Hybrid layer

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Robotics and Computer-Integrated Manufacturing 22 (2006) 113123

Hybrid adaptive layer manufacturing: An Intelligent art of direct

metal rapid tooling process

Sreenathbabu Akula, K.P. Karunakaran

Mechanical Engineering Department, Indian Institute of Technology, Bombay, India

Received 21 June 2004; received in revised form 2 February 2005; accepted 11 February 2005

Abstract

A direct metal rapid tool making process, hybrid-layered manufacturing (HLM), was developed for building metallic dies and

molds. This unique methodology has a numerical controlled system that integrates the TransPulse Synergic Metal Inert Gas (MIG)/

Metal Active Gas (MAG) welding process for near-net layer deposition and Computer Numerical Control (CNC) milling process for

net shaping. A customized software program was made to calculate the required adaptive slice thickness for the deposition of the

filler metal with welding process as successive layers from the lowest to the topmost layer direction and to generate the required NC

codes for machining from the top to the bottom layer direction of the deposited metallic layers for attaining the required contour

profile shape. To implement this proposed process, a low-cost three-axis manipulator was fabricated with stepper motor divers in

open-loop control and integrated with the weld machine. Adequate isolation to protect the motion control electronics from welding

spike was incorporated. Synchronization of this two-step processing of each layer, yielding near-net deposition with welding process

and near-net shaping with CNC milling operation offers a new accelerator way of building metal tools and dies.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: Rapid tooling (RT); Welding; CNC machining; Slicing; Molds and dies

1. Introduction

Building pre-production models of a product to test

various aspects of the aesthetic, ergonomic, functioning

and design are known as prototypes. With the concept

of globalization, the multinational corporations in the

open market system, the competition among the

industries has become very acute. The demand for

shorter development time, and reduced product life

cycle resulted in the emergence of a new paradigm calledTime Compression Techniques (TCT) or Rapid Proto-

typing (RP) [1,2].

The main process stages involved in fabricating

prototypes are common to most RP systems that are

currently available or under development, but the

mechanisms by which the individual layers are created

obviously depend on the particular system. A new

approach known as Direct RP through which a

prototype of the parent material can be generated has

emerged [3,4]. Some use laser welding whereas a

majority of them use Metal Inert Gas (MIG) welding.

In Laser Generating and High-Speed Milling process

developed at Fraunhofer Institute for Production

Technology (FhG-IPT), Germany, the raw material

used is binder-coated metallic powder, which whenpassed through the nozzle is melted by a laser beam

resulting in the deposition of the near-net layer. The

layer is then milled to net-shape. As this process makes

use of uniform slicing of 0th-order edge approximation,

it is not rapid enough [5].

Shape Deposition Manufacturing (SDM) process

developed at Carnegie Mellon University, US, uses an

additive process to deposit the rough material and a

machining process to get the desired accuracy. However,

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0736-5845/$ - see front matterr 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.rcim.2005.02.006

Corresponding author.

E-mail addresses: [email protected] (S. Akula),

[email protected] (K.P. Karunakaran).
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instead of sintering by a laser beam, they use a

deposition process called micro-casting, which is in

between metal spraying and welding processes. It uses 5-

axis machining that enables to make the profiles [6].

3D Welding process developed at the University of

Nottingham also uses an MIG welding process to build

metallic prototypes [7]. Using a 0.8 mm diameter wire,they report a building speed of 6500 mm/min, the bead

size being 4.5 mm wide and 1.4 mm thick. Although the

building speed of this process is very attractive, the poor

accuracy of 0.5 mm limits its applications. Similar

research is going on at several universities abroad

although they are not far away from commercialization.

Some of them are Southern Methodist University, US

[8]; Cranfield University, UK, Loughborough Univer-

sity, UK; Fraunhofer Institutes in Germany; Korean

Institute of Science & Technology (KIST) [9], University

of Kentucky, Lexington, US [10], and University of

Michigan, US. Each has its unique features and they

differ from one another in various ways. Similarly, they

differ in the type of slicing used, method of support

structure, application areas, etc. At the Indian Institute

of Technology, Bombay, it was proposed to develop a

hybrid-layered manufacturing (HLM) process with a

numerical controlled system that integrates the Pulse

synergic MIG/Metal Active Gas (MAG) process and

Computer Numerical Control (CNC) milling process to

build tools and dies [11].

