroud
powder feed tube
Chapter 2 Background
Chapter 2. 11 SDM Tooling Efforts 38
channels are then machined in the deposit. The passages are then filled with microcast
copper to preserve the integrity of the channels as the next layers of the part are
deposited.
The feature level was laser deposited and near net-shaped by CNC on layer by
layer basis. Layer thickness averaged .25 mm. When the structure was completed, the
copper channels where etched, removing the copper so that channels could be used.
many features because of size or taper angle had to be EDM.
The inserts where placed in a 10 ton injection molding machine. The inserts
were prepped to fit into the mold base. About 20 nylon parts were run to see if the
inserts worked. As expected with such a short run, no visible die wear occurred.
Figure 2.31 GM Tool
Chapter 2. 11.2 Alcoa Injection Molding ToolA set of injection molding inserts were made for Alcoa using the SDM process.
The inserts are a composite stainless steel tool. Residual stresses caused warpage or
deformation to the GM tool during the deposition process. This warpage added
additional machining and heat treating hours to the deposition process. To combat this
during the deposition of the Alcoa inserts, the interior of the insert is deposited with
invar instead of 316L stainless. Because of the intricacy of the inserts, only two
planning levels are available: the channel level and the feature level. Microcast
copper is deposited into cooling channels.
Sinker EDM Feature
Wire EDM Feature
Zone 1
Zone 2
Zone 3
Chapter 2 Background
Chapter 2. 12 Other Developing Laser Deposition Technologies 39
Once the bulk shape of the insert had been deposited and shaped, the copper
channels are etched out of the insert. Next specific tapers and part level features that
cannot be CNC machined were electro-discharge machined. The molding inserts
where completed and sent to Alcoa in December of 1998. Because of budget cuts the
tool was never tested.
Figure 2.32 Alcoa Injection Molding Inserts
Figure 2.33 The Interior of the Alcoa Tool
Chapter 2. 12 Other Developing Laser Deposition Technologies
There are other laser based technologies which could potentially be used to
StainlessSteel
Invar
Legend
Copper
Chapter 2 Background
Chapter 2. 12 Other Developing Laser Deposition Technologies 40
build die casting prototype inserts and other forms of rapid tooling. Some of the
outcomes of this research will be able to benefit not only SDM laser process but other
laser technologies as well.
Chapter 2. 12.1 Laser-induced Vacuum Arc DepositionLaser-induced vacuum arc deposition is a process which combines the
controllability of pulsed laser deposition with vacuum arc technology (2.22). This
allows for very small droplets which produces a fine films. Typically amorphous
carbon films are made. These films are very hard and have excellent wear resistance
and low friction. This method has been used to deposit hard films on metallic
substrates. This technology could potentially be used to face die inserts to make them
more wear resistant yielding longer insert die life. Potentially could allow softer steels
like 316L to become more wear resistant so they could potentially be used to prototype
die casting or glass-filled nylon injection molding.
Chapter 2. 12.2 CO2 Laser DepositionThis process uses a CO2 laser to solidify metallic powder (2.23). The substrate
translate in the X,Y, and Z directions as powder is fed into the interaction zone on the
substrate. The laser power ranges between 300-400W and is focussed on to the
substrate in a donut shape with a 600 mm diameter. Helium gas is used as a shield gas.
Stainless steel 304L parts have been built with this method. Tool steel inserts may also
be able to be made with this process.
Chapter 2. 12.3 Pulsed Laser DepositionExcimer Nd:YAG or CO2 lasers are used to produce vapor of plasma states to
deposit or grow thin films (2.24). Ceramic thin films have been grown by pulsed lasers
on stainless steel, hard metal, Si, SrTiO3, and ZrO2. This technology can be used for
hardfacing tools. If multiple layer films can be built without of loss of adhesion or
delamination, this technology may be able to build feature level of die inserts.
A similar process called laser implant deposition uses a KF excimer laser to
deposit and incorporate silicon on the surface of stainless steel.
Chapter 2 Background
Chapter 2. 12 Other Developing Laser Deposition Technologies 41
Chapter 2. 12.4 Laser Direct CastingLaser Direct Casting is a laser cladding process which uses a coaxial nozzle to
deposit and laze metal powders (2.25). Metal powder is injected into a laser generated
melt pool. The substrate translates in the X, Y, and Z directions in order to build 3D
parts. The laser power used in this process 400 W - 1400W and speeds of 500 - 1000
mm/min. Fully dense parts have been built with this process.
Chapter 2. 12.5 Laser CladdingLaser cladding is a process very similar to Laser direct casting (2.26). With
this process CAD.CAM systems are uses to develop the laser path also known as
cladding tracks. AISI 1045 steel plate and steel rollers have been deposited upon with
this process. In addition to metal depositions, metal matrix composites with ceramics
have been made. Cutting dies and stamping dies have been made with this process. A
similar process was developed at Los Alamos National Laboratory called Direct Laser
Fabrication (2.27). It is a near-net shape technology which uses CAD/CAM with a
high energy laser beam to produce fully dense parts.
Another laser cladding technique used powder blowing to place metallic
powder in the path of the laser for fusion to the substrate (2.33). A 5 KW CO2 laser is
used to solidify the powder. This process allows for very fine microstructure, no
porosity, uniform layer thickness and little dilution of material into substrate. The
HAZ produced is very small while the interface between cladding and substrate is very
sharp.
Chapter 2. 12.6 Laser induced Chemical Vapor DepositionAn argon ion laser beam is used to grow films of titanium nitride (2.28). These
films arise from direct laser pyrolysis of TiCl/4N/2/H/2 gas at atmospheric pressure.
These deposited films are hard, rough, and porous. Tool steel substrates have been
used in this process. The porosity of these coating may lend it unsuitable for tool
facing or build tool inserts. Other laser-induced chemical vapor deposition process are
at elevated pressures of 40 mbar and pulses at 100 to 600mW for nickel-iron films
(2.29). A similar process using an ArF excimer laser and low power CO2 laser to
produce pyrolytic laser chemical vapor deposition (2.32). This process has been used
to coat tool steels as well as small industrial tools.
Chapter 2 Background
Chapter 2. 13 Requirements for Die Cast Inserts 42
Chapter 2. 12.7 Laser Fused Spray ToolingMolybdenum powder is predeposited on a steel surface by plasma spraying.
This coating is then fused by a continuous wave Nd:YAG laser (2.31). In addition to
densification of the predeposited molybdenum, alloying with the steel substrate
occurs. Surfaces treated by this method have excellent wear properties. The process is
monitor for sound emissions to determine crack intensity during alloying to evaluate
laser and coating parameters as well as process quality. As long as deformation of the
substrate or die can be controlled, this process could be used to face die inserts.
Another process similar to the molybdenum spray process, is laser-surface
melting (2.30). A 3KW CO2 laser is used melt plasma spray surfaces of ceramics or
metal. These surfaces are produced by transferred plasma jet technology. The laser is
used to improve the homogeneity of plasma sprays which inherently contains voids,
cracks or pores. Plasma oatings100-200 µm thick have been homgenitized with laser
surface melting.
Chapter 2. 12.8 Hot wire laser depositionA laser beam is used to melt a hot wire electrode (2.34). Two millimeter thick
coatings have been built by this system. The high temperature gradients and intensity
of the laser interaction with the wire and substrate causes limited dilution and limited
penetration into the substrate. Corrosion resistant coatings have been made using this
technology. This technology could be used to hard face metal injection molding or
die casting inserts.
Chapter 2. 13 Requirements for Die Cast Inserts
To rapid tool die cast inserts, a designer must understand the material and
functional requirements that the inserts must have in order to successfully produce
castings. Understanding the service requirements of the environments in which the
inserts will be used is essential to designing viable parts.
Die casting results in abrupt thermal and pressure changes on the insert during
the injection of the molten aluminum. An insert for aluminum die casting may
encounter temperature changes from 150oC to 670oC and pressure changes from
ambient to 142 MPa in a cycle time as short as 20 seconds. To cycle through these
changes in temperature and pressure, die casting inserts must be made of material
Chapter 2 Background
Chapter 2. 13 Requirements for Die Cast Inserts 43
which possess the following characteristics:
•Low Coefficient of Expansion for high thermal fatigue resistance
•High hardness (44-48 Rc) for wear resistance (2.35)
•High modulus of elasticity or impact resistance to avoid deformation
from galling and heat checking
•Moderate thermal conductivity to produce castings of similar micro-
structures as H13 production inserts (on the order of 24 W/mK)
When designing die cast inserts, the material used must be able to survive these
conditions. Also, the material characteristics must have similar thermal properties to
insure proper microstructual development of castings produced by the inserts.
Additional requirements may be added because of processing requirements of laser
based deposition, particularly requirements of the SDM process. Initially the only
additional requirement is high corrosion resistance. SDM deposits sacrificial material
to support undercut features or preserve cooling channels shaped within the die.
Utilizing the 400 series martensitic stainless steel materials, will also add requirement
to insure the production of sound inserts with minimal deformation.
Chapter 2 Background
Chapter 2. 13 Requirements for Die Cast Inserts 44
2.1 Barkan, P and Iasiti, M. (1993). “Prototyping: A tool for Rapid Learning in Prod-uct Development.” Concurrent Engineering: Research and Applications 1: 125-134.
2.2 Barkan, P. (1991). “Strategic and Tactical Benefits of Simultaneous Engineering.” Design Manufacturing Journal (Spring): 39-41.
2.3 Kalpakjian, Manufacturing Engineering & Technology, 19952.4 Leu, Ming C., “Feasibility study of EDM tooling using metallized stereolithogra-
phy models,” Technical Paper - Society of Manufacturing Engineers, Proceed-ings of the NAMRX XXVI Conference Atlanta, GA, USA
2.5 Kai, Chua Chee, “Integrating rapid prototyping and tooling with vacuum casting for connectors,” International Journal of Advanced Manufacturing Technol-ogy, v14 n 9 1998. pp. 617-623.
2.6 Polosky, Quentin F., Mechanical property performance comparison for plastic parts produced in a rapid epoxy tool and conventional steel tooling, Annual Technical Conference - ANTEC, Conference Proceedings. Special Areas Annual Technical Conference - ANTEC, Conference, Proceedings v 3 1998, p 2972-2976.
2.7 Rahmati, Sadegh and Dickens, Philip, Stereolithography for injection mould tool-ing Rapid Prototyping Journal. Rapid Prototyping Journal v 3 n 2,1997. p 53-60.
2.8 Burns, David T., Malloy, Robert A, McCarthy, Stephen P., Analysis of metal coat-ing effects on stereolithography tooling for injection molding, Annual Techni-cal Conference - ANTEC, Conference Proceedings, Proceedings v 1 1998, p 888-892.
2.9 Janczyk, M., Rapid stereolithography tooling for injection molding: The effect of cooling channel geometry, Journal of Injection Molding Technology. Journal of Injection Molding Technology v 1 n 1 Mar 1997. pp. 72-78.
2.10 Lakshminarayan, U., McAlea, K., Girouard, D., and Booth, R., Manufacture of iron-copper composite parts using selective laser sintering (SLS**T**M), Advances in Powder Metallurgy and Particulate Materials, v. 3, p 13/77-13/85.
2.11 Klocke, F.,Celiker, T., and Song, Y.-A., Rapid metal tooling, Rapid Prototyping Journal. Rapid Prototyping Journal v 1 n 3, 1995. pp. 32-42.
2.12 Hornig, C. and Lohner, A., Direct laser sintering of metal powder, Kunststoffe Plast Europe. Kunststoffe Plast Europe v 87 n 11, Nov 1997. pp. 72.
2.13 Killander, Lena Apelskog, Rapid Mould: Epoxy-infiltrated, laser-sintered inserts, Rapid Prototyping Journal. Rapid Prototyping Journal v 2 n 1,1996. pp. 34-40.
2.14 Pak, Sung S, Prototype tooling and manufacturing through Laminated Object Manufacturing (LOM), International SAMPE Symposium and Exhibition (Proceedings), v43 n 1 1998. SAMPE, Covina, CA, USA. pp. 685-692.
2.15 Warner, Merlin C., Rapid prototyping for die casting: today's applications and future developments, Die Casting Engineer. Die Casting Engineer v 40 n 2 Mar-Apr.,1996. 4pp.
Chapter 2 Background
Chapter 2. 13 Requirements for Die Cast Inserts 45
2.16 Sachs, E, Williams, P., Brancazio, D., Cima, M., and Kremmin, K., Three-Dimen-sional Printing. Rapid tooling and prototypes directly from a cad model, Pro-ceedings of Manufacturing International '90. Part 4:, 1990, p 131-136.
2.17 Sachs, Emanuel, Cima, Michael, Brancazio, David, Curodeau, Alain, and Shalon, Tidhar, Three dimensional printing. Rapid fabrication of molds for casting, American Society of Mechanical Engineers, Production Engineering Division (Publication) PED. Advances in Integrated Product Design and Manufacturing American Society of Mechanical Engineers, Production Engineering Division (Publication) PED v 47. Publ by ASME, New York, NY, USA. pp. 95-10.
2.18 Rahmati, Sadegh, Rapid Prototyping Journal, Vol. 3, No .2, p. 53, (1997)2.19 Pintat, M. and Greulich, M., Proceedings Solid Freeform Fabrication Sympo-
sium, The University of Texas at Austin, pp. 74 (1995).2.20 Merz, R., Prinz, F. B., Ramaswami, K., Terk, M., and Weiss, L. E, Proceedings
Solid Freeform Fabrication Symposium, The University of Texas at Austin, p. 1, (1994).
2.21 Fessler, J. R., Merz, R., Nickel, A. H., and F. B. Prinz, Proceedings Solid Free-form Fabrication Symposium, The University of Texas at Austin, p.117, (1996).
2.22 Rebholz, C.,Scheibe, H.-J., Schultrich, B., and Matthews, A. Mechanical and tri-bological properties of hard aluminium-carbon multilayer films prepared by the laser-arc technique, Surface & Coatings Technology v107 n 2-3 Sep 10 1998. pp. 159-167,1998.
2.23 Kahlen, Franz-Josef,Kar, Aravinda, Watkins, Tom, and Burl, C., Stress analysis in rapid manufacturing, Laser Institute of America, Proceedings. Laser Institute of America, Proceedings v 83 n 2 1997. Laser Inst of America, Orlando, FL, USA. pp. E76-E83.
2.24 Kreutz, E.W., Pulsed laser deposition of ceramics - fundamentals and applica-tions, Applied Surface Science. Applied Surface Science v 127-129 May1 1998. pp. 606-613.
2.25 McLean, Mark A, Shannon, Geoff J., and Steen, William M., Laser Direct Cast-ing high nickel alloy components, Advances in Powder Metallurgy and Partic-ulate Materials v 3, 1997. Metal Powder Industries Federation, Princeton, NJ, USA. pp. 21-3-21-16.
2.26 Hu, Y.P., Chen, C.W., and Mukherjee, K., Analysis of powder feeding systems on the quality of laser cladding,Advances in Powder Metallurgy and Particulate Materials, Advances in Powder Metallurgy and Particulate Materials v 3, 1997. Metal Powder Industries Federation, Princeton, NJ, USA., p 21-17-21-31.
2.27 Lewis, Gary K., Lyons, Peter, Direct laser metal deposition process fabricates near-net-shape components rapidly, Materials Technology. Materials Technol-ogy v 10 n 3-4 Mar-Apr. 1995. pp. 51-54
2.28 Reisse, Guenter and Ebert, Robby, Titanium nitride thin film deposition by laser CVD, Applied Surface Science. Applied Surface Science v 106 Oct 2, 1996. pp. 268-274.
2.29 Maxwell, J.L., and Pegna, J., Deangelis, D.A., and Messia, D.V., Three-dimen-
Chapter 2 Background
Chapter 2. 13 Requirements for Die Cast Inserts 46
sional laser chemical vapor deposition of nickel-iron alloys, Materials Research Society Symposium Proceedings. Advanced, Laser Processing of Materials - Fundamentals and Applications, Materials Research Society Sym-posium Proceedings v 397 1996.,Materials Research Society, Pittsburgh, PA, USA. pp. 601-606.
2.30 Pujar, M.G., Dayal, R.K., and Singh Raman, R.K., Microstructural and aqueous corrosion aspects of laser-surface-melted type 304 SS plasma-coated mild steel, Journal of Materials Engineering and Performance v 3 n 3 June 1994. pp. 412-418.
2.31 Haferkamp, H., Gerken, J., Toenshoff, H.K., and Marquering, M., Laser alloying of molybdenum on steel surfaces to increase wear resistance, , Proc 1995 9 Int Conf Surface Modif Technol 1996. Minerals, Metals & Materials Soc (TMS). pp. 547-564.
2.32 Zergioti, I.Zervaki, A., Hatziapostolou, A., Haidemenopoulos, G., and Hontz-opoulos, E., Deposition of refractory coatings by LCVD, Optical and Quantum Electronics. Optical and Quantum Electronics v 27 n 12 Dec 1995. pp. 1377-1383.
2.33 Yellup, J.M., Laser cladding using the powder blowing technique, Surface & Coatings Technology. Surface & Coatings Technology v. 71 n 2 Mar 1995. pp. 121-128.
2.34 Bouaifi, Belkacem and Bartzsch, Jorg, Surface protection by laser beam deposi-tion with hot wire addition, Welding Research Abroad. Welding Research Abroad v 40 n 8, Aug-Sept 1994. pp. 31-33,1994.
2.35 Skoff, J. V., Die Casting Engineer, Vol. 31, n. 1 , p. 58, (1987)
Chapter 3 Testing Procedures
Chapter 2. 13 Requirements for Die Cast Inserts 47
Chapter 3 Testing Procedures
The investigative approach to designing successful carbon steel inserts or any
parts using SDM laser technology has been to understand, at a fundamental level,
microstructural development. Microstructure and processing conditions have been
shown to be highly correlated to material properties like strength and hardness.
Residual stress development has also been correlated to the microstructure.
Figure 3.1 Microstructure and grain growth.
To begin to understand the microstructure, a number of procedures had to be
Chapter 3 Testing Procedures
Chapter 3. 1 Grain Size 48
used to analyze SDM processed microstructure. Measurements of grain size, phase
volume, and material properties were studied to characterize SDM parts. Both manual
and automated processes were used.
Chapter 3. 1 Grain Size As Figure 3.1 illustrates, grain size influences material properties. With
increasing grain size, strength and hardness decrease. ASTM standard E112-96
describes procedures for measuring average grain size. These procedures characterize
two dimensional grain size which is exposed by sectioning test samples. Manual and
automated methods exist to measure grain size. Both methods were used to measure
grain sizes as well as verify technique accuracy.
Chapter 3. 1.1 Point Intercept MethodThree to five horizontal lines are placed across a metallographic image. The
length of the lines calibrated to the same scale as features within the image are drawn.
Next the number of times a grain boundary intersects one of the lines is summed. The
line intercept count (PL) is the number of intersections counted divided by the total
length of the lines.
Figure 3.2 Point Intercept Method
The PL is used to calculate the surface area in a unit volume. It is needed to
estimate the grain diameter. This method can also be accomplished with the use of a
circle. The circle must be larger than the largest grain. Total length is simply the
circumference of the circle. This method, also known as the Hillard method, and
reduces directional bias when counting intercepts.
Chapter 3. 1.2 Automated Point Intercept using Photoshop Plug InA program written by Reindeer Inc. uses pixel value measurements of grain
PL = counts / total length
PL = 5/ (3*100µm)
=.016 counts /µm
SV = 2* PL = .032 counts / µm
D = 8/(3SV) = 83µm
Chapter 3 Testing Procedures
Chapter 3. 2 Volume Measurement and Phase Confirmation 49
boundary images to measure the equivalent diameter and shape of grains in a
metallograph. The metallograph is prepared by thresholding the RGB (Red-Green-
Blue) image so that black represents the grain and white represents grain boundaries.
Depending on how the image is etched and photographed, the image may be inverted
so that the grain will be thresheld as black and the grain boundaries are white. Other
aspects of this program can measure grain perimeter, grain center of gravity in x and y
coordinates.
Figure 3.3 Grains of SDM Tool Steel Analyzed by Photoshop Plug In
Chapter 3. 2 Volume Measurement and Phase Confirmation
Chapter 3. 2.1 Point Intercept MethodThe ASTM E562-1995 procedure uses a grid of 36 points area to measure
phase volume fraction. The grid is a 6 x 6 square area which is placed over a
metallograph. The phase or features of interest which intersect grid lines are counted.
The number of intercepted grid points divided by the total number of grid points (36)
is an estimate of the volume fraction. The volume fraction of a phase or constituent is
the fraction of the volume of the structure that it occupies.
Accuracy depends upon selecting several grids. For 10% relative accuracy,
625 fields would need to be analyzed for a 2% volumetric phase and 63 fields would be
needed for a 20% volumetric phase using a 32 point grid.
