ME 3102 Manufacturing Science Module 1 2 Prepared for S5ME (B) by R. Ramesh, MED, NITC

ME 3102 Manufacturing Science Module 1

2Prepared for S5ME (B) by R. Ramesh, MED, NITC



Melting Equipments• In order to obtain the proper pouring and

melting temperature of the metal several furnaces are used:

• For ferrous metals:– Cupola furnaces– Open hearth furnaces– Electric furnaces

• For non-ferrous metals:– Pit Type – Stationary type– Tilting Type


Cupola Furnace• Used only for melting cast irons.

The iron is usually a mixture of pig iron (crude iron) and scrap.

• Coke is the fuel used to heat the furnace.

• Forced air is introduced through openings near the bottom of the shell for combustion of the coke.

• Coke is a fuel with few impurities and a high carbon content, usually made from coal.

• Flux is the carbonate of coal.


Cupola Furnace


Open Hearth Furnace

• In this the excess carbon and other impurities are burnt out of pig iron to produce steel.

• Since steel is difficult to manufacture due to its high melting point, normal fuels and furnaces were insufficient and the open hearth furnace was developed to overcome this difficulty.

• Once all the steel has melted, slag forming agents, such as limestone, are added.

• The oxygen in iron oxide and other impurities decarburize the pig iron by burning excess carbon away, forming steel. To increase the oxygen contents of the heat, iron ore can be added to the heat.






Indirect Electric Arc Furnace


Indirect Electric Arc furnace

• Arc is struck between two carbon or graphite electrodes and heating of the charge takes place due to radiation from the arc and contact with the furnace.

• It consist of a horizontal barrel shape steel shell lined with refractories. Melting is effected by the arcing between two horizontally opposed carbon electrodes.

• The barrel shaped shell is designed to rotate and reverse, in order to avoid excessive heating of the refractories above the melt .



Direct Electric Arc Furnace


Direct Electric Arc furnace• An arc is maintained between electrode and the metal

being melted. • The gap between the electrode and the charge is

adjusted continuously so that a steady state arc can be maintained.

• The furnace can be tilted for pouring the molten metal through the metal hole. It is used for melting steel, but occasionally cast irons are also melted.



Rotary Furnace


Rotary furnace

• The barrel is kept horizontal and revolved around its axis at about 1 rpm.

• The fuel is burnt at one end of the barrel by a burner and the hot flue gases pass through the other end.

• The molten metal is tapped through hole on the cylindrical surface at the middle of the furnace. It is used for melting grey and malleable irons in many locations.


Crucible furnace


Induction Furnace / Crucible Furnace


Induction Furnace / Crucible Furnace


Crucible furnace

• Employed for melting small quantities of nonferrous metals.

• Crucibles are made of clay, graphite or silicon carbide and it is kept in a cylindrical shell limited with refractory bricks.

• Tilting furnace is tilted to transfer the molten metal to the ladle.

• In non-tilting crucible is lifted out and taken to the moulds for pouring the metal.




Reverberatory furnace• A furnace in which the

material under treatment is heated indirectly by means of a flame deflected downward from the roof.

• It is used in the production of copper, tin, nickel and aluminum.

• It heats the metal to melting temperatures with direct fired wall-mounted burners.


• The primary mode of heat transfer is through radiation from the refractory brick walls to the metal, but convective heat transfer also provides additional heating from the burner to the metal.

• Advantages: – High volume processing rate – Low operating & maintenance cost

• Disadvantages: – High metal oxidation rates – low efficiencies & large floor space

requirements.

Reverberatory furnace


Pit type furnace


Pit type furnace



Solidification of Metals

• The process of becoming hard or solid by cooling or drying or crystallization

• Solidification involves the transformation of the molten metal back into the solid state.

• The solidification process differs depending on whether the metal is a pure element or an alloy.


Cooling curve for a pure metal during casting


Pure Metal

• A pure metal is metal that does not contain any other material in it. A pure metal is mostly soft, bristle or chemically reactive.


Pure Metal• A pure metal solidifies at a

constant temperature equal to its freezing point, which is the same as its melting point.

• The actual freezing takes time, called the local solidification time in casting, during which the metal’s latent heat of fusion is released into the surrounding mold. 35Prepared for S5ME (B) by R. Ramesh, MED,

NITC

Pure Metal

• The total solidification time is the time taken between pouring temperature and complete solidification.

• After the casting has completely solidified, cooling continues at a rate indicated by the downward slope of the cooling curve.


Pure Metal

• Due to chilling action of mold wall, a thin skin of solid metal is initially formed at the interface immediately after pouring.

• Thickness of the skin increases to form a shell around the molten metal as solidification progresses inward toward the center of the cavity.

Characteristic grain structure in a casting of a pure metal, showing randomly oriented grains of small size near the mold wall, and large columnar grains oriented toward the center of the casting.



• The metal which forms the initial skin has been rapidly cooled by the extraction of heat through the mold wall.

• Cooling action causes the grains in the skin to be fine and randomly oriented.