2. Proposed process

2.1. Hybrid-layered manufacturing

In order to manufacture tools more accurately and

rapidly, a direct rapid tooling process should have the

following characteristics:

sintering or melting of the hard material directly;

two-step processing of each layer, the first step yields

the near-net layer deposition/formation and the

second step machines the layer to the required

accuracy;

efficient Slicing technique;

elimination/minimization of staircase effect;

high rate of material deposition;

ability to build support structures.

Considering these essential features for the direct

rapid tooling process, it was proposed to develop a

unique methodology for building metallic dies and

molds by employing a numerical controlled system that

integrates the TransPulse Synergic MIG/MAG welding

process for near-net layer deposition and CNC milling

process for net shaping. The TransPulse Synergic MIG/

MAG provides the controlled heat and mass transfer

with precise depth of bead penetration and the CNC

machining enhances both the surface quality and

dimensional accuracy with great manufacturing

agility.

To implement this process, a programmable logic

controller (PLC)-based low-cost three-axis manipulator

was fabricated with stepper motor diver in open-loopcontrol. The tool head of the manipulator will hold the

welding torch and milling cutter. At any time either

milling or welding will take place, and for that the

welding gun can be moved up and down with a

pneumatic operated piston. The NC codes M08 and

M09 are used to invoke the switching functions of the

pneumatic piston to move up and down, respectively.

Further, the NC codes M03 and M04 are made to

control the on/off of the welding torch during the metal

deposition. The parameters related to welding processes

such as the speed of the welding filler wire, diameter of

filler wire, type of filler material, voltage, current, gap

between the electrodes, shielding gas, built style, etc. are

to be fine-tuned and frozen after performing

the experiments. While a majority of the welding

parameters will be controlled externally, the necessary

functions to integrate the welding process with the

machine motion will be carried out by user defined G

and M codes.

The framework of this research also consists of a

customized software program that uses the zeroth-order

edge approximation uniform [12] and adaptive slicing

strategy [13] to calculate each slice thickness to be

deposited with the required metal as successive layers

from the lowest to the topmost layer with the weldingprocess. Further, it generates the required CNC code for

machining from the top to the bottom layer direction of

the deposited metallic layers to attain the required

contour profile shape with user-specified accuracy.

The process does not pose any restriction or loss of

accuracy on the prototype as its size grows. Since the

size of the part is limited only by the traverse available

on the CNC machine, a larger CNC machine can be

used to produce large tools. In this context, it is

interesting to note that the die halves used in injection

molding, pressure die casting, sheet metal forming, etc.

will be free of reentrant features overhanging features

since they need to open and close in operation.

Therefore, building such dies in the proposed process

will not require support structures. Furthermore, since

the fatigue loading they suffer in operation is consider-

ably less, these tools will serve the purpose even without

any homogenization operation such as Hot Isostatic

Pressing (HIP) process [14]. The tools produced using

this process may be inferior to their conventional

counterparts in composition and tool life but these will

generate the final products as accurately as any other

tool. This HLM process can be retrofitted to any CNC

machining center [15] (Fig. 1).

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3. Methodology

Preliminary work in the area of 3D welding has

shown that complex shapes can be formed, but the

results are not perfect. The shape and dimensions

of the weld bead are very important in the use of 3D

welding as an RP system, since these will determine the

limits to the wall thickness, which may be produced

and will also influence the quality of the surface

finish. Although MIG/MAG welding cannot produce

the required accuracy, it is economical, safe, portable

and easy to maintain. Since only near-net layer is

being deposited in a hybrid process, low accuracy is

acceptable [16].

The developed HLM process will have the following

three stages:

i. building the near-net shape of the tool;

ii. heat treatment for stress relieving and strengthening;

iii. machining the near-net shape of the tool to final

dimensions.

3.1. Building the near-net shape of the tool

The metal deposition is done using a pulse Synergic

MIG/ MAG welding machine. The steps of the

deposition process are as follows:

Step one: Generate the tool path required to build

uniform/adaptive layers of zeroth-order edge approx-

imation from the bottom to the top:

The tool path consists of the paths for the welding gun

and the face mill and the required switching functions

M08 and M09. The clad zone for each layer will be

larger than the bottom contours of the layer by a

machining allowance. This allowance is from 0.5 to

2.0 mm. The switching functions are required for

change over between welding and face milling,

activating the welding operation and change over in

tool offset. The zeroth-order edge uniform slices of the

die of a connecting rod (Fig. 2) are shown in Fig. 3.