Chapter 3. 2.2 X-ray DiffractionX-ray diffraction was used in two ways: Phase Identification and Stress
analysis. For randomly oriented samples, quantitative measurements of volume
fraction of constituents like martensite, or austenite can be made from X-ray
diffraction patterns because the total integrated intensity of all the diffraction peaks for
10µm Inverted and Thresholded Image
Chapter 3 Testing Procedures
Chapter 3. 2 Volume Measurement and Phase Confirmation 50
Figure 3.4 Point Intercept Method for Volume Analysis
each phase is proportional to the volume fraction of that phase. Moreover, if the
crystalline phases or grains of each phase are oriented randomly then the integrated
intensity of any single diffraction peak is proportional to the volume of the fraction of
that phase.
Copper A radiation was used for most of the quantitative measurements.
Because of background emissions or limited penetration depth of copper, the beam
count time was extended 30 sec. per .25o.
As figure 3.5 shows characteristic peaks of austenite and martensite existed
Figure 3.5 Analysis of Integrated X-ray Intensity for Phase Volume Analysis.
Point Count, Pp
Pp = 3/36 = 1/12 = 8%
of
C%austenite +C%martensite + C%carbides =1
v = volume of cell unitp=multiplicity factorF = structure factor, F=f(f)e-2M =temperature factor
Assumptions: polycrystalline specimens,
completely filling the incident beam at all angles.
form of flat plate of effectively infinite thickness, randomly oriented grains, making equal angles with incident and diffracted beams
0
50
100
150
200
250
X R
ay D
iffra
ctio
n C
ount
s 300
350
400
4 1 4 2 4 3 4 4 4 5
Two Theta Angle4 6 4 7 4 8
111 Austenite Plane
110 Martensite Plane
IausteniteImartensite
=Raustenite * C%austenite
Rmartensite * C%martensite
R = ( 1v2 )[F2 * p * ( 1+cos2 2Θ
sin 2 Θ*cosΘ)(e−2 M )
Where:
Chapter 3 Testing Procedures
Chapter 3. 2 Volume Measurement and Phase Confirmation 51
with in an x-ray scan of the SDM tool steel samples. For peak separation, higher
angles were used to calculate integrated intensities to determine phase percentages.
X-ray stress measurements were also made. X-ray methods permit the
determination of the surface stress components which characterize the existing stress
system. X-rays measure biaxial stress within the surface because no stress exists at the
free surface.
Figure 3.6 Full X-ray diffraction Pattern
The relation of stress and strain are shown in Figure 3.7 . Using this formula
we can determine σx by finding the difference of strain in two angle in plane xz, and
σy can be found by measuring strain in yz plane. Strain can be measured by x-ray
diffraction of lattice parameters by the use of Bragg’s Law as shown in Figure 3.8.
Lastly, stress can be approximated by measuring strain at number of angles in a
particular plane and plotting the strain versus the square of the sines of each angle.
From the slope of the line the stress can be attained as shown in Figure 3.9. This
method is often called the Sin2Ψ method.
0
5 0
1 0 0
1 5 030 50 70 90
11
0
Co
un
ts 11
1 M
art
en
site
2theta
11
1A
ust
en
ite
00
2A
ust
en
ite
21
0 A
ust
en
ite 20
0 M
art
en
site
02
2 A
ust
en
ite
11
2 M
art
en
site
22
0 M
art
en
site
31
0 M
art
en
site
21
1 M
art
en
site
31
1 A
ust
en
ite
2θ
Counts
111
Aus
teni
te
111
Mar
tens
ite
00
2 A
uste
nite
200
Mar
tens
ite
020
Aus
teni
te
211
Mar
tens
ite11
2 M
arte
nsite
311
Aus
teni
te
220
Mar
tens
ite
310
Mar
tens
ite
30 50 70 90 11090% Martensite5.3% Austenite
Chapter 3 Testing Procedures
Chapter 3. 2 Volume Measurement and Phase Confirmation 52
Figure 3.7 Relation of Stress and Strain
Figure 3.8 Braggs Law of X-ray Diffraction and Strain
Figure 3.9 Stress Approximation by Sines.
Another method of determining stress measurements is called Fastress. A
Fastress machine was used to determine stress and retained austenite. This system uses
Ψεψ
σxσy εψ = 1/E (σx[(1+v)sin2ψ -v]-vσy)
εψ =Strain in xz plainσx = stress in x directionσy = stress in y directionE = Youngs Modulusv = Poisson’s Ratio
εψ2-εψ1 = 1/E (σx[(1+v){sin2ψ2 -sin2ψ1} ]
σx = (E/(1+v))[1/{sin2ψ2 -sin2ψ1})(ε ψ2-εψ1)
Z
X
Y
Ψεψ
σxσy
εψ = (dψ-do)/doεψ2-εψ1 = (dψ2-dψ1)/do
d = 2 sin θ l λd = lattice spacing2θ = diffraction Angle of X-rayλ = X-ray wavelength employed
If Ψ1 = 0 then:
σx = (E /(1+v))[1/{sin2ψ2)]* (dψ2-dψ1)/do
=Bragg Law
sin2ψ
εψslope = σx (1+v)/E
Ψ1, Ψ2, Ψ3 ...
Chapter 3 Testing Procedures
Chapter 3. 2 Volume Measurement and Phase Confirmation 53
two chromium targets and four counters. The two x-ray beams are positioned so that
two incident beams strike the fixed specimens at 0o and 45o for simultaneous
measurement. When the counters are centered on the lines of diffraction, a voltage
proportional to the difference of the angular positions is recorded. Since the voltage is
a linear function of (2ψ1-2ψ2), the stress can be measured directly. The Fastress acts as
a stress gauge which must be calibrated with known stresses. It has been designed to
measure stresses in ferritic and martensitic material.
Figure 3.10 Equations of Stress for Fastress Machine
Cross sections of laser deposited tool steels where analyzed by the Fastress
technique to gain a stress profile of the material in the as-deposited condition. The
Sin2Ψ method was used to measure the average stress of a 316L stainless steel deposit
and the average stress of the substrate for the 316L deposit. All measurements were
made with the substrates still attached to the laser deposit.
Chapter 3. 2.3 Scanning Electron MicroscopeScanning Electron Microscopes (SEM) were patterned after light microscopes
and yield similar information about topography, morphology, and composition. SEM
can relay topographical information about the surface features of an object, its texture
or other detectable features limited to a few manometers. SEM can relay
morphological information about the shape, size and arrangement of the particles
making up the object that are on the surface or have been exposed by grinding or
chemical etching. All detectable morphological features are limited to a few
nanometers. Lastly, SEM can relay compositional information about elements and
compounds making up the sample relative to the surface can be determined but limited
to 1 micrometer in diameter area.
Light microscopes can also be used for topographical and morphological
Ψεψ
σxσy
If Ψ1 = 0 then:
σx = (E /(1+v))[1/{sin2ψ2)]* (dψ2-dψ1)/do
With negligible error replace do with dψ2 or dψ1[]σx = (E /(1+v))[1/{sin2ψ2)]* (dψ2-dψ1)/dΨ1differentiate Bragg Law : ∆d/d= - cot θ ∆2θ/2
σx = (E cotθ (2(2ψ1-2ψ2))/{2(1+v))[1/sin2ψ2)]}
Chapter 3 Testing Procedures
Chapter 3. 2 Volume Measurement and Phase Confirmation 54
information. However, the SEM has a much higher resolution. Resolution with a
light microscope is 0.0002 mm while with a scanning microscope it is 0.000000001
mm. SEM can attain higher magnifications. A light microscope can magnify and
object up to 1,000X, while the scanning microscope may go up to 400,000X.
The SEM images are obtained by using a very small electron probe (or electron
spot) scanning over the surface of the specimens and by mapping the detected electron
signals from each specimen pixel onto the corresponding pixel of the Cathode Ray
Tube (CRT) or Charge Coupled Device (CCD), i.e. the screen. Typically a device
called an electron gun at the top of the apparatus produces a stream of monochromatic
electrons. The stream is condensed by the first condenser lens which is usually
controlled by the “coarse probe current knob”. This lens is used to form the beam and
limit the amount of current in the beam and works with aperture of the condenser to
eliminate the high-angle electrons from the beam. A second condenser lens focuses the
electrons into a thin, tight, coherent beam and is controlled by the “fine probe current
knob” The aperture of the second condenser again eliminates high-angle electrons
from the beam. A set of coils then scans the beam The dwell time is typically
microseconds. The final lens which is the Objective, focuses the scanning beam onto
specimen. When the beam strikes the sample, electrons scatter or excited within the
specimen. These electrons are detected with various instruments and counted. A pixel
value corresponding to the number of counts is displayed on the CRT /CCD. The pixel
intensity corresponds to the counts: (the higher the count the brighter the pixel). This
process is repeated until the grid scan is finished and then repeated, the entire pattern
can be scanned 30 times per second.
For this research, SEM was used to examine fracture surfaces such as
solidification and stress cracks. The ease of specimen preparation, high resolution,
and extensive field of depth make the SEM an invaluable analysis tool. The emissive
mode of SEM which utilizes low-energy secondary electrons emitted from the
specimen surface produces high resolution images contrasting surface roughness or
height of topographic features making the examination of cracks or fracture to within
4 nm.
Samples are mounted in bakelite or conductive SEM plastic mount. If a non-
conductive mount is used, carbon leads are painted from sample to the backside of the
mount to allow for charging of sample for imaging. Sample are polished up to .05 µm
silicate slurry finish.
Chapter 3 Testing Procedures
Chapter 3. 2 Volume Measurement and Phase Confirmation 55
Chapter 3. 2.4 Transmission Electron MicroscopyTransmission Electron Microscopy (TEM) was used to confirm crystalline
structures of martensite and austenite. In the standard mode, bright field and dark
field images were taken. A bright field image is produced when only the direct beam is
used for image formation. A dark field image is formed when the diffracted beam is
used for image formation.
Lattice images were also taken. These images were used to index diffraction
patterns to confirm the existence of phases. Double diffraction patterns were
differentiated based upon intensities. Angles are measured from the intensity of the
patterns to determine poles and stereographic projects of planes. Bases on determined
planes fcc, bct, and bcc phases were found.
Images in transmission electron microscope form when incident electrons are
scattered by the specimen and focused by one or more electromagnetic lenses.
Electrons scatter elastically, without energy loss (velocity and wavelength remain
unchanged) if they hit the nuclei of specimen atoms. Electrons scatter inelastically
with loss of energy (velocity decreases and wavelength increases), when the orbital
electrons of the specimen atoms are hit. Inelastic events generally involve deflections
through small angles (<10-4 radians) and cause specimen damage . The amount of
scattering is proportional to the thickness of the specimen or atomic number. Thick
specimens, or those with large number atomic numbers, scatter more electrons than
thin specimens or ones with low average atomic number. TEM specimens are typically
thinner than 50 nm. This reduces the number of collisions because most electrons pass
through the specimens without scattering. However, the electrons that do scatter are
sufficient to produce images but without causing specimen damage.
TEM differs from light microscopy techniques in different ways. Optical
lenses are generally made of glass with fixed focal lengths. TEM uses magnetic lenses
which are constructed with ferromagnetic materials and windings of copper wire
producing a focal length which can be changed by varying the current through the coil.
Magnification in the light microscopes is generally changed by switching between
different power objective lenses mounted on a rotating turret above the specimen. or
by changing to different power oculars (eyepieces). The magnification of TEM arises
changing focal lengths but the objective remains fixed. While light microscopes have
small depth of fields, the TEM have large depth of fields allowing for the full sample to
be in focus simultaneously.
A bright field image is produced when only the direct beam is used for image
Chapter 3 Testing Procedures
Chapter 3. 2 Volume Measurement and Phase Confirmation 56
formation. In other words, unscattered electrons of the incident beam combine with
scattered electrons which have been modified or refocus by passage through objective
aperture. Dark areas in the bright field image arise from specimen regions which
scatter electrons widely and into the objective aperture.
A dark field image is formed when the diffracted beam is used for image
formation. If only scattered electrons are used (unscattered electrons are removed), t a
dark field image is produced. The viewing screen is dark unless there is a specimen
present to scatter electrons. Dark field images typically have higher contrast than
bright field images. However, since the intensity is greatly reduced, longer
photographic exposures required.
The objective aperture can be displaced sideways to intercept the main
unscattered electrons. To study specific crystallographic orientations the apertures may
be placed off-axis or the beam may be tilted. This dark field image may be of poor
quality because the aperture accepts off-axis electrons subject which are subject to
larger aberrations (spherical and chromatic) than those on the optic axis. However, if
the incident electron beam is tilted at such an angle that it is intercepted by the aperture
and the diffracted beam of interest travels down the objective lens axis, only minimum
aberrations exist. These aberrations are similar to those suffered by a bright field
image. However, both of these methods allow only certain diffraction spots/rings to be
transmitted so only specific crystallographic orientations will be highlighted in the
image.
These crystallographic or lattice images expose crystal lattices. Crystals are
composed of groups of atoms repeated at regular intervals in three dimensions with the
same orientation. This group of atoms or the collection of points form is the space
lattice or lattice of the crystal. A crystal lattice can be indexed so that material phases
can be identified.
Chapter 3. 2.5 Electron Microbeam ProbeElectron microprobe analysis (EMPA) is a non-destructive method for
determining the chemical composition. EPMA allows one to quantitatively determine
the chemical composition of nearly all solids on the micron scale. This is achieved by
collecting x-rays that are emitted from atoms which have become “excited” by a
primary beam of electrons. Localized specimen chemistry can be achieved which can
provide insight into the degree of chemical homogeneity which will affect bulk
mechanical, electrical and thermodynamic qualities.
Chapter 3 Testing Procedures
Chapter 3. 2 Volume Measurement and Phase Confirmation 57
EMPA uses a high-energy focused beam of electrons to generate X-rays
characteristic of the elements within a sample from a volumes as small as 3
micrometers diameter. The resulting X-rays are diffracted by analyzing crystals and
counted by detectors. Chemical composition is determining by comparing the
intensity of X-rays from standards of known composition with those from the
unknown materials. The measurements are corrected for the effects of absorption and
fluorescence within the sample.
EMPA uses an electron beam current from 10 to 200 nanoamps to excite X-
rays. This beam current is about 1000 times greater than the beam used in SEM
analysis. These higher beam currents produce more X-rays from the sample and
improve both the detection limits and accuracy of the resulting analysis. Analysis
locations upon the specimen can be selected by using a transmitted-light optical
microscope mode or SEM mode, which allows positioning accurate to about 1
micrometer. The resulting data can yield quantitative chemical information in a
textural context. Variations in chemical composition within a material, such as a
mineral grain or metal, can be determined.
In this research, EMPA was used to measure chemical composition of material
interfaces and areas with graded material transitions. EMPA was very important in
determining the sharpness of interfaces within the micrometer range.
Chapter 3. 2.6 EtchingTEM and X-ray diffraction are very timely/expensive operations to perform
phase identifications. TEM samples may take weeks to prepare. With one martensitic
stainless steel sample, to dimple the sample for TEM took 2 weeks in the ion mill.
Because of very low penetration in ferrous based samples, when using X-ray
diffraction, a sample must run for about 1 week for a full scan from 40-120o to gather
enough counts to differentiate peaks. The analysis which has gone into this research
has spanned over 300 samples.
Samples were mechanically polished to .05 micron with an AlO3 slurry. The
following etchants were used to analyze the samples to determine phase compositions.
Light microscopy techniques were used to take picture of the samples.
Chapter 3 Testing Procedures
Chapter 3. 2 Volume Measurement and Phase Confirmation 58
Table 3.1 Etchants Used to Analyze Laser Material
Chapter 3. 2.7 Blue: Automated Volume Analysis A program called BLUE was developed to quantify material phases in steel
samples. These steel samples were etched in different reagents which colored
different crystalline structures with specific colors. Optical light microscope
photographs were taken at 125X magnification to 200X magnification. These
photographs were all taken at similar cyan, magenta, and yellow levels and similar
contrast, brightness and sharpness levels as well. By standardizing the parameters used
during the light microscopy photographs, automated analysis is able to be completed.
This technique allows one to compare the amounts of the different phases among all
the samples.
This automation can occur by using a pixel value analysis technique. An
earlier form of Blue assigns different ranges of grayscale pixel value to martensite and
other two austenite. Calibrated samples which have all ready been categorized for
phases were analyzed. This earlier form of blue is able to predict the amount of
martensite and austenite to within 2% of values reported values.
However, to resolve or separate colored phases of brown (light brown and
dark), blue and red, full RGB color pixel values must be used. Thus, Blue was created.
It is designed to allow the user to standardize an input file to represent all pixel values
of a specific phase. For example, a filter called blue209 contains all pixel values which
correspond to sigma ferrite, which have been revealed by potassium metabisulfide
Etchants Application Technique Phase Exposure
Picral Reagent Swabbed Exposes Grain Bound-aries
Sodium Meta-Bisulfite Immersed Darkens as Quenched martensite
Potassium Meta-Bisulfite Swabbed Darkens All Martensite
Muramaki’s Reagent Boiling Etched Blue-Sigma Ferrite, Brown-Delta Ferrite
Klemm’s Reagent Swabbed Blue - Ferrite, Brown-Martensite
Chapter 3 Testing Procedures
Chapter 3. 2 Volume Measurement and Phase Confirmation 59
reagent. The optical photomicrograph and the filter are saved as xpm files, which is a
text format of the graphic values. The xpm format allows for programming logic to
analyze the filter and the optical photo . The accuracy of blue is once again within 2%
of X-ray diffraction measurements.
Table 3.2 Comparison of X-ray Diffraction and Blue results for SDMT Steel Sample
In addition to just matching pixels, the beauty of BLUE is that it is able
identify additional area of the phase of interest by identifying a pixels proximity to that
of a given phase. This searching parameter is controlled by user inputs, so the
program can be told how far to deviate from defined pixel values.
Technique Martensite Austenite
X-ray Diffraction 90% 5.3
Blue Analysis 89% 7%
Chapter 3 Testing Procedures
Chapter 3. 2 Volume Measurement and Phase Confirmation 60
Figure 3.11 Color Separtion of Blue Technique
Chapter 3. 2.8 Electron Backscattering ProbeIn the final stages of this research, a new technique was discovered which can
used by others in the future who want to understand the material phases within bulk
deposits. Electron Backscattering Probe (EBSP) is a technique similar to SEM. This
technique enables crystal orientation to be determined on thicker surfaces than TEM.
To produce measurements, a stationary beam interacts with the surface of the crystal.
The electrons backscatter in a direction opposite the incoming beam and are captured
by on a phospor screen with a low intensity video camera. Orientations can be
determined by Bragg’s law. Unlike TEM which takes large amounts of time to prepare
carbon steel samples, preparation of EBSP samples are quite simple. Bulk samples are
polished by mechanical means to a .05 µm silica finish. They are etched and then
Delta Ferrite Filter
Sigma Ferrite FilterColor Image
Chapter 3 Testing Procedures
Chapter 3. 3 Material Property Test 61
polished again by only the .05 µm solution. Samples can also be quite large (over
1cm2 in area) giving maximum localized orientation information. These
measurements can be automated to get a complete surface mapping of crystalline
information. However, special filters have to be written for BCT formation. Further
testing of this technique for use with carbon samples is undergoing.
Figure 3.12 Comparison of TEM and EBSP
Chapter 3. 3 Material Property Test
To determine the material properties of tools steels deposited by laser
deposition, testing had to occur. Three types of tests which were preformed: tensile
testing, Charpy Impact testing and hardness testing. Tensile testing are able to
measure yield and ultimate tensile strength. Impact resistance a property which is very
necessary for dies inserts can be measured by Charpy Impact testing. Hardness testing
can give information on a more localized area. Microhardness testing was used to
yield layer by layer information or even interfacial informations.
Chapter 3. 3.1 Tensile TestingASTM E8-1998 describes methods for producing samples for tensile testing.
Tensile specimens, when tested, are able to yield material property information like,
yield strength, yield point elongation, tensile strength, elongation and reduction of
area.
BCC/BCT EBSP Pattern
37.757.6
[211] BCTTEM Index Pattern
Beam = [011]
Chapter 3 Testing Procedures
Chapter 3. 4 Part Functionality Testing 62
Chapter 3. 3.2 Charpy Impact TestingASTM E23-1998 describes the method of using notched-bar impact testing of
metallic materials by using a Charpy apparatus. It is used to test impact resistance.
Chapter 3. 3.3 ASTM E18-1998 describes Rockwell Hardness testing. ASTM E10-1998
describes Brinell hardness testing. Also, ASTM E92-1982 describes Vickers hardness
testing.
Chapter 3. 4 Part Functionality Testing
To determine if a part can actually be used in service or if the part possess
functionality. For this research three characteristic of functionality were tested:
deflection, wear resistance and thermal resistance. Deflection is an indication of
whether a part is able to meet geometrical requirements. Wear and thermal resistance
are indications of whether a part was able to meet service or environmental
requirements.