Step two: Fix a substrate on the table:

This substrate will conform to the mountings on the

press or injection molding machine on which it will be

used. It is recommended to have as thick a substrate as

possible. This will reduce distortion as well as building

time. This is possible because the dies invariably have a

thick bottom, which can become part of the substrate.

Step three: Select the necessary parameters on the

welding system:

The weld parameters of attained well-defined

bead geometry and layer thickness with adequate

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Fig. 1. Developed HLM machine.

Fig. 3. Zeroth-order edge uniform slicing.

Fig. 2. Die of a connecting rod.

S. Akula, K.P. Karunakaran / Robotics and Computer-Integrated Manufacturing 22 (2006) 113123 115


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machining allowance are selected. The weld machine

is operated in pulsed synergic mode.

Step four: Deposit the bottom most layer on the

substrate:

The welding path in any section or layer is of two

types: one is area-filling path (Fig. 4a) and the other is

contouring path (Fig. 4b). First, the deposition takesplace using area-filling paths and this is followed by

contouring. The deposited geometry fully covers the

required layer. However, it will be required to

optimize the path so as to transfer heat uniformly

over the layer. This is essential in view of the large

amount of heat input. Furthermore, arrangement for

preheating the substrate may also be required for

desirable patterns of grain size distribution.

After completion of the metal deposition at the

bottom most layer, the switching functions are

invoked to change over between welding and face

milling process by halting the welding process and

activating the face milling process and vice versa. For

this operation, a pneumatic system is used to swivel

between the welding gun and the milling cutter.

Step five: Face mill the top surface of the layer to

attain the required layer thickness:

The instability of the arc welding process may cause a

malfunction/defect in the middle of the weld bead. To

minimize and correct the deviation in successive

multiple layer deposition, face milling operation is

performed. This process step ensures the vertical Z

accuracy of building metal layer. Furthermore, the

welded surface may have an oxidized layer that

influences the subsequent layer deposited on top of it.

When milling is done, welding happens on a nascent

surface giving good quality of welding. Therefore, it is

required to do face milling after every layer deposition

though techno-economically not feasible for each

deposited layer (Fig. 5).

Repeat the above two steps for the remaining layers

till a casting like rough shape is obtained.

At the end, we will attain the near-net shape of the

required tool on the substrate.

3.2. Heat treatment for stress relieving and strengthening

Depending on the pattern of heat input, the grain

structure may be non-uniform and there could be

considerable amount of internal stresses. To relieve

them, suitable heat treatment is performed by annealing

or normalizing. HIP can densify the component and

improve its mechanical properties and fatigue life.

3.3. Machining the near-net shape of the tool to final

dimensions

All the horizontal surfaces of the tool are finished by

the face mill during the deposition stage and the edges of

the layers are still rough. These edges are machined in

this stage. This is done with the help of an end mill. The

tool paths consist of the paths for milling the edges of

each layer using a ball end mill in scan milling mode

using a maximum of three axes. The type and diameter

of the cutter will be automatically selected for different

regions by analyzing the local geometry [17]. The

approach involves splitting the machining surface into

three groups for the purpose of generating the cutter

path for their automatic machining: (i) a set of

horizontal surfaces, (ii) a set of vertical surfaces and

(iii) the set of remaining surfaces. The first set of surfaces

has already been machined during face milling; the

surfaces may, however, require touchup if spatter falls

on them. The other two sets of surfaces will be machined

using a flat or bull or ball end mill depending on whether

they are connected to the neighboring surface patches

sharply or through fillets [18,19].

ARTICLE IN PRESS

Fig. 4. Area-filling styles: (a) direction-parallel (also known as zigzag),

(b) contour-parallel (also known as spiral).

Fig. 5. Face milling on weld deposition.



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Finally, after completion of all these stages, the

desired metallic tool is attained.

4. HLM software

The HLM process consists of a custom-made soft-ware program that uses the zeroth-order edge approx-

imation slicing strategy of the RP paradigm to calculate

each slice thickness to be deposited with the required

metal as successive layers from the lowest to the topmost

layer with the welding process and it also generates the

required CNC code for machining from the top to the

bottom layer direction of the deposited metallic layers to

attain the required contour profile shape with user-

specified accuracy. Synchronization of the welding

process with work-piece/substrate motion and CNC

milling operation offers a new accelerator way of

building metal parts and tools.

Three types of NC programs are required in HLM:

i. paths for the layered weld deposition;

ii. paths for face milling each layer;

iii. paths for finish milling.

The first step for generating the welding path is to

slice the stereolithography (STL) file of the Computer

Aided Design (CAD) model into uniform layers of

zeroth-order edge approximation [20,21]. This results in

a set of loops defining each layer. During the near-net

layer generation, more material deposition has to

account for the machining allowance. Therefore, theloops defining each layer must be offset by the

machining allowance [22]. Having offset the loops of

the layer, the welding torch has to move along the

contours of the loops as well as their interior as per the

area-filling program. The area filling could be direction-

parallel or contour-parallel as described earlier. The

surface obtained after weld deposition will be scalloped

and have spatter deposition. In order to maintain Z

accuracy as well as to provide a flat nascent surface for

the next layer deposition, face milling is done. This is

relatively simple and is same for all layers but for the Z

coordinates. Having thus obtained the near-net layer, it

has to go for heat treatment and then finish machining.

For this purpose, a program for the layered machining

of the CAD surfaces has been developed. This finishing

is done within the required scallop tolerance and it can

make use of ball, bull and flat end mills.

The customized software, termed as Hybrid Layered

Manufacturing Software (HLMSoft), was developed to

run under Microsoft Windows platform. The code was

based on using Microsoft Visual C++ language. Micro-

soft Foundation Classes (MFC) were used to develop the

menus, dialog boxes and icons and the graphic outputs

were rendered using OpenGL graphics library. The

output files generated from the software, i.e. the weld

deposition path, face mill cutter path and the coarse

slice-machining paths, are of standard NC format as

shown in Fig. 6. These output files are compatible with

the protocol of the controller card SC03. The controller

card passes on these signals to the Control Box that

amplifies the power of these signals and feeds them to

the drive system.

4.1. Input format

The CAD model in an STL format, which is the de

facto standard for most of the RP processes, acts as the

input format for the software. The STL file consists of

unordered triangular facets, representing the surface of

an object. The tessellated facets are described by a set of

x, y and z coordinates for each of the three vertices andan outward pointing unit normal vector (Fig. 7).

4.2. Sectioning of the STL body with a plane

A verified and correct STL file of the CAD model in

binary format acts as the input file. The first step in this

process includes sectioning the tessellated body at

different vertical heights (z_levels) in relation to the

layer thickness to attain the information about the

number of contour loops, orientation of each loop and

the vertices of each loop [23].

4.3. Coarse slicing

Coarse slicing of the CAD model is the first step for

processing the body. Coarse slicing comprises of

decomposing the complex object into slices of simpler

geometry and then passing them for finer slicing. In a

coarse slicing, the number of loops in the top z_level and

in the bottom z_level will be the same, i.e. for each loop

in the bottom z_level there will be a corresponding loop

at the top level. In this slicing process it is essential to

establish the mapping between the top and the bottom

contours of a coarse slice.

ARTICLE IN PRESS

N0001 G28 Z0

N0020 G28 X0 Y0

N0030 M08 // To move pneumatic piston down

N0040 G10 L2 P1 X40.0 Y60.0 Z-150.0

N0050 G55

N0060 G90 G01 G21 F1000

N0070 G00 X0 Y0

N0120 M03 //To initiate the welding process

: :: :

N0500 M04 //To turn off the welding process

: :

: :

: :

N3990 M09 //To lift the pneumatic piston up

N4000 M30

Fig. 6. Output NC File of HLMSoftware.



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Further, each coarse slice is divided in accordance to

the user-defined/required uniform slice or adaptive

slice thickness known as Fine Slicing [24]. Fine Slicing

is an iterative process to calculate slice thickness based

on the local curvature of the body. The data attainedthrough the Fine Slices are utilized for the path

generation of the MIG/MAG welding gun and the

face-milling operation.

4.4. Setting the process parameters

Using the dialog box as shown in Fig. 8, the user can

input the required parameters for welding as well as

milling processes. Welding-related parameters to be

input are as follows:

type of area-fill (direction-parallel or contour-paral-

lel);

slice thickness;

bead step over;

welding direction (applicable only to direction-paral-

lel area-fill);

machining allowance;

welding feed rate, i.e. torch speed.