Chapter 3. 4.1 Deflection TestingOne measure of part integrity is surface warp or deflection. When layered
manufacturing parts exhibit deflection, it is a symbol of residual stress or shrinkage
imbedded to the part during the deposition or building sequence. A part with low
warpage tends to have low residual stress conditions or has built in a manner to
compensate for shrinkage. Currently, the shape manufacturing process has deflection
measured from the curvature of the substrate on the order of 1-2 mm.
Warpage or deflection is also a concern for the silicon processing industry.
Warp can significantly affect the yield of semiconductor device processing. Producers
and consumers of silicon products use the measurement of warp to determine if
dimensional characteristics of a silicon wafer will satisfy geometrical requirements.
Likewise, warp in layered manufacturing parts, will mean that additional processing is
required to insure that outer part dimensions are within required tolerances. Although
additional processing steps like machining may bring outer dimension back in to
specifications, internal geometry cannot be repaired very easily.
Silicon warpage is measured by ASTM F657-1992. A granite plate is used to
define a surface indexing 3 points on the bottom of the silicon wafer. The maximum
Chapter 3 Testing Procedures
Chapter 3. 4 Part Functionality Testing 63
radius of curvature is measured using this plane as a base. Deflection of beam deposits
is measured in a similar way to silicon warpage. For manual measurements, the
substrate and deposit are positioned on fixture which supports the substrate level to
granite plate. The deposit is now pointing towards the granite plate. The now exposed
surface of the substrate is used to take measurements. The edge of the exposed surface
of the substrate is grounded at zero. A height gauge is then moved across the substrate
in a grid with measurements taken every 6 mm. The largest absolute deviation in
height from zero is defined as the maximum deflection.
A coordinate measuring machine at the Stanford Linear Accelerator Center
was used to measure deflection. The beam deposit was again fixtured with the deposit
point down toward a granite plate. A 3 x 24 point grid was used to measure deflection.
This method was very fast and very accurate.
Figure 3.13 Measurement of Deflection
Chapter 3. 4.2 Dunk Test Specimen The Jack Wallace Dunk test is a die casting simulation. This test has been
calibrated so that one cycle is equivalent to 100 die casting shots. A cycle consists of
lowering a prefabricated fixture into molten aluminum (667 oC) while cold water (20oC) is injected into the interior cavity of the fixture. This test is also a measurement of
the thermal fatigue of the material.
D=DeflectionD
Chapter 3 Testing Procedures
Chapter 3. 5 Modelling of Results 64
Figure 3.14 Thermal Dunk Test Specimen
Chapter 3. 4.3 Die Casting
Die Casting was described in Chapter 2. 3.9. Die casting was used to test an
insert built by Shape Deposition Manufacturing. This is the ultimate test of insert
viability.
Chapter 3. 5 Modelling of Results A problem as complex as modeling phase transformation in laser deposition
can be attacked in many ways each with limitations. Laser deposition is a 3D
dimensional problem which involves many variables: laser scanning speed, laser focal
length, laser spot size, laser path, the number of laser scans, nitrogen shroud gas flow
rate, cooling rate, material properties, material combinations, substrate material and
thickness, bolting conditions, layer thickness, machining parameters, subsequent
processing steps, process step order, etc. Design of experiments like the Taguchi
approach can be used if judgements can be made to simplify the system components to
key parameters which can be stringently manipulated to plan experiments [3.8, 3.9].
Parameters like scanning speed and laser power can be manipulated for a planned
experiment. It is more difficult and at times impossible to regulate levels of certain
variables like the chromium content of a stock alloy for a planned experiment.
For the laser deposition system, it is at times, difficult to separate endogenous
(input) variables or exogenous (output) variables. Sometimes effects can be masked
because it may take several endogenous variable to produce one exogenous response.
Often DOE’s require a basic understanding of the system relationships to produce
accurate results. Because of these problems: the inability to control certain variable
50.8 mm
177.
8 m
m
177.
8 m
m
50.8 mm 50.8 mm
38.1 mm
Chapter 3 Testing Procedures
Chapter 3. 5 Modelling of Results 65
inputs and the daunting inability to separate variables, typical DOE methods may be
inadequate. When a judgment about the magnitude of influence and interelationship
of variables is needed other non-traditional methods may need to assessed. In cases
like the laser deposition system, it may be impossible to make such judgement calls.
Laser deposition is a complex problem which can be looked at as an ill-defined
system because there is insufficient knowledge to throughly define the systems and
key interactions. At present no theory exists which can account for all of the primary
relationships or interaction of variables to define system responses such as phase
formations or grain sizes. Therefore, modeling the system based on a theory driven
approach is difficult and can often lead to errors because of inappropriate assumptions.
Often, when building the model, one has to know things about the system that are
generally impossible to know without extensive testing.
These unknowns may not only cause the inappropriate selection of key
controls or variables, but the model structure can be compromised by the insuffiencient
knowledge about interference factors or influencing factors. This uncertainty can
confuse the modeling in several ways:
1. The selection of variables as endogenous variables or exogenous variables. 2. The
functional form of the relationships between variables and system dynamics. 3.
Proper understanding of the origin or description of error.
A data driven approach can overcome some or all of the problems associated
with ill-defined systems. Often data is analyzed by statistical means for model
formation. However, this type of defining process typically needs to have priori
knowledge about the structure of the system to produce the mathematical model
[3.10]. Data mining techniques can be implemented to gain system knowledge and
modelling information. Data mining techniques include data visualization, tree-based
methods and methods of mathematical statistics like multivariate regressions as well
as those for knowledge extraction from data using self-organizing modelling [3.13,
3.13].
Data mining is an interactive and iterative process of numerous subtasks and
decisions-making steps such as data selection and pre-processing, choice and
application of data mining algorithms and analysis of the extracted knowledge. Many
automated data mining programs try to limit the involvement of users in the overall
data mining process and the inclusion of existing a priori knowledge. Thus, the
process becomes more automated and more objective.
To tackle the phase transformation element of the laser deposition problem
Chapter 3 Testing Procedures
Chapter 3. 5 Modelling of Results 66
which is complex and expense (in time, operations, etc.) to solve and to ascertain
primary influences of parameters the Group Method of Data Handling (GMDH)
procedure was used [3.11]. GMDH looks for simple relationships in multiple level
systems.
The goal of GMDH and other similar tools are to predict behavior by means of
parametric or nonparametric models. Parametric models are adaptively created from
data by the Group Method of Data Handling (GMDH) in the form of networks of
optimized transfer functions (Active Neurons). Nonparametric models are selected
from a set of variables analyzing one or more patterns of a trajectory of past behaviors
which are analogous to a chosen reference pattern. Both approaches of self-organizing
modeling include not only core data mining algorithms but also an iterative process of
generation of alternative models with growing complexity, their evaluation, validation
and selection.
At present, GMDH algorithms present a method to identify and forecast
relationships in cases of noisy and short input sampling. In contrast to neural
networks, the results are explicit mathematical models, obtained in a relatively short
time. KnowledgeMiner is a software tool which uses GMHD. It is an easy-to-use
modelling tool which realizes twice-multilayered neuronets and enables the creation
of time series, multi input/single output and multi input/multi output systems (system
of equations). Successful applications are shown in the field of analysis and prediction
of characteristics of stock markets in financial risk control modelling [3.10].
Chapter 3 Testing Procedures
Chapter 3. 5 Modelling of Results 67
3.1 Kalpakjian, Manufacturing Engineering & Technology, 19953.2 Norton, John T., “Review of Methods of X-ray Stress Measurement,3.3 Kocks, U.F., Tome,C. N. and Wenk H.-R., “Texture and Anisotropy,” New York:
Cambridge University Press, 1998, pp. 167 -177.3.4 E. Beraha and B. Shpigler, Color Metallography, (1977).3.5 B. L. Averbach, L. S. Castleman, and M. Cohen, “Measurement of Retained Auste-
nite in Carbon Steels,” Transactions of the ASM, (1949) Vol. 42, pp112-120.3.6 N. Williams and C. Carter, Transmission Electron Microscopy - Diffraction, Vol 2,
(1996), pp.267-288.3.7 Burgman, Patrick, “Design of Experiments The Taguchi Way,” Manufacturing
Engineering, May 1985, pp. 44-46.3.8 Ross, Philip J., “Taguchi Techniques for Quality Engineering,” McGraw Hill, New
York: 1996.3.9 Ranjit, R. A Primer on the Taguchi Method, Van Nostrand Reinhold, New York, pp.
145-155, (1990).3.10 Lemke, F.; Mueller, J.A. (1997): Self-Organizing Data Mining for a Portfolio
Trading System. Journal for Computational Intelligence in Finance, 5(1997) pp. 3
3.11 Farlow, S.J. (ed.) (1984): Self-organizing Methods in Modeling. GMDH Type Algorithms. Marcel Dekker. New York, Basel
3.12 Madala, H.R.; Ivakhnenko, A.G. (1994): Inductive Learning Algorithms for Com-plex Systems Modelling,CRC Press Inc., Boca Raton, Ann Arbor, London, Tokyo
3.13 Sarle, W.S. (1995): Neural Networks and Statistical Models. in: Proceedings of 19th Annual SAS User Group International Conference. Dallas. pp. 1538-1549.
Chapter 4 Material Selection and Design of Small Scale Die Inserts
Chapter 4. 1 The Need for A New Material 68
Chapter 4 Material Selection and Design of Small Scale Die Inserts
Chapter 4. 1 The Need for A New MaterialAs described in Chapter 2. 10, the SDM process is an additive and subtractive
process which involves the deposition and shaping of material. When used with
metals, part material is deposited by laser deposition and shaped by CNC machining.
A sacrificial material is often used to support over-hanging features or to maintain the
integrity of hollow internal geometries within the part. For example support features
are often used to fill cooling channel passages as well as to support undercut surfaces.
To remove sacrificial material, an etching process is used. This removal process
requires that part materials have high corrosion resistance and sacrificial material to
have low corrosion resistance.
Common materials used to produce parts in Shape Deposition Manufacturing
(SDM) have been 316L stainless steel, copper, copper bronze, and invar. Stainless
steel-316L is a soft machinable material with high corrosion resistance. It is often
used as part material. However, it has a very high coefficient of thermal expansion
(CTE). When 316L is deposited, the melted powder upon solidification shrinks. This
shrinkage cause 316L parts to deform.
Copper is used as a support material. It is often plasma deposited and shaped
to support 316L parts. The low corrosion resistance of copper allows, exposed
surfaces of copper to be etched away and removed. The high thermal conductivity
makes it an attractive option for depositing within multimaterial parts to improve heat
transfer.
Copper bronze can also be used as a sacrificial material, and it can be deposited
Chapter 4 Material Selection and Design of Small Scale Die Inserts
Chapter 4. 1 The Need for A New Material 69
by laser. Its low corrosion resistance make it ideal for this etching process. It also
soluble with 316L. Thus, for depositing gradient structures, copper bronze is a good
candidate. However, the thermal conductivity is very similar to 316L so no thermal
advantage is gained by using copper bronze within the deposited part.
Invar is used as part material and for graded structures. Functionally graded
parts have been researched by the Stanford Rapid Prototype Lab. Two materials are
blended in the power feeding unit by allowing material mixing in situ by changing
powder feed flow rates. For example, Invar has a low CTE from 400C to room
temperature. It is during this temperature range when most residual stress develops
which causes part deformation. Therefore, invar has been used in graded structures
called functionally gradient parts. The powder feeding system used for SDM consists
of 3 automated powder feeders. Each can be filled with different material and
controlled individually. Figure 4.1 shows the results of depositing both invar and
stainless in over a 48 mm span. The measurements for this graph were taken by
electron microprobe analysis. As one material decreases the other increased. A
smooth compositionally graded structure is created. This technique has been used to
build multi-material parts. The invar is deposited on the interior of the part while
stainless steel is deposited on exterior surfaces or as barriers near low corrosion
resistant materials.
Figure 4.1 Graded Structure of Invar and 316L - Measured by Microprobe
0
0.2
0.4
0.6
0.8
1
0 8 1 6 2 4 3 2 4 0 4 8
Percent Compositon of Stainless-Invar Bar
%316L %Invar
Per
cent
Com
posi
tion
mm 100% Stainless Steel 100% InvarVariation within 95% confidence Level is + .6%
Chapter 4 Material Selection and Design of Small Scale Die Inserts
Chapter 4. 1 The Need for A New Material 70
As graded material with 316L, Invar has been used to build many parts.
However, Invar has low corrosion resistance and low machineability. Because of these
two properties, production of parts completely from invar is limited.
While each of these materials have been successfully used to produce parts,
each is inadequate to produce die cast inserts. Stainless 316L, Invar, copper, and
copper bronze are not wear resistant enough to survive molten injection of aluminum.
Nickel in 316L and Invar are soluble in molten aluminum. Alloying and precipitation
of die material with casting material produces an unusable material combination.
Copper and copper bronze in addition to solubility, have melting temperatures in the
range of molten aluminum. Mold deformation or dissolution would occur if used in
aluminum die casting. Also none of these materials attain the hardness needed for
wear resistance specified for die casting.
Table 4.1 Current SDM Materials: Balance Iron
Also, in addition to the requirements needed for die casting listed in Chapter 2.
13, the material needs to meet requirements of laser deposition in SDM. Also, any
new material should yield more applicability than just aluminum die cast inserts. The
process to find new materials should attempt incorporating materials which have
properties that would lower part deformation, or improve deposition characteristics,
etc. For example, if it is possible, the new material should have a low CTE or have
improved wetting characteristics so that additional materials can be used with SDM.
C% Mn% Si% Cr% Ni% Mo% Cu% Al% Sn%
Copper 0 0 0 0 0 0 99.9 0 0
Copper Bronze
0 0 0 0 .5 0 86.5 6 .2
Invar .03 .3 .2 0 40 0 0 0 0
316L .02 1.74 .73 17.3 13.1 2.66 0 0 0
Chapter 4 Material Selection and Design of Small Scale Die Inserts
Chapter 4. 1 The Need for A New Material 71
Table 4.2 Typical Materials used with SDM
Therefore the requirements of materials incorporated in the SDM process to
build aluminum die casting inserts can be divided into to camps, Tier I and II. Tier one
requirements cannot be compromised. Tier II. requirements are additional
requirements which are to be sought but are not decision limiting criteria:
Tier I. Requirements
• Low Coefficient of Expansion for high thermal fatigue resistance
•High hardness (44-48 Rc) for wear resistance (2.35)
•High modulus of elasticity for high resistance to deformation to avoid
galling and heat checking
•Moderate thermal conductivity to produce castings of similar micro-
structures as H13 production inserts (on the order of 24 W/mK)
•Minimal to trace amounts of Nickel to reduce the potential alloying or
dissolution.
•Corrosion Resistance or the ability to alloy with corrosion resistant
materials
Tier II. Requirements
•Good Machineability
•Low CTE
•Commercially available from reliable source
Melting Temperature (C)
CTE(µm/m•K)
300oC -20oC
Copper 1083 16.9
Copper Bronze
1053 16.2
Invar 1427 1.6
316L 1385 17.5
Chapter 4 Material Selection and Design of Small Scale Die Inserts
Chapter 4. 2 Initial Selection of Tool Steels 72
Chapter 4. 2 Initial Selection of Tool Steels
The hardness and strength requirements were the guiding criteria to select
materials. The typical tool building material for SDM was 316L stainless steel. This
is an austenitic steel which has the crystal lattice structure of face-centered cubic
(FCC). A FCC alloy has many good characteristics, low-temperature toughness and
excellent weldability, and typically good corrosion resistance. However, they are often
susceptible to stress-corrosion cracking. However, the main problem is that the
relatively low yield strength allows that these FCC alloys can only be hardened by
coldworking, precipitation, or solid solution strengthening. Precipitation and solution
hardening involve special heat treatment processes to either change the bulk to one
phase or cause precipitation of a second phase in the part matrix. All three treatments,
would be difficult to perform on a SDM 316L part especially if it were a multi-material
part (with Invar and copper) because of the multiple melting point /thermal phase
characteristics or deformation to final part dimensions.
The low yield strength was actually one of the guiding principles for choosing
316L. When one looks at beam deformation, maximizing R or the radius of curvature
would minimize the amount of warp or deflection which the deposited structure would
have. Stainless-316L has a high E/σ ratio.
Figure 4.2 Maximizing R to Minimize Deflection
y
R
Eσ
Maximize R by Maximizing
Chapter 4 Material Selection and Design of Small Scale Die Inserts
Chapter 4. 2 Initial Selection of Tool Steels 73
Table 4.3 Comparison of E/σ with Common SDM Metals
This criteria though does not seem to help when trying to find hardenable
material for build die inserts. Two types of materials which did meet the hardness
criteria were the 400 series martensitic stainless steel and tool steels. The 400 series
martensitic stainless steels stainless. Theses material have high modulus, can attain
high hardnesses. They have moderate to high corrosion resistance. The high
hardnesses does pose a challenge for machining, but it has similar machining
characteristics to 316L. Therefore, the expertise of depositing and machine stainless
could be exploited for these new materials.
Tool steels will also be able to attain the hardnesses required for die casting
inserts. However, the corrosion resistance of these materials was questionable.
Chromium which forms carbides that reduces corrosion susceptibility is low in many
tool steels. Also many allowing agents are also included in the composition of tool
steels.
Eight materials were originally tested. The alloys consisted of two high speed
tool steels T15 and M2, H13 a hot working steel and five industrial alloys similar in
carbon and chromium compositions to the 400 series stainless: Polar, Cryotherm,
Pyro 1, Pyro 2, Spraco 2.
E x104 MPaσ MPa
(Annealed)E/σ
Copper 110.4 69 15977
Copper Bronze
96.5 207 4660
Invar 14.1 276 511
Low Carbon Steel
20.0 558 358
316L 19.3 234 825
Chapter 4 Material Selection and Design of Small Scale Die Inserts
Chapter 4. 2 Initial Selection of Tool Steels 74
Table 4.4 Initial Tool Steel Selection for SDM Processing
The materials were tested for hardness, deposition quality and corrosion
resistance. The material was SDM deposited in samples measuring 30 mm x 35 mm x
4 mm were deposited from each of the metal powders. Each sample was produced
using standard deposition parameters of 100% laser power, 25 mm/sec laser scanning
rate, and 30 g/min. powder feed rate. Each sample was evaluated on the basis of
deposition quality, hardness, surface wetting and acid etch resistance. Deposition
quality was evaluated through visual observation of cracking and surface oxidation.
Hardness testing was performed with a Rockwell Hardness Indenter (ASTM E18-
1995) and 10 individual tests per sample. Etch resistance testing was determined in
accordance to ASTM A262-93a-1996 by placing the sample in 65o C nitric acid for 30
minutes. After an etch test, the microstructure of each sample was analyzed for
corrosion susceptibility based on etch structure classification (ASTM A262-93a-
1996).
C% Mn% Si% Cr% Ni% Mo% Co% W% P% V%
T15 1.6 0 0 4 0 1 5 13 0 5
M2 .8 .3 .3 4 0 0 0 6 0 2
Polar .01 0 0 10.5 0 0 0 0 0 0
Cryo1 .01 0 0 15 0 0 0 0 0 0
Pyro1 .01 0 0 12 0 0 0 0 0 0
Pyro2 .01 0 0 14 0 0 0 0 0 0
H13 .4 0 1.05 5 0 0 0 0 0 1.1
316L .02 1.74 .73 17.3 13.1 2.66 0 0 0 0
Chapter 4 Material Selection and Design of Small Scale Die Inserts
Chapter 4. 2 Initial Selection of Tool Steels 75
Table 4.5 Testing Results of Tool Steel in SDM Processing
Spraco 2 was the only alloy to successfully meet all the requirements of
hardness, etch resistance and deposition quality. This alloy was selected as the SDM
Tool Steel. Analysis of photomicrographs of the Spraco 2 deposit after the nitric bath
showed no ditch structures or end grain pits which would indicate susceptibility to
intergranualar corrosion. M2 and H13 steels can only be considered if they can be
used with corrosion resistant functionally graded structures such as a structure with
H13 on the outside and 316L deposited upon faces of etchable surfaces.
Alloy Composition Hardness Deposition Etching
T15
4% Cr, 5% Co, 13%W
1.6% C, 5% V, 1% Mo 63 Rc Pitted Fast
M2
4% Cr, .3% Si, 6% W
.8% C, 2% V, .3% Mn 58 Rc Good Fast
Polaris
Industrial Alloy
10.5% Cr 42 Rb Good Slower
Etching
Cryo 1
Industrial Alloy
15% Cr 40 Rb
Visible
Cracking Slower
Etching
Pyro 1 12% Cr, 0% Co <1%
C
30 Rc Cracking Slow
Pyro 2 14% Cr, 0% Co <1%
C
44 Rc
Good
(Cracked
during cooling)
Slight,
Very Slow
Spraco 2 16% Cr, 0% Co <1%
C
40 Rc Good None
H13 5% Cr. 0% Co .4%C 50 Rc Good Etching
Chapter 4 Material Selection and Design of Small Scale Die Inserts
Chapter 4. 2 Initial Selection of Tool Steels 76
Spraco tool was selected as the new SDM tool steel. Metallographic analysis
showed that using the deposition parameters, as for 316L were not sufficient because
some deposits had many voids. To reduce the voids and create an efficient deposition
routine, the deposition parameters had to be optimized.