Face-milling-related parameters to be input are:

diameter of the face mill,

feed rate for face mill.

Finish-milling-related parameters to be input are:

cutter diameter (of the ball end millinitially only

ball end mill was used),

cutter feed rate, cutter speed.

Parameters for Z control are:

Z-clear,

Z-rapid.

Parameters for work offset are:

X-work offset,

Y-work offset,

Z-work offset.

4.5. Weld deposition path

With the consideration of the limitations of the

welding process, the path for the welding gun has to

be generated to deposit the filler metal for attaining the

required slice profile shape and thickness. The tool path

is optimized to transfer heat uniformly over the layer as

the heat build-up due to the welding process may result

in part malformation and collapse of the structure.

Control of parameters and trajectory is added to the

start and the end portions of the weld in order to make

ARTICLE IN PRESS

Fig. 7. Various coarse slices in connecting rod. (a) Cross section at Bottom Slice, (b) Cross section at Middle Slice, (c) Cross section at Top Slice.

Fig. 8. Dialog Box of settings.



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their thickness and width similar to that of the central

portion of the weld. The joints (cross-sectional se-

quence) of weld pass decrease precision of the weld

deposition. So, to improve the deposition speed and

precision, it is necessary to optimize the number of weld

passes and joints between them.

Shrinkage and machining allowances are to be addedto Fine Slice contours. The outer contours will have a

positive offset, while the inner ones negative. The top

and bottom z_level contours of a layer are merged

(obtain the union of the outer contour and intersection

of inner contours). The slice thus obtained after

offsetting and merging is passed for mesh generation

in order to achieve the zigzag tool path segments

[2527]. These area-fill paths are then fed through the

controller to the three-axis manipulator (on which work

piece is fixed) and the weld machine to initiate the metal

deposition of the filler wire in a desired layer-by-layer

fashion.

4.6. Machining path

With the welding process, attaining the accurate

contour profile shape of the coarse slice is difficult.

For that, end-milling operation is performed to attain

the shape and accuracy. The tool path of the end mill is

generated with relation to coarse slicing and in

accordance to first-order edge adaptive approximation.

Adaptive slicing with first-order edge approximation

makes use of ruled surfaces [28]. As the coarse slice

passes the information of loops between the top and thebottom layers, a ruled surface has to be foud within the

upper limit deviation. In order to calculate the devia-

tion, a line is fitted to connect a point in the bottom loop

and the corresponding point in the top loop without

violating the tolerance constraint cusp height. The

corresponding point in the top loop is the one having the

same surface normal vector and nearest to the bottom

point. This results in the formation of a zigzag tool path

with the tool moving from top to bottom and back

several times. However, this tool path is not useful in

actual practice since it is possible to use it only for a tool

of negligible dimensions, i.e. a point tool. Therefore, the

data generated by this approach have to be modified for

tool accommodation. Hence, every point to be calcu-

lated is to be offset normal by an extent of tool radius

(in the plane of the loop). If the loop is extracted from a

positive simple body, outward normal is used and if it is

extracted from a negative simple body, the inward

normal is used. The normal is calculated by considering

the immediate vertices on both sides of the calculated

point. Thus, we get a zigzag path for the center of the

tool that is written out in a cutter location (CL) file. The

tool path generated contains a list of vertices that

contains the co-ordinates of the points. These paths are

then fed to the HLM machine for the generation of the

final shape of the model.

5. Experiments and illustrations

Preliminary experiments are carried out on theTransPulse Synergic MIG/MAG machine, with 12 mm

diameter ER70S-6 filler wire and shielding gas composi-

tion of 82% Ar+18% CO2, to fine-tune the required

weld process parameters to attain the optimized weld

bead geometry. Unlike the conventional weld deposi-

tion, the HLM process requires low heat input and low

weld penetration [29]; so the preliminary weld para-

meters are constrained to the low current range (40 to

130 A) of short circuit and globular metal transfer

modes only. The excessive remelting of the previously

deposited metal will disrupt the geometry of the earlier

formed layers as the large droplet size will contribute

more heat to the substrate and result in a more

pronounced finger-shaped penetration [30]. Further,

the excess residual heat due to the delayed solidification

results in large amounts of porosity, poor surface finish

and increased material flow.

On a Mild Steel base plate, 10 uniform weld layers are

deposited by 1.2 mm ER70S-6 Filler wire at 1000 mm/

min weld speed maintaining 10 mm stick out distance

with a zigzag weld path to build a 80 80 12mm3

rectangular block. The final deposited specimen is cross

sectioned at different lengths to determine the hardness

and microstructure. The hardness is measured on the

Rockwell hardness-testing machine for each individualdeposited weld layer as shown in Fig. 9. The cross-

sectional views of the deposited specimen showed no

sign of porosity presence in between the layers. The

microstructure of the specimen was seen through the

ESEM for the identification of the growth of the

dendrites. The heat-affected zone of the weld pool

varies with the amount of heat input into the deposited

layer and influences on the formation of martensite,

which alters the hardness of the deposited layer.

Martensite, a hard brittle form of steel, has extreme

ARTICLE IN PRESS

18

18.5

19

19.5

20

20.5

21

21.5

22

22.5

1 2 3 4 5 6 7 8 9 10

Layer Number

RockWellHardness

Hardness

Fig. 9. Variation of hardness with weld layer.



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hardness and low ductility, and its formation is

controlled by decreasing the rate of cooling of the weld

bead (Figs. 10 and 11).

To predict the intermediate weld bead width and

height at various operating process parameters of wire

feed rate and weld speed, experiments are performed

based on the statistical optimizing techniques of

regressive analysis [32,33]. The region of exploration

for fitting the first-order model is (1.22.8) m/min ofwire

feed rate and (400800) mm/min of weld speed.

Controlling factors Levels

1 0 +1

Wire feed rate (m/min) 1.2 2.0 2.8

Weld speed (mm/min) 400 600 800

The collected response data are fitted in the first-order

model forthe weld bead width as

^yw 3:6425 1:09x1 0:72x2

and the reinforcement weld bead height as

^yh 1:18125 0:2875x1 0:2625x2,

where x1 is wire feed rate and x2 is weld speed.

Due to the consistence of the TransPulse Synergic

MIG/MAG welding controller, with the co-relation of

the above-derived equations the corresponding wire feed

rate and weld speed are estimated to build the welddeposition for various slice layer thicknesses in relation

to adaptive slicing [34] (Figs. 12 and 13).

6. Fabrication of HLM machine

In the implementation of the HLM process, the

following steps are involved in retrofitting:

fabrication of the low-cost three-axis manipulator

(details in Table 1);

mounting of the welding torch on the spindle head as

shown in the Fig. 14;

interfacing welding machine and the three-axis

manipulator so that the welding operations can be

initiated and stopped through the NC codes;

a fixture for cooling the substrate;

incorporation of shields so that the occasional spatter

does not affect the structure of the three-axis

manipulator.

ARTICLE IN PRESS

0

200

400

600

800

1000

1200

1400

1600

20 40 60 80 100 120

Diffraction Angle 2*theta (degrees)

ArbitraryUnit

Fig. 10. X-ray diffraction pattern of the deposited layer.

6.74

5.78

4.363.76

7.66

6.54

5.44.92

0

2

4

6

8

10

400 600 800 1000

Weld Speed

WeldBeadWidth

at 20 volt

at 21 volt

Fig. 12. Variation of weld speed with weld bead width.

2

1.4

1.2

1

2.2

1.6

1.4

1.2

0

0.5

1

1.5

2

2.5

400 600 800 1000

Weld Speed

WeldBeadHeight

at 20 volt

at 21 volt

Fig. 13. Variation of weld speed with weld bead height.

Fig. 11. ESEM image microstructure of the layer.



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With low investment, a three-axis manipulator was

fabricated for the HLM process as shown in Fig. 6. The

horizontal X, Y motions are attained by the movement

of the substrate table on the ball lead screw arrangement

and vertical Z motion is achieved by the movement of

the tool head. Each axis has its motion controlled by an

individual stepper motor in open-loop drive with rapid

and linear interpolations, such that all 3 axes can be

moved simultaneously. Initially, tool head was designed

to have the spindle motor and the face-milling cutter to

be on the same base plate for their simultaneous

movement. Later it was modified as shown in Fig. 14

to have a spline shaft arrangement for the power

transfer between the isolated geared motor that was

fixed to the frame of the HLM machine and the spindle

motor. During the vertical movement of tool head,

isolating the spindle motor from its tool head reduces

the net load on the stepper motor. Thus, the accuracy

and repeatability of the z-axis movement is enhanced

(Fig. 14).