Chapter 4. 2.1 Optimization of Deposition Parameters
A Taguchi based design of experiment was used to find optimal deposition
parameters for SDM Tool steel. The optimal parameters should provide the maximum
material deposition rate which produces fully dense deposits and material within the
acceptable hardness range. Remelting of the substrate should also be kept to a
minimum to preserve the geometry of previous layers. Three factors were
investigated: laser power, laser scan speed, and powder feed rate. A Latin square array
of 8 experiments varying the three factors at two levels was completed.
Table 4.6 Experimental Parameters
The full factorial design was chosen to directly characterize the two factor and
three factor interactions. Tool steel samples of 30 mm x 10 mm x 5 mm were
deposited at each of the 8 experimental settings. Hardness was measured as described
in Chapter 3. Metallographs were taken at optical magnification from 50-1000x.
From these photomicrographs, the laser penetration depth, porosity and pore density
were calculated. D. Gentry, et al. (4.1). showed that optical measurements of porosity
were within 93% of the accuracy of traditional porosimetry methods.
A Taguchi function for maximizing the outcome, F was chosen to correlate the
observed measurements with the optimal condition (4.2). The optimal setting is found
when F is maximized with the following equation :
F = .4 H* + .4(1- P) +.2 D*
Parameter Level 1 Level 2
Laser Power 100% 90%
Laser Scanning Rate 25 mm/sec 35 mm/sec
Material Feed Rate 30 g/min 15 g/min
Chapter 4 Material Selection and Design of Small Scale Die Inserts
Chapter 4. 2 Initial Selection of Tool Steels 77
where H* is the Hardness normalized by 48 Rc, Pi is the porosity, D* is the depth of
penetration normalized by 100 µm. Once the optimized results were obtained, tensile
specimens were deposited. ASTM Standard E8-1995 was used for the tensile testing
of specimens.
The factors, (A = Laser Power, B = Laser Scanning Rate, and C = Powder Feed
Rate) contribute to the optimal deposit conditions as described in . Using the
optimizing function, (equation 1), the following deposition parameters were selected:
100% laser power, 35 mm/sec scan rate, and a powder feed rate of 15 g/min.
According to the data, the laser scan rate is the single most important factor in the
optimizing function, as well as for controlling hardness, penetration depth, and
porosity individually. Specifically, the faster the scanning rate within the limits
examined, the harder the material, the lower the porosity and the lower the penetration
depth. Laser power independent of the other factors made little contribution to the
outcome.
Table 4.7 Analysis of Variance Table for Optimal Response: A = laser power, B = scan rate, C = powder feed rate
The signal to noise ratio of the data is 37.9 and mean squared deviation is
.0002. These value indicate a minimal scatter of data and a highly reliable optimum
(4.1).
Degree
of
Freedom
Sum of
Squares
Mean
Squares
(Variance)
Variance
Ratio
Pure Sum of
Squares S'
Percent
Contribution
A 1 0.0006 0.0006 1.23 0.0001 Pooled
B 1 0.07 0.07 143.27 0.07 47.39
C 1 0.0001 0.00007 0.13 -0.0004 Pooled
AxBxC 1 0.05 0.05 96.72 0.05 31.88
AxB 1 0.004 0.004 7.23 0.003 2.08
AxC 1 0.02 0.02 36.19 0.02 11.72
BxC 1 0.004 0.004 7.45 0.003 2.15
Total 15 0.2 0.0000
ERROR 0 0.004 0.0005 1.00 0.008 4.78
Chapter 4 Material Selection and Design of Small Scale Die Inserts
Chapter 4. 2 Initial Selection of Tool Steels 78
Chapter 4. 2.2 Material PropertiesAt this optimized level, the material structure and properties were analyzed.
Metallographs were made of cross sections of the SDM tool steel deposits made at the
optimum conditions.
The metallograph shown below shows a cross section of SDM Tool steel etched with
prical acid. The material structure is primarily martensite (dark, fine regions) with
retained austenite (light bands) . The material exhibited a hardness of 45 Rc. and was
found to be 99.9 % dense. The pore density is .002 pores/ mm2 with an average pore
diameter of 29 µm (4.3). The mechanical properties from tensile testing are shown in
Table 4.8 compared with standard H13 tool steel.
Table 4.8 SDM Tool Steel (as deposited) Vs. H13 (tempered)
As compared to H13 which has been heat treated , SDM tool steel has a lower
yield and tensile strengths and less elongation (4.4). However, these properties for the
SDM Tool steel are in the as deposited condition. For prototype inserts, these
property levels may be sufficient. The high modulus of the SDM tool steel is a result
of the martensitic matrix. The retained austenite should add increased toughness and
reduce the possibility of cracking or galling (4.5).
Mechanical Property SDM Tool Steel H13
Ultimate Tensile Strength 879 MPa 1482 MPa
.2% Offset Yield Strength 510 MPa 1344 MPa
Reduction of Area 2% 38%
Modulus of Elasticity 254 MPa 210 MPa
Elongation (in 25 cm gage
length)
2% 14 % (50 cm gage
length)
Chapter 4 Material Selection and Design of Small Scale Die Inserts
Chapter 4. 3 Building A Small Scale SDM Die Cast Insert 79
Figure 4.3 Metallograph of Austenite (light) and Martensite (dark) in SDM Tool Steel
Chapter 4. 3 Building A Small Scale SDM Die Cast InsertTo test the SDM process and the new SDM tool steel, a die cast insert needed
to be built. To minimize the possibility of failure within a die casting machine, a small
scale insert was built. To design the insert a set of guidelines was imposed. The both
the part thickness and feature depth would be minimized while maximizing the ratio
between the two specifications. These constraints accomplish two things, the
deposition time would be minimized and the complexity of the die insert would also be
simplified. A ratio of 4 to 1, part thickness to feature depth was maintained.
An insert was built using the optimal parameters for SDM Tool steel and
cycled in an overflow of a production die set in a 600 ton die casting machine. The
insert, the Stanford paper clip (Figure 4.4) had outer dimensions of 6.4 cm x 6.4 cm x
1.9 cm. By using the SDM additive and subtractive processes, the insert was
deposited and fabricated in layers. A pocket of aluminum bronze, 1 cm x 1 cm x.32
cm, was deposited in layers .32 cm below the S feature of the paper clip. The
aluminum bronze was added to illustrate the multi-material capability of SDM.
100 µm
Chapter 4 Material Selection and Design of Small Scale Die Inserts
Chapter 4. 3 Building A Small Scale SDM Die Cast Insert 80
Figure 4.4 SDM Tool Steel Insert
A number of tests were performed on the insert prior to die casting. Melted
wax was poured in the die to test part removal. Thermal fatigue testing was performed.
The insert was thermally cycled by pouring molten aluminum 380 alloy at 670oC into
the insert, allowing the casting and insert to air cool to about 100oC and removing the
casting. The insert was then air cooled to room temperature. This would be repeated
seven times with different amounts of insert preheat.
The original 5o taper was found to be insufficient for the removal of the wax
from the insert. Ejector pins, gating, and venting were added to facilitate material
removal (Figure 4.7). The insert was tested for thermal fatigue resistance with gravity
poured aluminum.
Figure 4.5 shows the thermal cycling history of the insert. Aluminum 380
castings without warpage or dimensional shifts, proper surface finish and low porosity,
were produced with an insert preheat of 660oC and a 5 minute dwell time in the closed
stand. Figure 5 compares the microstructures of the gravity poured insert and a die
cast part from a traditional H13 insert. The gravity poured parts showed more
columnus structure due to dwell times on the order of 25 times longer. However, the
hardness of the castings from the SDM insert and H13 insert are similar (55 Rb and 57
Rb, respectively).
Chapter 4 Material Selection and Design of Small Scale Die Inserts
Chapter 4. 3 Building A Small Scale SDM Die Cast Insert 81
Figure 4.5 Gravity Casting produced Thermal Cycling
The die casting machine parameters used to produce castings are listed in Table
4.9. Castings were evaluated microstructural homogeneity under the optical
microscope and compared to castings produced by an H13 insert. The average grain
diameter was calculated by the intercept method in accordance with ASTM Standard
E112-1995.
Table 4.9 Die Casting Parameters
Parameter Setting
Shot Pressure (No Intensifier) 103 MPa
Intensifier Pressure
27.6 MPa
Die Preheat (2 hours) 121oC
Die Temperature while running 82oC
30
130
230
330
430
530
630
730
0 50 100 150 200 250T
empe
ratu
re (
C)
Time (Min.)
Chapter 4 Material Selection and Design of Small Scale Die Inserts
Chapter 4. 3 Building A Small Scale SDM Die Cast Insert 82
Figure 4.6 A. Die Casting of Aluminum B. Gravity Cast Structure
For actual die casting, the features were remachined with a 15o taper to
facilitate easy removal of the part without ejector pins. The insert was positioned in an
overflow of a valve body die set where ejector pins could not be used. The increased
taper angle resulted in sharp thin edges in the “S” feature as shown in Figure 4.7.
a b
Figure 4.7 Remilled Insert: 15o Taper added to all walls (a) and close up of fine f
Figures 4.8 and 4.9 show, the position of the insert in an overflow section of a
production H13 die casting tool. The cycle time for producing the casting shown in
Figure 4.10, from injection of the aluminum to ejection of the casting was 70 seconds.
The SDM insert was run in the die cavity for over 150 shots with no visible evidence
of wear, fatigue, or cracking.
Both the castings from the SDM insert and H13 overflow sections had a
hardness of 57 Rb. Metallographs of a castings from the two sections are shown in
Figure 4.10.
A. B.60 µm 60 µm
10 mm 1 mm
Chapter 4 Material Selection and Design of Small Scale Die Inserts
Chapter 4. 3 Building A Small Scale SDM Die Cast Insert 83
Figure 4.8 SDM Insert in Overflow
Figure 4.9 SDM Insert placed in overflow of Saturn Valve Body
have a similar structure with agglomerated cell patterns. The ASTM E112-1995
intercept method was used to calculate average grain diameter. The average grain
diameters for the castings from the SDM insert and the H13 insert were 8.5 µm and
8.6 µm, respectively.
Chapter 4. 3.1 SDM Tool Steel use for small inserts The SDM tool steel used for the die casting insert exhibited higher hardness
than expected for this material. This material deposited through flame spray or
conventional welding operations will have a hardness in the range of 20-28 Rc. The
material experienced rapid quenching during air cooling which initiated a martensitic
transformation without going through the other diffusion limited phases. The
martensitic phase is responsible for the high hardness of this low carbon alloy. Alloys
with higher carbon content should produce material with even higher hardnesses, but
10 mm
Chapter 4 Material Selection and Design of Small Scale Die Inserts
Chapter 4. 4 Deflection Vs. SDM Tool Steel 84
such materials would be difficult to mill. The low carbon content in SDM tool steel
will allow the SDM process to produce structures within the range of 35-45 Rc.
The corrosion resistance of the SDM tool steel assures that the rapid
transformation is not causing sites for intergranular corrosion (4.6). Certain heating
conditions can allow chromium to be depleted from the alloy matrix and form
chromium carbides on the grain boundaries. This depletion weakens corrosion
resistance. This condition is corrected by tempering and other heat treatments. The
rapid cooling does not allow this precipitation to occur. This is one reason that SDM
tool steel artifacts could be used without heat treatments. In addition to giving good
corrosion resistance, the high chromium content will also give the insert high heat
resistance up to 1000oC (4.7).
These experiments show that SDM is a viable method for rapidly producing
die casting inserts. SDM die cast inserts can produce time savings over conventional
tool and die methods. Multi-material dies with better heating properties can be used
to shorten dwell times and still attain proper cell structures. Moreover, the
possibilities of producing small prototype die inserts “as deposited”, without heat
treatment, can reduce die production lead times further.
Figure 4.10 Die Cast Part from the SDM Mold.
Chapter 4. 4 Deflection Vs. SDM Tool Steel
An interesting observation was made with beams deposited with SDM tool
steel. One beam was built at 20 mm/s and another at 30 mm/s. The beams were 127
mm x 4 mm x 12.5 mm on a 152 mm x 52,4 x 6.25 mm substrate. The beam built at 20
mm/s had an average hardness of 46 Rc, and the beam built at 30 mm/s had an average
hardness of 38 mm/s. The beam seemed to possess less deflection with increasing
Cast from H13InsertCast from SDM Insert
30 µm
Chapter 4 Material Selection and Design of Small Scale Die Inserts
Chapter 4. 4 Deflection Vs. SDM Tool Steel 85
hardness. This is exactly the opposite effect seen in 316L beams. Increasing hardness
in 316L beams seamed to increase deflection.
Figure 4.11 Increasing Hardness reduced deflection in SDM Tool Steel
The test suggests that increasing hardness may have influence upon deflection.
Further testing would have to help calibrate whether the change in deflection was truly
the result of hardness or whether it can be attributed to smaller grain size, phase
percentage or other factors which can contribute to increased hardness.
Figure 4.12 Increased Hardness increased deflection in 316L beams
-0.2
0
0.2
0.4
0.6
0.8
20 40 60 80 100 120 140
SDM Tool Steel (46 Rc )
SDM Tool Steel (38 Rc)
1.5
1.0
0.5
0.0
Distance Along Beam (mm)
Def
lect
ion
(mm
)
-0.2
0
0.2
0.4
0.6
0.8
20 40 60 80 100 120 140
Stainless Steel (316L) (90 Rb)
Stainless Steel (316L) (86 Rb)
defle
ctio
n (m
m)
distance (mm)
Increasing Hardness
1.5
1.0
0.5
0.0
Distance Along Beam (mm)
Def
lect
ion
(mm
)
Chapter 4 Material Selection and Design of Small Scale Die Inserts
Chapter 4. 5 Attempting to Build Larger Scale Inserts 86
Chapter 4. 5 Attempting to Build Larger Scale Inserts
The unheat treated SDM die insert made from the alloy known as SDM tool
steel work well with the small scale die insert, producing over 150 parts without any
signs of wear or cracking. However, when the a much larger insert was attempted, the
Dunk test specimen, catastrophic failure occurred. This insert has a part thickness to
feature level thickness of 177 mm to 171 mm. In an attempt to minimize the build or
deposition time, the insert was deposited on its side, changing the deposition part
thickness to feature ratio to 55 mm to 40 mm. A dunk test specimen was being built in
1mm thick layers at the optimized setting for SDM tool steel. Every 10 layers, the part
was allowed to cool before more deposition occurred. With only 5 mm left to be built
the 50 mm part fractured during deposition.
Figure 4.13 Cracked Deposit: 50 layers of SDM Tool Steel Deposited
Metallographic examinations were performed upon crack surfaces. Etchants
were used to determine the amounts of martensite, austenite, and ferrite were located
in the area of the crack. The first interesting finding was the stridation of the regions of
tempered to as-quenched martensite. The grain size of the tempered martensite was on
average 3 times larger than grains within the as-quenched region. The tempered
region has cementite precipitation on the former austenitic boundaries of the tempered
martensite. The larger tempered martensite packets are an indication of extended
reheat temperatures or extended heating times. Very close to the crack sigma ferrite
stringers were found. Also along the crack face sigma ferrite seemed to be propagated.
The metallographic examination helped us to determine that the heating
characteristics of layers is retained as the deposit is built. Tempering of layers occurs
but is not complete. Grain size differences is quite apparent. The initiating crack
surface started in area of brittle martensite. Moreover, sigma ferrite was found along
the crack interface. It is difficult to determine if sigma ferrite existed before crack
Chapter 4 Material Selection and Design of Small Scale Die Inserts
Chapter 4. 5 Attempting to Build Larger Scale Inserts 87
failure. However, the appearance of sigma ferrite does indicate under certain
deformation or heating conditions sigma ferrite can develop.
It became quite obvious that by building parts with tools steels, new designing
or planning rules are required. Unlike our experience with austenitic material, these
carbon steels can develop multiple phases. Our understanding of these phases and
phase development would also need to be challenged.
Figure 4.14 Microstructure of the cracked layers of SDM tool steel deposit
Figure 4.15 Sigma Ferrite has developed along crack edge
1 mm
Dark regions representas-quenched martensite.
Light regions representtempered martensite and other phases.
100µm
Delta Ferrite formed oncrack interface.
Delta Ferrite Stringerwithin deposit.
Chapter 4 Material Selection and Design of Small Scale Die Inserts
Chapter 4. 5 Attempting to Build Larger Scale Inserts 88
Figure 4.16 Close up of tempered and as-quenched layers of SDM deposit.
When examining the crack surface in the interior of the part, voids were also
discovered. This void growth had not been encountered before. Smaller samples were
then built at the previously optimized levels. Void growth was now at 30%. When
samples of the original powder was tested at the optimal settings, void percentage was
under 1%. The powder characteristics or composition had changed.
100 µmAs-Quenched
Tempered Martensite and
Martensite
Other phases
Chapter 4 Material Selection and Design of Small Scale Die Inserts
Chapter 4. 5 Attempting to Build Larger Scale Inserts 89
Figure 4.17 500 and 1000 X Views of Cracked SDM Deposit.
Chemical analysis showed that the silicon content of the powdered alloy had
changed from initial specifications. Also, the particle size had become coarser. When
refining the powder mesh to between 90 µm-120 µm, the void percentage reduced or
was eliminated. However, instability of the powder source and cleanliness of powder
was now questionable. A new material for carbon steel deposition would have to be
developed. However, the profoundly different microstructure of carbon steels,
required that a more fundamental understanding of the deposition process needed to
developed.
20 µm40 µm
40 µm20 µm
Chapter 4 Material Selection and Design of Small Scale Die Inserts
Chapter 4. 5 Attempting to Build Larger Scale Inserts 90
.
4.1 Gentry, D., Humbert, L., and Burlot, Rene, C. R. Academy of Science, Paris, Vol 309:II, p. 1481, (1989)
4.2 Ranjit, R. A Primer on the Taguchi Method, Van Nostrand Reinhold, New York, pp. 145-155, (1990).
4.3 Homand-Etienne, F., and Houper, R., International Journal of Rock Mechanics and Mining Sciences and Geomechanics, p. 125, (1989)
4.4 ASM Engineering Property of Steel, ASM, pp. 458-461, (1982).4.5 Wayman, Michael, The Metals Black Book: Ferrous Metals, Vol. 1., 2nd Edition,
Cast 1 Publishing Inc., Edmonton, Alberta, pp. 14-37, (1995).4.6 Metals Handbook, ASM, Vol 2, pp. 253-254, (1974).4.7 Metals Handbook, Lyman, Taylor, Vol. 2, 8th Edition, American Society of Metals,
Metals Park, Ohio, pp. 221-283, (1964).
Chapter 5 Characterization Tool Steels
Chapter 4. 5 Attempting to Build Larger Scale Inserts 91
Chapter 5 Characterization Tool Steels
In this chapter, the characterization of carbon steels on a layer by layer basis
was preformed to understand the cell and layer morphology of depositing tools steels.
A generalization of carbon steel (>.05%C) morphology in layer manufacturing is
made which can be aid in designing and building carbon steel parts. The term carbon
steel is used to designate the set of steels characterized. The austenitic steel 316L is
also characterized to serve as a comparison. New tools for determining microstructural
phases are adapted for layered manufacturing with laser deposition: Shaeffler Welding
Diagram and the Solidification Modes Diagram.
When originally attempting to deposit carbon steels, the same optimization
strategy designed for laser deposition of 316L stainless steel and aluminum bronze
was used. This strategy emphasizes trying to maximize layer thickness and minimize
the build completion time. Using this laser deposition strategy which emphasized
maximizing throughput was simply inadequate for depositing carbon steels because of
phase transformations. Phase transformations of materials had not been encountered
before. In order to improve tool steel deposition a more fundamental understanding of
the deposition of tools steels from a microstructual level needed to occur. Also, since
SDM Tool (SDMT) steel was no longer the most reliable powder choice, more
materials had to be tested. The opportunity to investigate higher carbon materials
would be beneficial to the quest to understand the affect of parameters on material
phases and layered deposition microstructure.