Heat buildup due to the welding processes results in

partial malformation or collapse of the structure. A

cooling channel plate has been fixed to incorporate

effective heat control management. This plate is

mounted on the machine table and cold compressed

air is circulated through the duct. The substrate is

mounted on top of this while the weld deposition occurs.

The temperature variation within the deposition layerwith the severity of cooling influences the generation of

internal stresses and the resulting microstructure of the

deposited layer (Fig. 15).

To relieve these undesirable residual stresses, a

suitable heat treatment is performed using normalizing

and annealing processes. As these residual stresses are

unchecked, they may induce warping, loss of edge

tolerance and delaminating, thereby reducing the

strength and influence on the tool life.

The material homogeneity of the tools obtained with

this HLM process is between those of cast and machined

parts. Thus, this process is not suitable for making

forging dies where very high impact forces are

encountered. But the die used in injection molding,

die casting and sheet metal forming undergoes con-

siderably less fatigue loading during the operation,

so these tools can serve the purpose even without any

homogenization operation such as the HIP process.

Further, these die halves are free from overhanging

features, as they need to open and close in operation.

Building such dies and mold with free from re-entrant

profiles by this novel methodology will not require any

support structure, thus making the process more

attractive (Table 2).

ARTICLE IN PRESS

Table 1

Specifications of the low-cost PC-based three-axis manipulator

Traverse X400 mm, Y300 mm, Z300 mm

Accuracy on each axis 70.05 mm

Rapid speed 2500 mm/min (max)

Speed during interpolation 1000 m m/min (max)

Interpolations required Rapid and linear motions simultaneously on all three axesNumber of switching functions Eight controllable through NC program such as M codes

Lead screw type Ball lead screws on all three axes

Drive type Stepper motors in open loop

Kinematics X, Y motions by moving the table and Z by the tool head.

Structure Machined and fastened structure

Maximum load on the table 100 k g (the job being built)

Spindle 1 H P motor with gear reduction to give 200 r pm. Face mill is clamped through collet and draw bolt

Attachment for welding torch A pneumatically operated slide with 50mm traverse

Fig. 14. HLM machine with modified tool head.



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7. Conclusions

Direct production of the metal part is unique among

current RP techniques. With the proposed and devel-

oped HLM process, the overall cycle time of tools and

dies can be developed much faster than the current

existing commercial RP systems. The HLM process can

be developed entirely as a new RP system or even

retrofit to the existing three-axis CNC machine, thus

minimizing the investment cost. Strength of the depos-

ited metal layer depends mainly on the availability of

suitable filler wires and all the desired material proper-

ties cannot be attained with the welding process. The

tools produced using this process may be inferior to

their conventional counterparts in composition and tool

life period but these will generate the final products as

accurately as any other tool. Flooded with competitors

and thronged by customer demands, manufacturing

industries find Direct Metal RP as a golden goose for

their new product development.

References

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ARTICLE IN PRESS

Fig. 15. Fabrication of the connecting rod with the HLM process. (a) Deposited Weld Layer for Connecting Rod, (b) Face Milled weld deposited

Connecting Rod.

Table 2

Comparison of tool making using laser sintering and HLM process

Characteristic SLS and 3DP HLM

Principle Powder metallurgy (PM) Welding and millingwell-known processes

Density and strength Porous part (without compaction). Since it is not

totally steel, not very strong

Under stable operating conditions, density close

to 100% is possible. Strength depends only on the

availability of suitable filler wires which are

available in a variety of choices

Pos t-pro ce ssi ng Impr egnatio n with cop per i n a fu rnac e is re quir ed

which takes several hours

No post processing for density improvement.

However, Hot Iso-static Pressing (HIP) improves

fatigue strength

Accuracy Limited by particle and layer sizes Same as CNC machining

Slicing type Only uniform slicing of zeroth-order edge Adaptive and visible slicing is possible. The whole

die is a single visible slice

Overall cycle time Slower than HLM Much faster than SLS and 3DP

Safety Hazardous due to use of laser Not hazardous

Available as A complete and costly machine A retrofit or complete machine



11/11

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Further Reading

[31] Gas Metal Arc Welding, Welding Handbook AWS, 1996.

ARTICLE IN PRESS


Documents

Hybrid layer