Chapter 5 Characterization Tool Steels
Chapter 5. 1 The Addition of 400 Series Stainless Steels and Other Carbon Steels92
Chapter 5. 1 The Addition of 400 Series Stainless Steels and Other Carbon Steels
With the quality of SDMT alloy now in question, the opportunity to change or
add materials to the characterization search was now presented. Martensitic stainless
steel alloys can attain the hardnesses need for die cast tooling, and were readily
available. Multiple sources of powder could be found. The 400 series stainless can
possess several different quantities of carbon, so the possibility of developing a
understanding of the relationship of material constituents to factors like hardness or
strength now existed. For conventional processes, like quenching and tempering,
correlations currentlt exist to quantify the effects of material composition upon
hardness levels. Thus, by using the 400 series, one could develop a more meaningful
understanding of carbon steel deposition than just developing a carbon steel for die
casting. The composition of tested martensitic steels are compared against the other
carbons steels investigated and 316L in the table below:
Table 5.1 Carbon Steels: Martensitic, Hot Working, Tool Steels, and Commercial Alloys
When one looks at the E/σ ratio for these steels, we can see that only one has a
ratio higher than 316L stainless. However, most are much worse:
C%Mn%
Si% Cr% Ni%Mo%
Nb%
Al% P% W V S%
410 .06 .17 .53 12.5 .07 0 0 0 .017 .007
420 .45 .49 .54 13.6 0 0 0 0 .017 .007
431 .18 .69 .57 15.6 1.78 0 0 0 .016 .003
SDM Tool Steel I
.01 1 1.23 21.5 2.8 0 .84 0 0 0
H13 .4 0 1.05 5 0 0 0 0 0 0 1.1 0
M2 .8 .3 .3 4 0 0 0 0 0 6 2 0
Rock .15 16 1
316L .02 1.74 .73 17.3 13.1 2.66 0 0 0 0
Chapter 5 Characterization Tool Steels
Chapter 5. 1 The Addition of 400 Series Stainless Steels and Other Carbon Steels93
Table 5.2 Comparison of E/σ of Tool Steels
The coefficient of thermal expansion of these materials are lower than 316L but
not lower than Invar in the 20oC - 400oC range. It is during this range the residual
stress begin to develop. At temperatures above this range, the yield strength of the
material has been exceeded so the plastic deformation of the material relieves any
additional stress accumulation. SDM tool steel is similar in composition to 440C.
Thus, 440C is included to approximate SDM tool steel’s coefficient of thermal
expansion.
The corrosion resistance of most of the 400 series stainless is a bit lower than
316L. From the initial tooling search, a criteria of at least 12% chromium had been
designated to insure corrosion resistance. However, since SDM has the capability of
building gradient structures, the ability to deposit a high corrosion resistant material
next to a low corrosion resistant material, this concern was removed.
E x104 MPaσ MPa
(annealed)E/σ
410 22 241 913
420 20 345 580
431 20 665 301
M2
H13 21 1250 168
SDM Tool Steel
25.4 510 498
316L 19.3 234 825
Chapter 5 Characterization Tool Steels
Chapter 5. 1 The Addition of 400 Series Stainless Steels and Other Carbon Steels94
Figure 5.1 Comparison of the Coefficient of Thermal Expansion of Tool Steels and SDM Mate-rials
For example, suppose cooling channels need to be deposit in a tooling insert.
Typically, the part would be planned with copper (the sacrificial material) being
deposited as the cooling channel and the hard carbon steel deposited around the
copper, forming the walls of the cooling channel. However, during the etch removal of
the copper, some of the low corrosion resistant carbon steel would also be etched.
However, if 316L stainless is deposited as a barrier between the copper and carbon
steel interface, the carbon steel interior of the die insert would be protected. Figure 5.2
shows the cooling channel arrangement. Figure 5.3 shows that it is possible to achieve
smooth gradients by changing the percentage of each metal during deposition. The
figure shows a graded deposition with SDM tool steel and 316L by laser deposition.
The dotted line represents the composition as detected by electron microprobe
measurements and calibrated to represent the increasing carbon content. The solid line
represents the hardness of carbon steel alloys with similar compositions. The smooth
transition enables the use of martensitic stainless or even lower corrosion resistant
carbon steels, like H13.
0
5
10
15
20
0 200 400 600 800 1000 1200 1400
Coefficient of Thermal Expansion
CT
Eµm
/(m
K
)
Temperature (K)
316
410
420440C
414430
Invar (Fe -36Ni)
Chapter 5 Characterization Tool Steels
Chapter 5. 2 Differences in Depositing Carbon Steels 95
Figure 5.2 Graded Structure with Cooling Channel
Figure 5.3 Graded Deposition of 316L and a .15% Carbon Steel
Chapter 5. 2 Differences in Depositing Carbon Steels Our understanding of deformation, grain size, cell morphology, and other
aspects of laser layered manufacturing had developed over many years of laser
depositing invar, aluminum bronze, and 316L. However, none of these materials
undergo solid state phase transformation during laser deposition. These materials
remain face centered cubic or body centered cubic.
Chapter 5. 2.1 Stress States of Carbon SteelsTwo noticeable aspects of deposition changed: stress state and hardness. In
316L, tensile stresses, measured by X-ray diffraction, were located within the deposit
and compressive stresses were measured within the substrate. This is predominantly
from solidification shrinkage. The melted 316L powder will shrink upon solidification
and cooling. The coefficient of thermal expansion insures that the shrinkage will be
A A
Section A
Copper
316L
CarbonSteel
Die Insert
Cooling Channel
0
10
20
30
40
50
0 0.05 0.1 0.15
Hardened StainlessSS-Tool Steel Deposition
Roc
kwel
l C
% C
100% 316L100%
Carbon Steel
Chapter 5 Characterization Tool Steels
Chapter 5. 2 Differences in Depositing Carbon Steels 96
upon the order of the thermal gradient. However, since the deposit is constrained by
the substrate it is placed in tension. The substrate is simultaneously placed in
compression.
Figure 5.4 Material Shrinkage
However, in carbon steels, some solid-state phase transformations can induce
either tension or compression within the laser layered deposits. Stress measurments
of cross sections of laser deposited 316L stainless steel and carbon steels were taken
by use of the Sin2Ψ and Fastress methods, respectively [Section 3. 2.2]. Figure 5.5
shows the results for the stress testing. Steels with percentage weight carbon between
.2-.8% can impose compressive stresses within the deposit and tensile stresses within
the
Figure 5.5 Stress development in SDM Deposits
substrate. When considering multiple phase depositions, one can consider it having a
As the materialshrinks the molecules
of the deposit are placed in tension
These tension forces cause
the moleculesof the substrate
to be in compression.
because it is constraint by substrate.
- 4 0 0
- 2 0 0
0
2 0 0
4 0 0
- 4- 3- 2- 101234
.2% C -Stainless Steel
.8% C - M21.7% C - Grey Cast Iron.1% C 316L
Str
ess
(M
PA
)
Depth (mm) - Note: 0 is substrate-deposit interface
σzz
Cross Sectionof Layers σ z
z
Chapter 5 Characterization Tool Steels
Chapter 5. 2 Differences in Depositing Carbon Steels 97
matrix phase and inclusion phases. The balance between hydrostatic pressure arising
from the inclusion phase upon the matrix and stress from thermal contraction of the
matrix will dictate the magnitude and sign of the stress [6.17].
Figure 5.5 is a graph of residual stress measured at different points within the
bulk of a deposit which is still constrained by the substrate. The austenitic 316L
stainless steel deposit exhibits tension, while the substrate is in compression which is
common to single phase depositions. The austenite (20%) and martensite deposition
of a .2%C martensitic stainless steel has the deposit in compression and the substrate
displaying both compression and tension at different points. The two phase (matrix
and inclusion) deposit has changed the stress profile exhibited by the deposit.
The high carbon steel (.8% C) also displays mixed modes. At the top of the
deposit (the last deposited layer) the material is primarily austenite and martensite and
the material exhibits compression. The middle and bottom layers consist of 3 phases,
austenite, martensite and ferrite. The middle and lower regions of the deposit exhibit
tension.
The possibility of different stress states causes our current understanding of
laser deposition to be challenged. However, these different stress states could be
beneficial. The possibility of compression in the deposited layers could beneficially
affect layered deposition by improving crack resistance but it could also be detrimental
by reducing fatigue strength.
Chapter 5. 2.2 Hardness of Laser Deposited Carbon SteelsHardness for 316L and Invar were 90-96 Rb (200-230 HK) and 80-90Rb (175-
200 HK). This level of hardness indicates typically soft and reasonably ductile
material. Hardnesses for the carbon steels deposited by SDM’s 2400 W Nd:YAG laser
was now in the range of 35-65 Rc (350-700 HK). The higher the carbon content, the
100
200
300
400
500
600
-8-6-4-202468
410-Left Side 410-Middle
Mic
roha
rdne
ss (
HK
)
Distance (mm) Note: O is Deposit Interface
Deposit Substrate
Chapter 5 Characterization Tool Steels
Chapter 5. 2 Differences in Depositing Carbon Steels 98
Figure 5.6 Microhardness of a 410 (SP) deposit on a low carbon steel cold roll substrate
higher the hardness ranked. Figures 5.6-5.9 are microhardness graphs of 410 and 420
martensitic stainless steels. Each structure is from 5-6 layers thick. The measurements
are taken in two places: 2 mm on the left edge of a deposit and in the middle of the
deposit. Each deposit is about 20 mm wide. Different build styles were used to try to
increase hardness by creating more martensite within the bulk structure. A single pass
style (SP) occurs when each layer is built with powder first being placed on the
substrate or previously deposited layer and then scanned by the laser. This sequence
continues over and over until the part is completed. A double pass style (DP) is very
Figure 5.7 Microhardness of a 410 (DP) deposit on a low carbon steel cold roll substrate
Figure 5.8 Microhardness of a 420 (SP) deposit on a low carbon steel cold roll substrate
isimilar with one important difference. Powder is preplaced upon the substrate or
previously deposited structure and then scanned by the laser. Immediately, the laser
scans the part again, a double pass. No additional powder is placed on the newly
deposited layer before the double pass occurs. This procedure of a powder-laser scan
combination followed by a powderless-laser scan is continued to produce each layer of
100
200
300
400
500
600
-8-6-4-202468
410R Left410R-Middle
Mic
roha
rdne
ss (
HK
)
Distance (mm) Note: O is Deposit Interface
Deposit Substrate
100
200
300
400
500
600
-8-6-4-202468
420-Left 420-Middle
Mic
roha
rdne
ss (
HK
)
Distance (mm) Note: O is Deposit Interface
Deposit Substrate
Chapter 5 Characterization Tool Steels
Chapter 5. 3 Changes in Solidification Modes 99
the part. It was hoped that the second scan would reduce the retention of secondary
retained phases or extend the heat affected zone of the laser pass to harden the
material.
The left side of deposits seem to have much more variability in hardness than
the middle of the deposit. The bottom layers of the SP deposits in the middle of the
deposits are typically harder than the top layers of the same area. In 410, the DP
sequence hardened the deposit significantly, while the 420 DP deposit seems to have
soften a bit.
Figure 5.9 Microhardness of a 420 (DP) deposit on a low carbon steel cold roll substrate
Chapter 5. 3 Changes in Solidification ModesWhen building parts or die inserts by using a layer laser manufacturing
technique, there are two steps which should dictate the resultant phases: Solidification
and Layering. Solidification or the solidify of molten metal particles into a solid
deposit is a significant affect. With our laser deposition process, we are able to attain
solidification rates between 104 and 105 K/s. This is rapid solidification. By
undergoing rapid solidification, metastable phases can now exist at room temperature.
Austenite, martensite, and even sigma ferrite may exist. Under slower cooling
processes, martensite will not form, austenite should not be retained, and sigma ferrite
may not exist in iron based steels. If we can understand the solidification modes
produces by rapid solidification, then we may be able to predict in advance phases
produced in the deposition process and begin to prescribe processing changes or even
heat treatments for minimal layer deposits (1-3 layers).
Tools have been developed by Shaeffler (5.1) and Delong (5.2) to aid in
predicting solidification modes. These tools are dependent upon knowing the
100
200
300
400
500
600
-8-6-4-202468
420R-Left 420R-Middle
Mic
roha
rdne
ss (
HK
)
Distance (mm) Note: O is Deposit Interface
Deposit Substrate
Chapter 5 Characterization Tool Steels
Chapter 5. 3 Changes in Solidification Modes 100
composition of the metals. The solidification diagram is based upon the fact that as
most iron based metals solidify from the liquid state, the will either solidify as
austenite or ferrite. Typically , the determining factor resides within the composition.
The amount of nickel or chromium within the metal is typically the determinant.
Nickel is an austenite stabilizing element and chromium is a ferrite-stabilizing
element. The ratio of Chromium to nickel is a good indication of which phase the melt
will solidify first. If the Cr/Ni ratio is higher than 1.5 the melt will solidify as ferrite.
If the Cr/Ni ratio is lower than 1.5 it will solidify as austenite (5.3).
Other elements may also be classified as austenite or ferrite-stabilizing.
Carbon and manganese are strong austenite stabilizing elements. Molybdenum,
silicon, and niobium are strong ferrite stabilizing elements. Austenite forming element
may extend the austenite range on the Fe-C phase diagram to lower temperatures.
Ferrite stabilizing element may extend the ferrite zone or shrink the austenitic phase
field (5.1).
The solidification diagram displays four modes of solidification which are
quantified by the calculation of nickel (Ni-Eq) and (Cr-Eq) chromium equivalence.
The nickel equivalence (Ni-Eq = Ni + 30 C + .5 Mn) and the chromium equivalence
(Cr-Eq = Cr + Mo + 1.5 Si + .5 Nb) attempt to incorporate the effects of the strongest
ferrite or austenite-stabilizing elements.
Figure 5.10 Solidification Diagram
The Solidification Diagram was designed to be used to determine the
solidification modes of weld metal. It uses the nickel and chromium equivalents to
determine the mode preference. Four solidification modes are represented on the
= Cr, Nb, Mo
Ni-E
quiv
alen
t = C
, NI
0
510152025
3035
40
5 10 15 20 25 30Cr-Equivalent
A
AFFA
F
Chapter 5 Characterization Tool Steels
Chapter 5. 3 Changes in Solidification Modes 101
diagram: Austenitic (A), Austenitic-ferritic (AF), Ferritic (F) and Ferritic-austenitic
(FA). Austenitic solidification is a mode in which austenite solidifies as the primary
and only mode. No ferrite is present in the structure. Segregation occurs only to grain
boundaries. Austenitic-ferritic solidification has austenite solidifying as the primary
phase with ferrite forming at grain boundaries. The ferritic second phase may
partially transform to austenite at subsolidus temperatures. Ferritic solidification
occurs when ferrite solidifies at the primary and only phase. Ferritic-austenitic
solidification occurs when ferrite is the primary phase but a second phase, austenite
grows at grain boundaries.
Seven carbon steels were examined. M2, H13, Rock, SDMT, 410, 420, 431
were compared against 316L. Table 5.3 shows the calculated nickel equivalents.
Table 5.3 Nickel and Chromium Equivalents
Figure 5.11 Solifidification Modes as Determined by Nickel and Chromium Equivalents
Metal Cr-Eq Ni-Eq
410-Anval 12.500 2.1700
420-Anval 13.600 15.750
431-Anval 15.600 8.0800
SDMT 21.520 3.1450
M2 4.0000 28.000
H13 6.3500 14.000
Rock 21.000 1.5500
316L 19.960 13.905
0
5
1 0
1 5
2 0
2 5
3 0
3 5
4 0
5 1 0 1 5 2 0 2 5 3 0
Nic
kel E
qu
iva
len
t
Cr-Equivalent
410
420
431
316LH13
M 2
RockSDMT
A
AFFA
F
Chapter 5 Characterization Tool Steels
Chapter 5. 4 The Influence of Layering upon Laser Deposited Carbon Steels102
However, rapid solidification of the laser melt pool allows for most of the
modes to solidify as austenitic and austenitic-ferritic (Figure 5.12). The rapid
solidification seems to suppress the ferritic transformation. When comparing these
results against other rapid solidification techniques, the suppression of the ferritic
transformation is supported for similarly composed metals especially stainless steel
(5.5,5.6).
Figure 5.12 Redrawn Solidification Diagram
Chapter 5. 4 The Influence of Layering upon Laser Deposited Carbon Steels
A single layer typically is not enough material deposition to build a part.
Several layers have to be deposited in order to build parts. Figure 5.13 shows a
deposition which has been etched with sodium meta-bisulfite to reveal the individual
weld passes.
Figure 5.13 SDM deposited 410 Stainless steel Vs. a diagram showing layered manufactur-ing
With each additional layer, prior layers begin to be reheated. The layering
process allows for multiple temperature gradient to exist within the deposition. Thus,
Ni-E
quiv
alen
t
05
10
1520
2530
35
40
5 10 15 20 25 30Cr-Equivalent
431
A
AF
FA
420
410
316LH13
M2
RockSDMT
2 mm
Build PlatformSubstrate
NN+1
N+2 ∆T1
∆T2
∆T3
Chapter 5 Characterization Tool Steels
Chapter 5. 4 The Influence of Layering upon Laser Deposited Carbon Steels103
the phases of the solidified structure will change. Solid state phase transformations
may occur, grain growth, or tempering may occur. To account for just solidification
phases is insufficient.
The SDM deposition process is very similar to multipass welding. Tools like
the Shaeffler welding diagram could be potentially used to benefit SDM designers by
aiding them to understand phase development. By knowing phase developement,the
SDM designer can now make a more informed decision on material selection and
potentially gain insight upon necessary heat treatment regimes to remove unwanted
phases. However, the Shaeffler welding diagram was designed for lower solidification
rates. Conventional welding processes like acetylene torches or GMAW solidify
between 100 to 102 K/s. Solidification for this application has been calculated to be
between 104 to 105 K/s. Other laser based process can be as high as 107.
Figure 5.14 is a plot of the test metals on the traditional Shaeffler Welding
Diagram. The phases that the diagram predicts are only correct in one incident. Using
the nickel and chromium equivalents calculated above, one can see the predicted
phases are inadequate. For the Shaeffler Diagram to be useful for layered
manufacturing, it would have to be changed.
Figure 5.14 Plotting the test metals based on the Nickel and Chromium Equivalents
Other researches investigating rapid solidification have suggested altering the
Shaeffler diagram. Vitek et. (5.7) al and Katayama et. al. (5.8) have actually proposed
modifications of the Shaeffler Welding diagram to reflect rapid solidification. Instead
of modify the areas of the diagram, Self et. al (5.9) proposes modifying the definition
of the nickel equivalent. This approach seems to be stronger, in that trends to
modifying the definitions of nickel equivalents can be accomplished more easily than
redefining the boundaries of the entire diagram. In this study, the researched focused
Ni-E
quiv
alen
t
431 = A+M+F420 = A+M+F316L = A410 = A+MSDMT = A+MH13 = A+MM2 = A+M+F
The Prediction
Actual
H13
M2
0
5
10
15
20
25
30
0 5 10 15 20 25 30 35 40Cr-Equivalent
420
A+M
MartensiteA+M+F
δ-Ferrite
431
410M+F(δ)
Austenite
316L
SDMT
431=A+M+F
420 = A+M+F316L = A
410 =A+MSDMT =A+M
H13 =A+MM2 =A+M+F
Actual
A=Austenite, F=Ferrite, M=Martensite
Chapter 5 Characterization Tool Steels
Chapter 5. 4 The Influence of Layering upon Laser Deposited Carbon Steels104
upon the martensitic regions of the Shaeffler Diagram. Other researcher’s data who
have looked at other rapid solidification processes or laser multipass welding
phenomena have predominantly studied metals falling in the austenitic and ferritic
regions. Combining their analysis with this research could provide a more
comprehensive revision to the Shaeffler welding diagram and a more robust solution.
Utilizing the group handling data method (GMDH), the Cr-Eq and Ni-Eq were
modified to place the rapid solidification metals in to the correct areas. The GMDH
was not only given the compositional elements of the metals to choose from but
melting temperatures, thermal conductivities, etc. to find the relevant modifications to
the Shaeffler Diagram. Also, other researchers data was also included in the pattern
recognition search (5.6,5.10,5.11).
Figure 5.15 Modified Shaeffler for Layered Laser Manufacturing
Figure 5.15 shows the modified Shaeffler Welding Diagram. Notice how the
the over all trend is to shift down and to the right for the more martensitic and tri-
phase steels and up and left for the more austenitic steels. (Figure 5.17 shows more of
the modelled data and includes other researchers data: Katayama (5.8), Vitek (5.7),
David(5.6) , Anjos (5.9), Wei (5.10). )
Figure 5.16 shows the proposed New Ni-Eq and Cr-Eq. Both the New Ni-Eq
and the New Cr-Eq show a dependence upon the Cr-Eq. This is similar to results
found by Self et al (5.9). Self found that the amount of chromium was relevant in the
retention of austenite and the formation of martensite . At high chromium the phase
change of interest in Self’s work is austenite-ferrite, but at low chromium, it becomes
austenite-martensite.
ST = SDMT
Ni-E
quiv
alen
t
Cr-Equivalent
0
5
1 0
1 5
2 0
2 5
3 0
0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0
A+M
MartensiteA+M+F
δ-FerriteM+F(δ)
Austenite
LS H13
4 1 04 2 0
4 3 1M2
ST309A
316L
Chapter 5 Characterization Tool Steels
Chapter 5. 4 The Influence of Layering upon Laser Deposited Carbon Steels105
Researchers have shown that the level of chromium can become an agent to
determine how certain phase-stabilizers behave. In addition to the strong austenite-
stabilizers listed earlier, there are others: Co, Cu, N, Al. There are also additional
ferrite-stabilizers: Ta, Ti, W, V, Z. However, during cryogenic solidification or high
service temperatures, some of these element along with Mn, Al, Mo, and Si may
change and stabilize the opposite modes (5.12, 5.13). At high solidification rates,
ferrite-stabilizers may act as austenite-stabilizers and vice-versa. One indicator of this
mode change, may be the level of chromium. In low chromium systems, ferrite-
stabilizers act like ferrite-stabilizers and austenite-stabilizers act like austenite-
stabilizers. In high chromium systems the “switching: properties has been shown to
occur.
Figure 5.16 The proposed NI-Eq and Cr-Eq
Figure 5.17 shows the limits of the proposed model. The distinctive separation
of high ferrite- low austenite content welds from fully ferritic welds is not very clear.
312B was fully ferritic in rapid solidification experiments. However, with the
traditional Shaeffler diagram. However, Katayama’s 100% Ferrite line may be more
appropriate for the true boundary line (5.8).
Figure 5.17 The Extents of the Laser Layered Manufacturing Shaeffler Model
(New Ni-Eq) = 11.977810 -0.368111 [W] + 1.598563 [Mo] + 0.579666 [Ni] - 0.262729 [Cr-Eq]
(New Cr-Eq) =0.666030 + 0.578787 [V] +5.386530 [Si] + 2.048034 [W] + 0.811128 [Cr-Eq]
Instability in the Model: Modelin accurately predicts the austeniteferrite content in this region.
100% Ferrite Line proposed by Katayama,et. al.
0
5
1 0
1 5
2 0
2 5
3 0
0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0
A+M
MartensiteA+M+F
δ-FerriteM+F(δ)
Austenite
LS H13MS
R
410420
431 M2ST
446
312B309A
316L316B
304A
316A
Ni-E
quiv
alen
t
Cr-Equivalent Solidification Rate: 104-106
Chapter 5 Characterization Tool Steels
Chapter 5. 5 Phase Location 106
Chapter 5. 5 Phase LocationThe improved Shaeffler Welding Diagram for Layer Manufacturing does not
account for the exact percentage of austenite, martensite or ferrite within the deposited
material except for 100% single consituency regions. Regions of multiple phases now
only account for the existence of a phase as oppose to the quanitity. This exists for two
reasons: 1. The data is being fit to the Shaeffler Welding Diagram thereby eliminating
the phase percentage and diagram location relationship. 2. The data is an average of
the amount of phases which exist within the bulk deposit.
The original Shaeffler Welding Diagram was designed to aid designers to not
only predict the existence of phases within the welded materials but also to predict the
phase percentages. Because data is being fit into diagram, it was very difficult to
maintain the phase percentages as described by the diagram and successfully redefine
the Ni & Cr equivalences to accurately describe a wide set of material compositions
and encompass the full effects of rapid solidification. To produce a diagram which can
acurately describe phases percentage would require redrawing the entire Shaeffler
Diagram.
The model uses the average phase values to position the material within the
correct Shaeffler zone. A laser deposit which is more than 3 layers thick can be
divided into 3 disctinct regions: bottom, middle and top.
Figure 5.18 Top, Middle, and Bottom sections of carbon steels
Figure 5.18 shows two sets of pictures of microstructural zones of laser
A1. Top 420
B1. Top 420
A2. Middle 420
A3. Bottom 420
B2. Middle 420
B3. Bottom 420
50 µm
50 µm
50 µm
50 µm50 µm
50 µm
SS-Single Pass
SS-Double Pass
SS-Single Pass
SS-Double Pass
SS-Single Pass
SS-Double Pass
Chapter 5 Characterization Tool Steels
Chapter 5. 5 Phase Location 107
deposited stainless steel. The micrographs labeled “A” were built with single scans.
Single pass scans are a deposition sequence of placing the powder upon the substrate
or previously deposited layer and then scanning the powder with the laser heat. The
micrographs labeled “B” were built with double scans. Double pass scans are a
deposition sequence of placing the powder upon the substrate or previously deposited
layer, scanning it with the laser heat, and then immediately scanning the surface again
with laser heat without the addition of new powder. These two samples are included to
show how different the microstructure can look if the laser solidification-reheating
sequence is changed even with the same lasing conditions (laser power, scanning
speed, powder feed rate, etc.).
The bottoms of both carbon steel deposits have a significant amounts as-
quenched martensite, characterized by dark brown to black because of reactive
etching. The lighter brown shown in all 6 photos is tempered martensite. Blue
indicates ferrite. The shiny or unreacted boundaries are mixtures of retained austenite
and ferrite. Table 5.4 and 5.5 show the average phase percentage for the 420 deposits
Table 5.4 Phase Averages for each location in a 420 Single Pass Laser Deposit
Table 5.5 Phase Averages for each location in a 420 Double Pass Laser Deposit
Martensite Ferrite Austenite
Top 86% 7% 5%
Middle 82% 6% 9%
Bottom 76% 4% 12%
Average 81% 6% 9%
Martensite Ferrite Austenite
Top 77% 4% 14%
Middle 94% 0% 2%
Bottom 79% 2% 15%
Average 83% 2% 10%
Chapter 5 Characterization Tool Steels
Chapter 5. 6 Grain Size of Laser Deposited Layered Carbon Steels 108
of both conditions (See section 3. 1.1 -3. 2.2 for methodology used to measure
phases). The differences in microstructure based upon location can be dramatic and is
determined by proximity to substrate ( which acts as a heat sink), the number of layers
deposited, and laser conditions. A single tool which could meaningfully characterize
all of these effects and still provide quantitative phase percentages is extremely
difficult to build. Multiple tools for each region, each lasing condition, and each
material would have to be constructed. Also, the regions may not scale as easily to
larger builds. When the total thickness of deposited layers extends beyond 25 mm,
significant changes may occur to the phases present. The continual reheating of lower
layers begins to form more equilibrium phases like cementite and banite. Therefore,
another set of Shaffler Diagram’s would have to be built.
As recounted in Chapter 2, there are several competitive laser deposition
processes which can also benefit from this knowledge. However, each of the processes
uses different deposition sequences, laser conditions, etc. A specific regional tool for
SDM could not be leveraged very well for their applications. A universal tool that can
inform the designers of potential phases that can arise during laser deposition, is much
more useful.
Chapter 5. 6 Grain Size of Laser Deposited Layered Carbon Steels
Grain size is indicative of the material properties of the laser deposit. The
grain structure of austenitic steels is very different from the laser deposited carbon
steels. Austenitic steels formed lenticular shaped cells while the carbon formed from
lathe to plate -like structures. No direct correlation could be found between powder
size to grain size. However, by using GMDH a relation ship was developed which can
predict equivalent diameters of the grains. Equivalent diameters are based on the area
of the grain.
Using GMDH, a heuristic was developed to predict grain size. The factors
included at the beginning of modeling were material composition, powder size,
solidification rate, thermal conductivity, and melting point. Figure 5.20 displays the
relevant terms. The heuristic will enable designers to get an estimate of the grain size
of the structure if they are using a layering process with solidification rates from 104-
105 K/s.
Chapter 5 Characterization Tool Steels
Chapter 5. 6 Grain Size of Laser Deposited Layered Carbon Steels 109
Table 5.6 Powder Diameter Vs. Grain Diameter
Figure 5.19 Examples of Grain Sizes
The heuristic’s components seem to be quite applicable. The carbon content
will influence phase formation as well as aid carbide forming elements. Carbide
forming element, particularly Mo and V, because they can act as grain size controlling
elements. The principal effect of the grain size controlling elements is to extend the
recrystallizing time of austenite. The longer time the it takes for austenite to
recrystallize, the more nucleation sites for ferrite to form. The more ferrite nucleation
site the smaller the ferrite grains will be (5.14). The chromium and Ni-Eq
(30*C+Ni+Mn), all aid in determining the phases which exist in the solidify melt. The
Metal Phase(s) Powder
SizeEquivalent Diameter
316L 100%-A 74 +34 µm 9.64 µm
410 SS 30%A+M 55 +32 µm 10.29 µm
431 SS 9%A+M+2%F
122 +49 µm 5.63 µm
420 SS 14%A+M+6%F
100 +60 µm 5.04 µm
H13 3%A+M 45 +20 µm 4.76 µm
SDMT 20%A +M 90 +20 µm 4.11 µm
Laser Deposited 420 SS
100 µm100 µm
Laser Deposited 316L Stainless
100 µm
100 µm
Laser Deposited 431 SS
100 µm
Laser Deposited 410 SS
100 µm
Laser Deposited 316L
Chapter 5 Characterization Tool Steels
Chapter 5. 7 Layer Thickness 110
melting kinetics related to powder size, thermal conductivity and melting temperature
will all influence grain size.
Figure 5.20 Equation for Grain Size Predictor
Figure 5.21 Grain Size Comparison Actual Vs. Heuristic
Chapter 5. 7 Layer ThicknessLayer thickness is another characteristic of deposition which needs to be
understood. The ability to predict layer thickness will enable designers to accurately
forecast the number of layers needed to build a part and predict completion times.
Also, understanding the nature of layer morphology will enable more insightful
planning.
Since SDM is deposited at room temperature and without preheat, much of the
energy of the first layer of deposition is used to bring the substrate and build platform
to a steady state temperature with the fresh deposit. The low carbon steel substrate and
aluminum build platform act as a heat sink. As more layers are deposited, and the
distance from the top layer to the substrate increases, the upper layers of the deposit
begin to get warmer. The weld pool can be larger and more powder is able to solidify.
Thus layers size increases with z-height. However, the layer thickness does begin to
achieve a mean thickness. The last layer of the deposit is about 40-50% larger than
Average Grain Diameter (µm)= 22.8(µm) + {0.3 [V] + 3.7 [C] -0.0008/K [Melting Temperature] + 0.6[Cr] -0.1/µm [Powder Size] -0.8 [Mo]-0.3 [NI-EQ] -0.5ms/K [κ]}µm
0 2 4 6 8 1 0 1 2
4 1 0
4 2 0
4 3 1
SDMT
H13
3 1 6 L
Invar
Experimentally MeasuredCalculated Values
Equivalent Diameter (µm)
Chapter 5 Characterization Tool Steels
Chapter 5. 7 Layer Thickness 111
mean size thickness. The last layer is not remelted like the prior layer so it maintains
the thickness of individual weld passes.
Table 5.7 displays layer values for 4 steels used in the research. Five layer
structures were built of each material. Two examples of 420 are shown. One was built
with single pass layers while 420-DP was built in a double pass condition:
Immediately following a powder melting laser pass, the laser scans the part again, this
time with no powder. All of the other beams were built with a single pass. The double
pass builds seem to slightly thicken the latter layers of the build.
Table 5.7 Layer Thickness for SDM Carbon Steels
Every time a deposit is allowed to cool to room temperature, the layer size
starts almost back over, as if it were a brand new deposit instead of a continued build.
Figure 5.22 shows a 60 layer (55 mm) structure. Notice how the layer thickness
deviates as the build gains in z-height. Figure 5.23 shows the same graph but this time
it is identified with every time with the break that was taken during the deposition.
The ability to control layer thickness seems to stem from scheduling breaks. If a
certain layer size is critical to part strength or critical internal location, like the
deposition strategy for the location of internal geometry, a cooling/deposition
sequence during the build process need to occur. Lastly, if more than 12 layers are
deposited at any point in time, the grain size of bottom layers may grow as well as
incur phase transformations.
Layer316L(µm)
SDMT(µm)
431(µm)
420(µm)
420-DP (µm)
5th Layer 1200 1286 1624 732 755
4th Layer 937 856 703 468 550
3rd Layer 899 904 797 413 503
2nd Layer 604 647 557 442 470
1st Layer 550 292 470 174 208
Chapter 5 Characterization Tool Steels
Chapter 5. 7 Layer Thickness 112
Figure 5.22 Sixty Layer Deposition
GHDM was used to determine what the most significant factors effecting the
mean layer thickness. In addition to the material composition, material properties like
thermal conductivity, melting point and solidification rate, two addition factors were
added: fill rate and thermal diffusivity length.
Figure 5.23 The Effect of Cooling on Deposition
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 5 10 15 20 25 30
Ave
rage
Lay
er T
hick
ness
(m
m)
Z-Height (mm)
20 25 30 35 40 45 50
10.80.6
0.4
0.2
02
4
6
8
2
4
0 5 10 15 20 25 30
A
B
B
B A
A
Z-Height (mm)
Laye
r T
hick
ness
(m
m)
A = Overnight Cooling
(12-24hr breaks)B = Short Cooling Periods
(1-2hr breaks)
20 25 30 35 40 45 50
Chapter 5 Characterization Tool Steels
Chapter 5. 7 Layer Thickness 113
Figure 5.24 shows the results of the model. Metals with various deposition fill
rates and scanning speeds were put in the model. However, Figure 5.25 just shows one
setting. Figure 5.25 shows a comparison between the model and the actual values of
mean layer thickness.
Figure 5.24 Heuristic for Predicting Mean Layer Size
Figure 5.25 Comparison of Mean Layer thickness: Predict Vs. Actual
Even thought the solidification rate does include the effects of the scanning
speed, it is the powder fill rate which make more of an impact upon mean layer
thickness. As Figure 5.26 shows, from 15 mm/s to 40 mm/s there is an insensitivity
to laser scanning speeds. The amount of energy that the SDM puts into weld is so
large that regardless of the speed adequate layer solidification is achieved. At speeds
higher than 40 mm/s, the 407 W/mm2 is no longer sufficient to get good layer
solidification with no porosity or retention or secondary phases.
The fill rate is extremely sensitive. Depositions below 23 g/min tend produce
too thin of layers and more secondary phases from solid state phase transformations
tend to occur. Depositions above 23 g/min produce too thick of layers, secondary
phases are retained from the melt.
Layer (µm) = 1176.7µm + {80.8 [V] -0.0008s/K [Solidification Rate] +11.1 [Ni] - 40.3 [Ni-EQ + 27.8 min/g [Fill Rate] - 5.5/µm [Powder Size]} µm
0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0
4 1 0
4 2 0
4 3 1
SDMT
H13
3 1 6 L
Invar
Experimentally MeasuredCalculated Value
Thickness (µm)
30 mm/s at 23g/min
Chapter 5 Characterization Tool Steels
Chapter 5. 7 Layer Thickness 114
Figure 5.26 Optimizing Deposition Parameters
Fill Rate (g/min)
Laser Scan Speed (mm/s)
%R
etai
ned
Aus
teni
te 0 5 10 15 20 25 30 35 40
0
5
10
1
5
20
2
5 3
0
35
4
0
15 20 25 30 35 40 45 50 55
Speed
Fill Rate
Chapter 5 Characterization Tool Steels
Chapter 5. 7 Layer Thickness 115
5.1 Shaeffler, A. L., “Constitutional Diagram for Stainless Steel Weld Metal,” Metal Progress 56(5): 680-680B.
5.2 Delong, W. T. “Ferrite in Austenitic Stainless Steel Weld Metal,” Welding Journal 53(7):273-s -286-s.
5.3 Suutala, N. Takalo,T.,Moisio, “The relationship Between Solidification and Micro-structure in Austenitic and Austenitic-Ferrite Stainless Steel Welds,” Metallur-gical Transactions 10 (A), 512-514 (1979).
5.4 Robert, George, Krauss, George, and Kennedy, Richard, “Tool Steels,” ASM Inter-national, 1998.
5.5 Elmer, Walter, “The Influence of Cooling Rate on the Microstructure of Stainless Steel Alloys,” Lawrence Livermore Laboratory, September 1988.
5.6 David, S. A., Vitek, J. M., and Hebble, T. L., “Effect of Rapid Solidification on Stainless Steel Weld Metal Microstructures and Its Implications on the Shaef-fler Diagram,” Welding Research Supplement, October 1987, pp. 289-s- 300-s.
5.7 Vitek, J. M. , Dasgupta, A. , and David, S. A., “Microstructual Modification of Austenitic Stainless Steel by Rapid Solidification,” Metallurgical Transactions 14 (A), 1833-1841 (1983).
5.8 Katayama, S. and Matsunawa, “Solidification Microstructure of Laser Welded Stainless Steels”, Proc. ICALEO, p. 60, 1984.
5.9 Self, J. A., Matlock, D. K., and Olson, D. L., “An Evaluation of Austenitic Fe-Mn-Ni Weld Metal for Dissimilar Metal Welding,” WRC Bulletin, September 1984, p 282-s - 288-s.
5.10 Wei, M. Y. and Chen, C., “Fatigue Crack Growth Characteristics of Laser-Hard-ened 4130 Steel,” Scripta Metallurgica et Materialia, Vol 31., No. 10, 1994, pp. 1393-1398.
5.11 Rieker, C., D. G. Morris, and Morris, M. A., “Microcrystalline Surface Layers Created by Laser Alloying” Journal of Less-Common Metals, 145 (1988) 595-600.
5.12 Wallace, W., Trenouth, J. M., Daw, J.D., “Microstructual Instabilities in An Industrial Engine Vane,” Metallurgical Transactions A (Physical Metallurgy and Materials Science), Vol 7A, No 7, Jul 1976 p. 991-997.
5.13 Sakamoto, T., Nakagama, Y., and Yamauchi, I, “Effect of Mn on the Cryogenic Properties of High Austenitic Stainless Steels,” Advances in Cryogenic Engi-neering: Materials, Volume 32, 1986, p 65-71.
5.14 Reed-Hill, Robert E. and Abbaschian, Reza, Physical Metallurgy Principles, PWS-Kent Publishing: Boston, pp. 661-685.
Chapter 6 Applications of Characterization
Chapter 6. 1 High Impact Resistant Materials 116
Chapter 6 Applications of Characterization
Now that the formation and transformation of carbon steels in layered laser
deposition is better understood, one can use this new understanding to solve
deposition problems. Knowledge of grain size and phase composition are used to
select high impact resistant materials and to reduce part deflection. These are just two
examples of how understanding the marriage of material microstructure and
deposition parameters can benefit layered manufacturing
Chapter 6. 1 High Impact Resistant MaterialsDie cast inserts need high impact resistant materials in order to survive the die
casting service environment. Figure 6.1 shows a pressure trace of a 6 ton die casting
machine . Dies in this machine will exhibit pressure changes from ambient to 20,000
psi and temperature changes of 150_C to 667_C in 12 seconds or less
Figure 6.1 Pressure Trace from a 6 ton Die Casting Machine
Shot Pressure (PSI)
14000
15000
16000
17000
18000
19000
20000
Time (Sec)
150m C
667m C
1.0 1.4 1.8 2.2 2.6
One second aftermetal injection.
Chapter 6 Applications of Characterization
Chapter 6. 1 High Impact Resistant Materials 117
If molten aluminum is injected into a die with low impact resistance, the die
will gall or initiate heatchecking. Thin features will degrade, and the die inserts will
be rendered useless quickly.
Impact resistance is typically measured by charpy impact specimens (see 3.
3.2) at room temperature. Die casting die inserts are typically made out of H13 and
are austenitized and tempered. After these heat treatments, charpy specimen’s can
typically endure from 10-20 ft-lb of impact resistance.
Chapter 6. 1.1 Engineering Material SelectionsOur typical engineering understanding would have us select the material by
looking at the ratio of the modulous of elasticity to yield strength (Figure 6.2 and Table
6.1). According to this SDMT should have the best results. Although better than most,
Figure 6.2 Maximizing the E / σ relationship to increase Impact Resistance
Table 6.1 Comparison of E /o Tempered Relationship
E x104 MPaσ MPa -
(Tempered)E/σ
410 22 1005 219
420 20 690 290
431 20 738 271
SDM Tool Steel
25.4 510 498
H13 21 1344 156
316L 19.3 415 465
Minimize the Deformation By Toughening the Materialε *L =δδ h σ/E Minimize δ by maximizing E /σ
δ
Ιmpact TestingCharpy Impact Specimen
Chapter 6 Applications of Characterization
Chapter 6. 1 High Impact Resistant Materials 118
431 was clearly the best. On the half size specimens, it showed almost twice the
impact resistance. If E/σ is not the appropriate measure to use, perhaps the
microstructual information can help.
Figure 6.3 Results of Charpy Testing
Chapter 6. 1.2 Microstructual Material SelectionImpact resistance is a function of grain size just as yield strength and many
other properties are. Also, the phases within the deposit will determine, impact
resistance. Martensite has a bulk modulus of 17.3 x 104 MPa while austenite has a
bulk modulus of 16.4 x 104 MPa and ferrite has a bulk modulus of 17.1 x 104 MPa
(6.1). The added toughness of martensite may be swaying the results. However, when
compare grain size and phase retention, we see that it is a combined result which gets
the best results. A small grain size and the reduction of secondary phases gives the
best impact resistance (Figures 6.4 and 6.5).
Figure 6.4 Grain Size (Equivalent Diameters) vs Impact Resistance
The additional heat treatment of H13 did not improve the results. This is a
0 2 4 6 8 1 0
4 1 0
4 3 1
4 2 0
SDMT
H13
Impact Resistance (ft-lb)
Note: H13 was austenitized.All other specimens are inthe as deposited condition.
Half Size Charpy Specimens(5mm x 10 mm x 55 mm T x W x L)
0 2 4 6 8 10
10.29
5.63
5.04
4.11
410
431
420
SDMTGra
in S
ize
(µm
)
Impact Resistance (ft-lb)
Chapter 6 Applications of Characterization
Chapter 6. 1 High Impact Resistant Materials 119
trend seen in other metals. A series of experiments were run on the 420 martensitic
stainless steels (Figure 6.6). During conventional tempering temperatures between
225- 500 Co the formation of sigma ferrite is quite high. Sigma ferrite is a brittle
phase of ferrite . Because it is brittle, it often acts as a crack initiation point in
materials or reduce material properties. Typically, sigma transforms from metastable
delta ferrite at temperatures of 600-700 Co(6.2). Because conventional heat treatment
cycles do not seem to work, each metal will have to be tested to find appropriate heat
treatments.
Figure 6.5 Secondary Phase Constituents Vs. Impact Resistance
Figure 6.6 Sigma Ferrite Formation during Tempering Cycles of 420 Stainless Steel.
The metal 431 had the highest impact resistance. Even though its E/σ ratio is
not very high, the small grain structure and limited amount of secondary phases cause
its material properties to be superior. When comparing the result of 431 with full size
specimens, we see that the 431 results are comparable to conventionally rolled or
forged H13 which has been tempered (Figure 6.7). The results do suggest that there is
0 2 4 6 8 10
3 0
2 0
1 7
1 1
410
431
420
SDMT
% 2
nd P
hase
C
ompo
sitio
n
Impact Resistance (ft-lb)
Dan
gero
us H
eat
Tre
atm
ent
Zon
e
0
1 0
2 0
3 0
4 0
5 0
0 2 0 0 4 0 0 6 0 0 8 0 0 1000
Delta Ferrite Sigma Ferrite
Pha
se P
erce
ntag
e
Tempering Temperatures
Chapter 6 Applications of Characterization
Chapter 6. 2 Characterizing Material Composition to Induce Martensite Transforma-tion 120
a possibility to produce die inserts which can work without heat treatment. This would
be an additional time savings for the designer and manufacturer.
Figure 6.7 Comparison of Layered Laser Deposited Carbon Steels and Conventionally formed and tempered H13.
Chapter 6. 2 Characterizing Material Composition to Induce Martensite Transformation
Laser deposition produces rapid solidification or quenching because of fast
heat conduction from the laser melt pool into the base metal. This solidification rate
can also be influenced by changing deposition parameters such as scanning speed and
laser power. When depositing carbon steels, rapid solidification can cause metastable
phases to occur, primarily martensite. Martensite formation is a diffusionless process
which occurs when the deposit cools from high temperatures (like the steel melting
point) to below the martensitic start temperature in less than 10 sec. The arrows shown
in Figure 6.8 illustrate the isothermal transformation of H13 and 410 stainless steel to
martensite by rapidly cooling to room temperature. Cooling slower than this would
not produce martensite but result in the production of ferrite and cementite.
When the laser melts the powder, the first solid metal to nucleate within the
liquid is austenite. Austenite has a face center cubic structure. The rapid quenching
causes the carbon in the austenite phase to transform to body center-tetragonal (BCT)
martensite trapping carbon in the BCT lattice which has not had a chance to diffuse.
0
5
10
15
20
25
25 30 35 40 45 50
Cha
rpy
V-n
otch
ed
Impa
ct e
nerg
y (
ft•lb
f)
Hardness (R c)
420/410 H13 (Tem, L)
H13 (2 Tem, L)
H13 (Tem, L)
H13 (Tem, T)H13-VAR (2 Tem, T)
H13-ESR (2 Tem, T)
431
410 420SDMT
Chapter 6 Applications of Characterization
Chapter 6. 2 Characterizing Material Composition to Induce Martensite Transforma-tion 121
Figure 6.8 Isothermal Transformation Diagram of H13 and 410 steels. Compiled from [6.9].
This transformation results in an expansion in volume. This expansion can be
as big as 4%. This is the same order of magnitude as the thermal shrinkage which
arises from cooling of the metal. If martensite transformation can be induced and
controlled by layered laser deposition, then the potential to balance the solidification
shrinkage with phase volumetric expansion may exist.
The use of lasers to change part or surface microstructure has been extensively
researched for laser cladding and surface hardening. Several researchers have shown a
relationship between laser parameters and microstructual evolution for laser cladding
and surfacing. Wang et al. [6.5] showed that metastable phases can be produced on
material surfaces by laser quenching, a process in which laser melting and quenching
of a material occurs by using a very short laser pulses. Yang et. al. [6.6] used a CO2
laser to show that case depth or the depth of phase transformation and morphology of
materials can be influenced by laser power and laser scan rate. Fouquet et. al [6.7]
used a continuous wave CO2 laser to transform the surface of grey cast iron from
austenite to a mixture of austenite, cementite and martensite by using overlapping
multiple laser scan paths. Rieker, et. al [6.8] uses a remelting second pass of the laser
to homogenize the chemistry and microstructure of a laser hardened surface on ferritic
stainless steel.
200
400
600
800
1000
1200
1400
1600
1800
1 10 100 1000
T
empe
ratu
re (
C)
Time (s)
410
H13
A+9%FA+>9%F
A+F+C F+C
AA+F A+F+C
F+C
Ms
A=AusteniteF=FerriteC=CementiteM=Martensite
Chapter 6 Applications of Characterization
Chapter 6. 2 Characterizing Material Composition to Induce Martensite Transforma-tion 122
Once the deposition of the beams was completed, the beams were tested for
deflection and phase composition. Table 7 lists the average results for each type of
material tested.
Table 7: Results of Deposition Experiments
Table 6.2 Phase Analysis and Deflection ResultsSelective etchants revealed martensitic and austenitic phases in all the metals
with the exception of 316L which is only austenitic. Martensitic stainless steel, 420,
had three phases: austenite, martensite, and ferrite (Figure 6.9 and Figure 6.10).
Figure 6.9 Pictures of 431 stainless: 1. Optical microscope with light brown reflecting tem-pered martensite, black - as-quenched martensite, white - retained austenite. 2. Bright-field image of TEM with white regions - martensite, black regions - austenite. @30,700 X.
Microhardness for the specimens ranged between 400-540 HK. For each of
Lase
r P
ass
Defl
ectio
n (m
m)
% A
s-qu
ench
edM
arte
nsite
(MA
Q)
% T
empe
red
Mar
tens
ite(M
T)
% A
uste
nite
% F
errit
e
MA
Q/M
T
410 1 .97 7 55 34 0 .125
420 1 1.26 16 65 9 6 .249
431 1 .432 29 62 4 2.3 .471
SDM -Tool 1 .89 9 68 20 0 .129
316L 1 .97 0 0 98 0 .0
410 2 1.14 4 88 6 0 .047
420 2 .33 28 55 10 2 .506
431 2 .33 30 58 6 3 .51
1. 2
M
A
A100 µm
Chapter 6 Applications of Characterization
Chapter 6. 2 Characterizing Material Composition to Induce Martensite Transforma-tion 123
the metals, the hardness ranges were typically below the value of typical as-quenched
or hardened steel ranges. Figure 6.11 compares the quench/hardness ranges for the
martensitic stainless steels used in the experiments.
For certain materials increasing the martensite percentage reduced deflection
while it increased it in others. Figure 6.12 shows that by increasing the ratio of as-
quenched martensite to tempered martensite, deflection is reduced.
More as-quenched martensite is found in base layers than in N+1 layers
(middle and top layers). This suggests that the quenching rate is much faster near the
substrate than for subsequent layers. The double laser pass condition increases the
quenching rate for middle sections of the beams than the single pass condition. The
inset in figure 6.12 compares middle and bottom layers of 420 in single pass and
double pass condition.
Figure 6.10 Optical Microscope pictures of :1. 410 - SP, 2. 410 - DP, 3. 420 - SP, 4. 420 - DP.
Chapter 6. 2.1 As-Quenched Martensite MaterialsIncreasing the amount of as-quenched martensite retained in the beams
reduced the deflection of SDM deposited beams. As-quenched martensite allowed for
maximum volumetric expansion. Tempered martensite allowed the carbon to diffuse,
shrinking the lattice and reducing the amount of volumetric expansion achieved. SDM
laser manufacturing induced as-quenched martensite. However, reheating of layers
during the deposition of subsequent layers tempered much of the as-quenched
martensite. The addition of a second laser pass increased the quenching rate and
helped maintain more as-quenched martensite for most of the metals tested. Attaining
a ratio of at least .45 as-quenched to tempered martensite produced a 30-75%
reduction in deformation in the SDM deposited beams.
1. 2. 3. 4.20 µm
20 µm
20 µm
MT A MAQ A MAQ MT A F MAQ MT A MT MAQ
20 µm
Chapter 6 Applications of Characterization
Chapter 6. 2 Characterizing Material Composition to Induce Martensite Transforma-tion 124
Figure 6.11 Comparison of the hardness values for martensitic stainless steel beams: 1 - Sin-gle Pass, 2-Double Pass. Quenching range compiled from [6.15].
These tests upon the SDM layered process indicate that continuous deposition
of material layers influences the microstructure of N, N-1, N+1 layers. The layer that
is being deposited, N, is obviously affected because of the melting and solidifying of
the material. The N-1 microstructure is affected because of the reheating of the layer
due to the newly deposited layer. The next layer, N+1 is affected because its grain
growth direction is seeded from the prior layer N. As more layers are deposited, more
heat is retained in the bulk. Grain growth in both austenitic and martensitic steels
increases with each additional layer. Grain size in layer N+1 is larger than in N.
However, in martensitic steels, this increased heat retention produces more tempered
martensite in the bulk and upper layers than near the interface between substrate and
first layer. The difference in the heat characteristics is an important element to
modelling and understanding the phase development within layered metal part.
Quench/Hardening Range for 410
300
350
400
450
500
550
600
650
700
0 5 10 15 20 25 30 35
Har
dnes
s (H
K)
Percentage of As-Quenched Martensite
410 (1)
410 (2)
420 (2)431 (2)420 (1)
431 (1)
Quench/Hardening Range for 431
Quench/Hardening Range for 420
Chapter 6 Applications of Characterization
Chapter 6. 2 Characterizing Material Composition to Induce Martensite Transforma-tion 125
Figure 6.12 The effect of increasing MAQ/ MT Ratio on deflection.
Multiple phases may be both detrimental and beneficial to material properties.
A fully martensitic beam that is in the as-quenched phase may be too brittle to use. A
beam that has ferritic phases could develop sigma ferrite which is a brittle phase often
related to crack initiation points in metal. However, the retention of austenite or delta
ferrite has been shown to increase the toughness of martensitic parts [6.16].
Charpy impact specimens show a 20% drop impact resistance at the maximum
martensite condition. However, after heat treating a beam with negligible changes in
deflection at 200Co for 12 hours, the difference was ony 10%. Therefore, if the as
deposited condition is not sufficient, this special heat treatment may be enough to give
a prototype die insert the impact resistance that it needs.
Chapter 6. 2.2 Double Laser Pass Affects the Amount of As-Quenched Martensite
The double pass in most cases increased the amount of As-Quench martensite
retained in the deposition. As Figure 6.13 shows the double pass seems to improve the
ratio of as-quenched to tempered martensite significantly. The single pass has room
temperature powder with air gaps to heat and solidify, limiting the depth of the heat
affected zone (HAZ) where most of the as quench martensite populates. The double
pass has 250 Co fully solid material (98-99.5% dense with minimal pores) to remelt,
0.2
0.4
0.6
0.8
1
1.2
1.4
0 0.1 0.2 0.3 0.4 0.5 0.6
De
flect
ion
(m
m)
MAQ
/MT Ratio
420 (1)
420 (2)431 (1)
431 (2)
410 (2)
410 (1)
N-L
ayer
s
Primary Sitesfor As-QuenchedMartensite in 420
(1) SP (2) DP
MT
MA
Q-M
T
MA
Q-M
T
MA
Q-M
T
Substrate
20µm
Chapter 6 Applications of Characterization
Chapter 6. 2 Characterizing Material Composition to Induce Martensite Transforma-tion 126
extending and densify the depth of the HAZ. Figure 6.14 shows the difference in a
single pass and double pass on 410 stainless. The dark brown is as-quenched
martensite, lighter brown is tempered martensite and white is retained austenite.
Notice the reduction in the amount of retained austenite in the HAZ of the double pass
deposition.
Table 6.3 Heat Treatment of 431 Laser Deposited Steels
Figure 6.13 Double Pass Vs Single Pass Laser Depositions
Specimen ConditionImpact
Resistance (ft-lb)
431-SP As Deposited 9.25
431-DP As Deposited 7.5
431-SP Heat Treated
12h @ 200Co 8.5
431-DP Heat Treated
12h @ 200Co8.5
0
0.1
0.2
0.3
0.4
0.5
0.6
0 5 10 15
Double Laser ScanSingle Laser Scan
Rat
io o
f As-
Que
nche
d M
arte
nsite
/T
empe
red
Mar
tens
ite
Ratio of Crequ
/ Niequ
Chapter 6 Applications of Characterization
Chapter 6. 2 Characterizing Material Composition to Induce Martensite Transforma-tion 127
Figure 6.14 410 Stainless with Single Pass and Double Pass Configurations
200 µm
2 mm
200 µm
2 mm
Single Pass Double Pass
Chapter 6 Applications of Characterization
Chapter 6. 2 Characterizing Material Composition to Induce Martensite Transforma-tion 128
6.1 Pan, H. H. and Weng, G. J., “Thermal Stress and Volume Change During A Cool-ing Process Involving Phase Transformation,” Journal of Thermal Stresses, 15:1-23, 1992, pp. 1-23.
6.2 Gill, T. P. S., Shankar, V., Pujar, M. G., and Rodriguez, P., “Effect of Composition on the Formation of d-Ferrite to s In Type 316 Stainless Steel Weld Metals,” Scripta Metallurgica Materialia, Vol. 32, No. 10, pp 1595-1600.
6.3 L. Samuels, Metallographic Polishing By Mechanical Methods, 3rd Ed.1982.6.4 Fourney, “Surface Engineering of Tool and Die Steel,” ASM Specialty Handbook:
Tool Materials, (1995), pp.391-392.6.5 W. K. Wang, C.J. Lobb, and F. Spaepen, “Formation of Metastable Nb-Si Phases
by Picosecond and Nanosecond Pulsed Laser Quenching,” Materials Science and Engineering, 98, (1988) 325-328.
6.6 L.J. Yang, S. Jana, S. C. Tam, and L. E. N. Lim, “The Effects of Process Variables on the Case Depth of Laser Transformation Hardened AISI 01 Tool Steel Spec-imens
6.7 F. Fouquet and E. Szmatula, “Laser Surface Melting of a Pearlitic Grey Cast Iron,” Material Science and Engineering, 98 (1988) 305-308.
6.8 C. Rieker, D. G. Morris and M. A. Morris, “Microcrystalline surface layers created by laser alloying,” Journal of Less-Common Metals, 145 (1988) 595-600.
6.9 Crucible Steel (1949), Rickett, Waltin, Butler (1952), Wang, Lobb, & Spaepen, (1988)
6.10 R. Merz, F.B. Prinz, K. Ramaswani, M. Terk, and L.E. Weiss, “Shape Deposition Manufacturing,” Proc. of Solid Freeform Fabrication Symposium, The Univer-sity of Texas at Austin, (1994) pp.1-8.
6.11 J. R. Fessler, Merz, A.H, Nickel and F. B. Prinz, Proceedings of Solid Freeform Fabrication Symposium, The University of Texas at Austin, (1996) pp.117.
6.12 E. Beraha and B. Shpigler, Color Metallography, (1977).6.13 B. L. Averbach, L. S. Castleman, and M. Cohen, “Measurement of Retained Aus-
tenite in Carbon Steels, ”Transactions of the ASM, (1949) Vol. 42, pp112-120.6.14 N. Williams and C. Carter, Transmission Electron Microscopy - Diffraction, Vol
2, (1996), pp.267-288.6.15 D. Olson, T. Siewert, S. Liu and G. Edwards, “Selection of Wrought Martensitic
Stainless Steels,” ASM Handbook: Welds, Brazing, and Soldering, Vol. 6, (1993), pp.432-442.
6.16 G. R. Link, J. Fessler, A. Nickel and F. Prinz, “Rapid Tooling Die Case Inserts Using Shape Deposition Manufacturing,” Material and Manufacturing, Vol 13, No, 2, (1998), pp. 263-274.
6.17 H.H. Pan and G. J. Weng, “Thermal Stress and Volume Change During A Cooling Process Involving Phase Transformation,” Journal of Thermal Stress, Vol. 15, (1992), pp. 1-23.
Chapter 7 Designing For Microstructual Manipulation
Chapter 7. 1 Aggregate and Localized Microstructual Influence 129
Chapter 7 Designing For Microstructual Manipulation
This chapter will focus upon designing with microstructure in mind. This
approach builds upon the ability of layered manufacturing to influence microstructure
on a localized as well as aggregate basis. Several design rules are introduced to aid
designers working with carbon steel deposition to produce viable parts .
Chapter 7. 1 Aggregate and Localized Microstructual Influence
Influencing or altering the microstructure of a formed object to achieve certain
materials properties is a common aspect of producing viable parts. Heat treatment is
often used to homogenize the microstructure of newly formed sand castings or
machined metal prototypes to attain uniform hardness and reduce residual stress. The
surface microstructure of metal parts like gears may be deformed by peening to
impose compressive stress to increase wear resistance. Die surfaces may be
impregnated with alloying agents to improve die life. Laser hardening has been used
on camshafts to improve wear resistance.
Layered manufacturing techniques of solid freeform fabrication have the
ability to influence or control the microstructure of the prototype. Like heat treatment
and casting processes, laser deposition can affect the microstructure of the part.
However, unlike these bulk processes, laser deposition techniques like surface
cladding can have a more localized effect (Figure 7.1). Yang et al. [6.6] showed that a
CO2 laser hardening process can produce a microstructually altered region near a
part’s surface 350 µm thick.
Chapter 7 Designing For Microstructual Manipulation
Chapter 7. 1 Aggregate and Localized Microstructual Influence 130
Figure 7.1 Surface effects of processing techniques.
The ability of laser processing to affect both aggregate and localized
microstructure as well as the ability of layered manufacturing to affect both aggregate
and localized geometry lends itself naturally to begin to think about designing with
microstructural constraints or enhancments in mind. Often the designer is limited in
her ability to improve her design because they are only focusing upon the feature level
of design. They may focus upon the need for a part’s surface to be a bearing surface or
that two mating parts will rub, and not the underlying microstructure.
If one could begin to think about the design on a microstructural level one may
find opportunities to improve the design. For example , a designer is building tensile
bars. They need to be stiff but still have good elongation properties. With
conventional processes, his choices are limited. With layered laser technique like
SDM, one could improve elongation by placing austenitic grains within strategic
locations and martensitic in areas requiring stiffness (Figure 7.2). Considering
microstrure, or Designing for Microstructual Manipulation (DFM2) adds an additional
element of freedom to design.
Four thermal fatigue specimens were built without possessing a complete
0 500 1000 1500 2000 2500 3000
Fine Al2O3 Polish
Abrasive Paper
Surface Grinding
Turning Lathe
EDM
Laser Cladding
Laser Welding
Laser Surface Alloying
Cutoff Wheel
Hacksaw
Heat Treatment
Casting
Laser Deposition
Steel Brass
Micrometers of Surface Influence from Deformation or Alloying Thickness
(.1 µm)
Chapter 7 Designing For Microstructual Manipulation
Chapter 7. 2 Applying DFM2 to Internal Geometry of Die Inserts 131
knowledge of the microstructure (Figure 7.3). Each one failed because the deposition
strategy was not superior to the effects of the microstructure. However knowledge
gained by these experiments, have increased the knowledge of carbon steel deposition
100 fold.
Figure 7.2 Freeing Designer with Microstructure
Figure 7.3 Four Thermal Fatigue Specimens
Chapter 7. 2 Applying DFM 2 to Internal Geometry of Die Inserts
Copper cooling bars can be deposited in die inserts to improve the thermal
conductivity of the dies. With improved conductivity, faster injection cycles can be
attained without compromising casting integrity or microstructure. Placing sharp
corners in the interior of a part builds in stress concentrations which can induce
solidification cracking (Figure 7.4). If the corners could be avoided entirely the
Constrained Designers
Part Needs Elongation needs elongation
•Choose Austenitic Material
Unconstrained Designer
Interior Section needs ElongationExterior needs StiffnessChoose Graded Material Deposition:•Martensitic Exterior•Austenitic Interior
more than stiffness.
SDMT Steel“Cracky”
SDMT Steel“Splity”
410 Steel“Brittle”
H13/410 Composite“Holey”
Chapter 7 Designing For Microstructual Manipulation
Chapter 7. 2 Applying DFM2 to Internal Geometry of Die Inserts 132
Figure 7.4 Sharp Internal Corner of 316L and Copper
situation would improve, but the difference in thermal conductivity can still produce
very sharp thermal gradient differences which can also produce stress concentrations
(Figure 7.4). However, inducing delta ferrite near the sharp or thermally divergent
interface may be a better way to avoid solidification cracking. Figure 7.5 shows a
corner which was removed by the laser only to form to additional corners.
Figure 7.5 Using laser to eliminate sharp corners is not always successful.
A better way to eliminate solidification cracking is to introduce delta ferrite
near the stress concentration. From the carbon steel characterization, we know that
420 forms delta ferrite more easily than any of the other metals tested. The 420
material was applied after an initial powderless pass to warm the copper and insure
good bonding. Figure 7.6 shows the etched surface of the interface. Delta ferrite
forms near the interface. No cracks were observed.
500 µm
316L
316L
Copper
Solidification cracknear sharp CopperCorner.
100 µm
Original Corner
Additional Corner Created
Chapter 7 Designing For Microstructual Manipulation
Chapter 7. 3 Applying DFM2 to Part Building Strategy 133
Figure 7.6 (A.) Corner Solidification crack in SS-CU. (B.) Corner Prep with 420 Stainless
Chapter 7. 3 Applying DFM 2 to Part Building Strategy
Chapter 7. 3.1 Part Substrate InterfaceGood bonding between the parts substrate and first layers of the part are
essential to prevent delamination. As the earlier characterization shows, the first layer
is the thinnest. This thin layer is subject to both residual stress in the substrate as well
as stress within the deposit. Plasma spray coatings are applied with pressure and heat
to coat a surface. However most of the bonding is purely mechanical than
metallurgical. Mechanical bondings are easier to rupture than metallurgical bonding.
Figure 7.7 shows the similarities between a plasma coating and a laser layered deposit
of H13 on low carbon cold roll steel.
Figure 7.7 A. Plasma Spray Coating, B. Single Pass interface of H13 and low carbon steel
50 µm50 µm
316L
Cu Cu
420
316L
δ
δ
Α. Β.100 µm
100 µmCu
Stainless Steel
100 µm
Good Bonding Poor Bonding
A. B.
H13
Cold Roll Steel
Chapter 7 Designing For Microstructual Manipulation
Chapter 7. 3 Applying DFM2 to Part Building Strategy 134
Figure 7.8 A prepass before depositing improves substrate and deposit bonding.
One way to get a better bond is by doing a prepass of the laser before
deposition. A prepass is a powderless laser scan of the substrate immediately before
depositing. The prepass warms the substrate and roughens the surface to further aid in
bonding the first layer of deposition. The bonding region doubles in size making a
much stronger bond.
This is not the same as doing a double laser pass (a powder scan followed by a
second powderless laser scan). The interfacial bonding region of a single pass vs a
double pass is about the same, on the order of 100 µm.
Chapter 7. 3.2 Part Layering StrategyBecause carbon steels will continue to transform as multiple layers of laser
deposition are attempted no more than 25 mm of layers of continuous deposition
should be attempted. If one is trying to design a die insert which is 50 mm in
thickness, try to incorporate the substrate within the design. Many die insert designs
can be split into a feature level and a bulk part. The feature level includes the casting
surface and features beneath the part surface like conformal cooling channels or other
internal features. The bulk part is simply the rest of the tool which forms the base.
The bulk part should be machined from a similar metal and used as a big substrate for
the rest of the deposited features. Special care should be used to insure proper
bonding occurs.
100 µm
Good Bonding
H13
Cold Roll Steel
Chapter 7 Designing For Microstructual Manipulation
Chapter 7. 3 Applying DFM2 to Part Building Strategy 135
Figure 7.9 Conceptual Splitting of a Die Insert
If this two part build system cannot be done, anneal the part and substrate every
25 mm of deposition. The only way to insure that unwanted phases do not occur
within lower parts of the deposition, or that unwanted residual stress arising phase
transformations or mismatches in coefficients of thermal expansion, is to anneal the
entire deposit. However, annealing is another form of heat treatment. Special care
must be taken to characterize the appropriate annealing conditions and procedures.
Certain temperatures may cause chromium carbides to form or redistribute upon grain
boundaries which can lead to integrannular corrosion during etching procedures or
part service. Cooling procedures from the annealing temperatures must also be well
characterized. If one does not understand the affects of certain heat treatments upon
the particular carbon steel, avoid depositing over 25 mm. In this authors opinion, this
caution can be applied not only for SDM but, for any high energy - rapid solidification
procedure attempting layered manufacturing of carbon steels.
Lastly, the number of layers which are deposited at any given time should be
limited to no more than 7-12 layers. The divergence of thicknesses within the
deposition can weaken the strength of the part. Uniform grain sizes and layer
thicknesses allow parts to have more homogenous properties and attain higher
strengths. Continuous deposition beyond these limits cause severe inhomogenity in
grain size and layer thicknesses. For example, charpy impact specimens were taken
from different positions within a large deposit of SDM Tool Steels (Figure 7.10 and
Table 7.1 ). Charpy 1 samples a have a slightly higher value but a very high deviation.
Tooling Insert Bulk Part Feature Level
= +
Chapter 7 Designing For Microstructual Manipulation
Chapter 7. 3 Applying DFM2 to Part Building Strategy 136
Figure 7.10 Charpy impact specimens taken from large deposit
Table 7.1 Charpy Impact Resistance for variable layer sizes (Sample 1 Area) and consistent layer sizes (Sample 2 Area).
Charpy samples taken from region 2 have much more uniform properties. The slightly
lower measurements are probably because they were taken from a lower position
within the laser deposition and have been tempered inappropriately by subsequent
deposition layers. (The big deposit violates the 25 mm rule.)
Chapter 7. 3.3 Deposition Parameters
For the 2400 Watt ND:YAG laser which delivers an incident power of about
400 Watt, the speed as was shown earlier is not a significant factor. With almost all
Impact Resistance (ft-lb)
Average Charpy(ft-lb)
Standard Deviation
(ft-lb)
Charpy 1-a 9.5 7.83 1.53
Charpy 1-b 6.5
Charpy 1-c 7.5
Charpy 2-a 7.5 7.5 0.00
Charpy 2-b 7.5
10.80.6
0.4
0.2
02
4
6
8
2
4
0 5 10 15 20 25 30
A
B
B
B A
A
Z-Height (mm)
Laye
r T
hick
ness
(m
m)
A = Overnight Cooling
(12-24hr breaks)B = Short Cooling Periods
(1-2hr breaks)
Charpy 2Charpy 1
20 25 30 35 40 45 50
Chapter 7 Designing For Microstructual Manipulation
Chapter 7. 3 Applying DFM2 to Part Building Strategy 137
speeds below 40 mm/sec, SDM is still delivering enough power to insure solidification
of the deposit. However, the fill rate is critical. Figure 7.11 shows a graph which
compiles at speeds from 15 to 50 mm/sec the effects of changing fill rate. Only one fill
rate reduces the amount of 2nd phase constituents at all speeds.
Figure 7.11 For Laser Deposition, Fill Rate is a Critical Parameter
Therefore, fill rate can be used as a way of reducing or increasing the amount
or retained or transformed secondary phases. Depending upon the nature of parts
design and ultimate use, fill rate can be an important way of achieving different
material characteristics. However, unless one is sure of the affects of secondary
phases, one should find a setting which minimizes the effects. For this research that
setting was 23 g/ min.
% R
etai
ned
Aus
teni
te
0
5
10
15
20
25
30
35
10 15 20 25 30 35Fill Rate (g/min)
Layers Too Thick-Maximum Retention of 2nd Phase
Layers Too Thin-Maximum Reheating-Multiple Phase Transformations
Chapter 8 Conclusions
Chapter 8. 1 Characterization of Carbon Steels 138
Chapter 8 Conclusions
Deposition of carbon steels is feasible and delivers a whole range of potential
applications and capabilities to Shape Deposition Manufacturing which cannot be
achieved by traditional deposition metals. However, the addition of carbon steels
requires a more detail understanding of the microstructure than was necessary for
other deposition materials. The added knowledge benefits SDM and creates new
unique opportunities to improve deposition and more importantly designed objects.
Chapter 8. 1 Characterization of Carbon SteelsKnowledge of how phases forms and how grains and layers evolve are essential
to develope effective strategies to deposit tools steels. Because SDM involves a high
energy deposition source which induces rapid solidification and uses layered
manufacturing techniques which induce multiple temperature gradients through out
the deposition, traditional designer’s tools are ineffective. The Shaeffler Welding and
Solidification Diagrams have been used by low energy welding sources to predict
phase evolution. However, for SDM they were highly inaccurate. Proposed in this
research are two new modified versions of the Shaeffler and Solidification Diagrams
which can be used by designers to more accurately select metals for layered high
energy deposition.
A heuristic for predicting grain size and layer thickness have been proposed for
SDM. The heuristic identifies the specific variables in composition, and other more
process oriented variables like, the fill rate, solidification rate and powder size as
determinants for grain and layer size. Although the heuristics are directly applicable to
SDM, the parameters may lend incite in to other high energy processes. Also, the
Chapter 8 Conclusions
Chapter 8. 2 Application of Characterization 139
understanding of how layer evolve within SDM may also aid others in understanding
there own processes.
Chapter 8. 2 Application of CharacterizationTwo applications of using characterization to improve deposition have been
highlighted by this research. Choosing high impact resistant materials for building die
casting inserts is dependent upon understanding the microstructure in addition to bulk
material properties. Because high energy deposition processes produce material
which are very different than traditional materials, they will display unusual
properties. The small grain sizes of the deposited materials lend to these unique
properties. However, knowledge of phase evolution is important. Certain phases in
addition to the grain size can improve properties while other may reduce properties.
Delta ferrite can help prevent solidification cracking in some metals while serving as a
crack initiation point in others. Martensite can add toughness and reduce deflection,
but can also reduce elongation and yield strength. Therefore, traditional material
property knowledge must be supplemented by knowledge of the microstructure when
working with nontraditional part forming methods.
Chapter 8. 3 Designing with Microstructure
Now that an understanding of how the microstructure of laser deposited carbon
steels develops within layered manufacturing techniques has been extracted, and how
this knowledge can be used to improve deposition, this knowledge can be further
extended to aid designers. Designers can use this knowledge and improve designs
intended for layered manufacturing. Improved part strength, wear resistance, and
resistance to deformation can be accomplished if one allows the microstructure to
guide design decisions. Sooner or later, the microstructure will win, and lesser designs
which do take microstructual concerns into account will fail. Design rules which
accomplish this have been proposed in this research.
Chapter 8. 4 Leverage of This KnowledgeAs indicated by Chapter 2, there are several other high energy deposition
processes, which are also attempting build structures in a layered fashion. They two
can benefit from understanding the nature of the microstructure. Metals prototyping
has lagged behind its plastic counterparts even though they share many similar
problems linked to thermal gradients and shrinkage. Yet, plastics rapid prototyping
Chapter 8 Conclusions
Chapter 8. 4 Leverage of This Knowledge 140
has been quite successful in changing the design process for many designers world-
wide. Leveraging the nature of metal microstructure may be the key to advancing this
technology to every designer’s door.
Appendix
Chapter 8. 4 Leverage of This Knowledge 141
Appendix
Appendix
Chapter 8. 4 Leverage of This Knowledge 142
Chapter A. 1 Selecting a Laser Scanning Speed
When trying to determine a laser scanning speed knowing the thermal
diffusion length is a potential starting point. Thermal diffusion length measures the
penetration or diffusion of heat of a laser while it is incident upon a particular material.
To determine the interaction time, or the time a moving laser is incident upon a spot,
an approximation is provided by dividing the beam with by the velocity of the beam.
An approximation for the interaction time, tp, is spot size divided by scanning speed.
The thermal diffusivity, αth, should be known for each metal. For 2000 W absorbed
into substrate or metal at a 2.8 mm spot size, the calculated thermal diffusion length
values are listed in Table :
Table A.1 Thermal Diffusion Length for Several Carbon Steels
Select an initial speed which is approximately thickness of layer which you are
attempting to deposit. This may not be the most optimized speed but it is good starting
point for optimization and characterization.
20 mm/sec 30 mm/sec
316L 1.01 mm .824 mm
SDMT 1.31 1.07
410 1.33 1.08
420 1.28 1.05
431 1.19 .97
H13 1.31 1.07
LT 2αth
tp
( )=
Appendix
Chapter 8. 4 Leverage of This Knowledge 143
Chapter A. 2 Martensitic Start Temperature
The martensitic start temperature can be calculated with knowledge of the
constituents of the materials.
Ms=521-350 C-13.6Cr -16.6 Ni -25.1 Mn -30.1Si -20.4 Mo-1.07CR * Ni +21.9(CR+.73Mo)C This formula was developed by Self and Carpenter, “Phase Transformations and Alloy Stability,” (1986).
Table A.2 Martensitic Start Temperature for Materials Used in this Research
Material Ms in Co
410 324
420 284
431 214
H13 306
SDMT 208
M2 240
316L -298
Appendix
Chapter 8. 4 Leverage of This Knowledge 144
Chapter A. 3 Theoretical Volumetric Expansion
Theoretical Percentage of Volume Increase based upon 100% martensitic
transformation.
Lyman, Metals Handbook, 8th Ed.
C%
Aus
teni
te to
Mar
tens
iteVo
lum
e P
erce
ntag
e
Dim
ensi
onal
mm
per
mm
Aus
teni
te to
Fer
rite
and
cem
entit
e
Dim
ensi
onal
mm
per
mm
Sph
erod
ite -
Tem
pere
d m
arte
nsite
to e
utec
toid
- F
eC3
glob
ules
in
410 0.06 4.6082 0.0154 4.507 0.0144 -4.5074
420 0.45 4.4015 0.0147 3.646 0.0142 -3.6455
431 0.18 4.5446 0.0152 4.242 0.0155 -4.2422
SDMT 0.15 4.5605 0.0152 4.309 0.0155 -4.3085
H13 0.4 4.428 0.0148 3.756 0.0153 -3.756
1010-low carbon
0.1 4.587 0.0153 4.419 0.0155 -4.419
Appendix
Chapter 8. 4 Leverage of This Knowledge 145
Chapter A. 4 Grain Size Heuristic
For the chosen data basis the following best model was generated:
X26= + 9.84e-2z61 + 2.30e+0z62 + 5.82e+0
z61= + 1.00e+0z51
z51= + 1.00e+0z41
z41= + 1.00e+0z31
z31= + 1.00e+0z21
z21= + 1.00e+0z11
z11= + 2.89e+0X14 - 3.54e-1
z62= - 2.40e-1z51 + 1.18e+0z52
z51= + 1.00e+0z41
z41= + 1.00e+0z31
z31= + 1.00e+0z21
z21= + 1.00e+0z11
z11= - 5.14e+0X1 + 1.49e-3X19 - 6.23e-1
z52= - 2.78e-1z41 + 1.26e+0z42
z41= + 1.00e+0z31
z31= + 1.00e+0z21
z21= - 2.24e-1z11 + 8.88e-1z12
z11= + 5.42e+0X1 - 1.30e+0
z12= + 2.91e-1X7 - 4.79e-2X24 + 4.63e-1
z42= - 1.79e-1z31 + 1.09e+0z32
z31= + 1.00e+0z21
z21= + 1.00e+0z11
z11= + 1.94e-1X7 - 2.21e+0
z32= - 1.84e-1z21 + 1.12e+0z22
z21= + 1.00e+0z11
z11= + 1.12e+0X9 - 5.01e-1
z22= + 7.91e-1z11 + 4.42e-1z12
z11= + 2.91e-1X7 - 4.79e-2X24 + 4.63e-1
z12= - 1.52e-1X16 - 2.64e-1X20 + 7.66e+0
Appendix
Chapter 8. 4 Leverage of This Knowledge 146
Mean Absolute Percentage Error (MAPE): 3.07 %
Approximation Error Variance: 0.01669
OUTPUT VARIABLE:
X26 - avg cell
RELEVANT INPUT VARIABLES:
X14 - V
X1 - C
X19 - Melting
X7 - Cr
X24 - powder
X9 - Mo
X16 - NI-EQ
X20 - K
CHOSEN HEURISTICS:
Data Length: 9
Number of Input Variables: 20
Max. Lagged Time: 0
Model Type: input-output-model / exclusively linear / static
The embraced linear model is:
X26 = 22.838715 + 0.284510X14 + 3.738204X1 -0.000820X19 + 0.642496X7 -
0.125312X24 -0.769774X9 -0.278010X16 -0.483130X20
Appendix
Chapter 8. 4 Leverage of This Knowledge 147
Chapter A. 5 Layer Thickness
For the chosen data basis the following best model was generated:
X25= + 2.50e+1z41 + 2.10e+2z42 + 7.45e+2
z41= + 1.00e+0z31
z31= + 1.00e+0z21
z21= + 1.00e+0z11
z11= + 9.19e-1X14 - 1.12e-1
z42= - 1.59e-1z31 + 9.96e-1z32
z31= + 1.00e+0z21
z21= + 1.00e+0z11
z11= + 1.84e+0X2 - 1.25e+0
z32= + 2.14e-1z21 + 1.02e+0z22
z21= + 1.00e+0z11
z11= + 1.99e-1X7 - 2.18e+0
z22= - 4.51e-1z11 + 1.37e+0z12
z11= + 1.46e-5X27 - 3.68e+0
z12= - 1.01e-1X16 - 2.68e-2X24 + 3.29e+0
Mean Absolute Percentage Error: 9.40 %
Approximation Error Variance: 0.1993
OUTPUT VARIABLE:
X25 - avg layer
RELEVANT INPUT VARIABLES:
X14 - V
X2 - Mn
X7 - Cr
X27 - solid rate
X16 - NI-EQ
X24 - powder
Appendix
Chapter 8. 4 Leverage of This Knowledge 148
CHOSEN HEURISTICS:
Data Length: 9
Number of Input Variables: 22
Max. Lagged Time: 0
Model Type: input-output-model / exclusively linear / static
The embraced linear model is:
X25 = 2003.830933 + 22.995251X14 -61.247478X2 + 8.895004X7 -0.001405X27
-29.596924X16 -7.855000X24
Appendix
Chapter 8. 4 Leverage of This Knowledge 149
Chapter A. 6 New Chromium Equivalence Definition
For the chosen data basis the following best model was generated:
X17= + 3.11e-1z21 + 6.19e+0z22 + 1.72e+1 z21= + 1.00e+0z11 z11= + 1.86e+0X14 - 3.61e-1 z22= + 2.76e-1z11 + 9.99e-1z12 z11= + 3.15e+0X4 - 1.36e+0 z12= + 3.32e-1X5 + 1.31e-1X15 - 2.29e+0
Mean Absolute Percentage Error: 6.57 %Approximation Error Variance: 0.0512
OUTPUT VARIABLE: X17 - new Cr
RELEVANT INPUT VARIABLES: X14 - VX4 - SiX5 - WX15 - Cr-Eq
CHOSEN HEURISTICS: Data Length: 16Number of Input Variables: 15Max. Lagged Time: 0 Model Type: input-output-model / exclusively linear / static
The embraced linear model is:
X17 = 0.666030 + 0.578787X14 + 5.386530X4 + 2.048034X5 + 0.811128X15
Appendix
Chapter 8. 4 Leverage of This Knowledge 150
Chapter A. 7 New Nickel Equivalence Definition
For the chosen data basis the following best model was generated:
X18= - 5.35e-1z21 + 4.52e+0z22 + 1.14e+1 z21= + 1.00e+0z11 z11= + 6.89e-1X5 - 2.58e-1 z22= + 3.37e-1z11 + 7.53e-1z12 z11= + 1.05e+0X9 - 5.97e-1 z12= + 1.70e-1X8 - 7.72e-2X15 + 3.93e-1
Mean Absolute Percentage Error: 19.96 %Approximation Error Variance: 0.3663
OUTPUT VARIABLE: X18 - new NI
RELEVANT INPUT VARIABLES: X5 - WX9 - MoX8 - NiX15 - Cr-Eq
CHOSEN HEURISTICS: Data Length: 16Number of Input Variables: 17Max. Lagged Time: 0 Model Type: input-output-model / exclusively linear / static
The embraced linear model is:
X18 = 11.977810 -0.368111X5 + 1.598563X9 + 0.579666X8 -0.262729X15