• As cooling continues, further grain formation and growth occur in a direction away from the heat transfer. Since the heat transfer is through the skin and mold wall, the grains grow inwardly as needles.

Pure Metal

Characteristic grain structure in a casting of a pure metal, showing randomly oriented grains of small size near the mold wall, and large columnar grains oriented toward the center of the casting.


Dendritic Solidification




Alloys

(a) Phase diagram for a copper–nickel alloy system

(b) Associated cooling curve for a 50%Ni–50%Cu

composition during casting. 43Prepared for S5ME (B) by R. Ramesh, MED, NITC

Alloy• A metal made by combining two or more

metallic elements, especially to give greater strength or resistance to corrosion.


• Most alloys freeze over a temperature range rather than at a single temperature.

• The exact range depends on the alloy system and the particular composition.

• As temperature drops, freezing begins at the temperature indicated by the liquidus and is completed when the solidus is reached.

• The start of freezing is similar to that of the pure metal.

Alloys


• The solid–liquid region has a soft consistency that has motivated its name as the mushy zone.

• Depending on the conditions of freezing, the mushy zone can be relatively narrow, or it can exist through out most of the casting.

• The latter condition is promoted by factors such as slow heat transfer out of the hot metal and a wide difference between liquidus and solidus temperatures.

Alloys


Characteristic grain structure in an alloy casting, showing segregation of alloying components in the center of casting.




Solidification Patterns

(a) Solidification patterns for gray cast iron in a 180-mm (7-in.) square casting. Note that after 11 min. of cooling, dendrites reach each other, but the casting is still mushy throughout. It takes about two hours for this casting to solidify completely. (b) Solidification of carbon steels in sand and chill (metal) molds. Note the difference in solidification patterns as the carbon content increases. 50Prepared for S5ME (B) by R. Ramesh,

MED, NITC

Solidification TimeSolidified skin on a steel casting. The remaining molten metal is poured out at the times indicated in the figure. Hollow ornamental and decorative objects are made by a process called slush casting, which is based on this principle.

2

aSurfaceAre

volumeCtionTimeSolidifica

where C is a constant that reflects mold material, metal properties, and temperature

Chvorinov’s rule – empirical law for estimating solidification times. Allows comparisons between different shaped castings in the same material and mold types to be made.


Progressive & Directional Solidification

• The condition of having a partly solid, partly liquid zone growing from the outside inward is referred to as progressive solidification.

• Gating design must control this progressive solidification in such a way that no part of the casting is isolated from active feed channels during the entire freezing cycle and is known as directional solidification.

• Progressive solidification is a product of freezing mechanism and cannot be avoided. The degree of progressive solidification can how ever be controlled.


• A rapidly cooled casting which results in a short distance between the start and end of freezing is said to have a high degree of progressive solidification, whereas which is slowly cooled would possess a low degree of progressive solidification.

Progressive & Directional Solidification


Types of Solidification

Directional Solidification• Directional solidification

describes solidification that occurs from farthest end of the casting and works its way towards the Sprue.


Progressive Solidification• It is also called as parallel

solidification.• In this the solidification

starts at the walls of the casting and progresses perpendicularly from that surface.

Types of Solidification



• Nucleation and Grain Growth


• Critical size of the nucleus

• The casting product is of length 150mm, breadth 210mm and height 120mm. It has a hole of diameter 100mm. Calculate the dimensions of the patterns used. Consider shrinkage allowance to be 21 mm/min.




• Dimensional allowance for any dimension can be calculated using Da=Do x a

• Thus, the final dimension will be Df=Do±Da (Or) Df=Do(1±a)

• Where, Do is the original dimension of the casting, and a is the shrinkage allowance.

• The final dimension will be:• Df=Do(1±0.021)


• The final dimension will be:• Df=Do(1±0.021)

• Length of the pattern • lf=lo(1+0.021) = 150 x 1.021 = 153.15 mm

• Breadth of the pattern• bf=bo(1+0.021) = 210 x 1.021 = 214.41 mm

• Height of the pattern• hf=ho(1+0.021) = 120 x 1.021 = 122.52 mm

• Diameter of hole in the pattern • df=do(1-0.021) = 100 x 0.979 = 97.9 mm



Types of cores

• Vertical core• Horizontal core• Balanced core• Kiss core• Ram up core• Hanging core• Drop or stop off core







Hanging core



Pressurized gating system

• The total cross sectional area decreases towards the mold cavity

• Back pressure is maintained by the restrictions in the metal flow

• Flow of liquid (volume) is almost equal from all gates

• Back pressure helps in reducing the aspiration as the sprue always runs full

• Because of the restrictions the metal flows at high velocity leading to more turbulence and chances of mold erosion


Pressurized gating system


• The total cross sectional area increases towards the mold cavity

• Restriction only at the bottom of sprue

• Flow of liquid (volume) is different from all gates

• Aspiration in the gating system as the system never runs full

• Less turbulence.

Un-pressurized gating system


Un-pressurized gating system


• Assuming uniform cooling in all directions, determine the dimensions of a 90 mm cube casting after it cools down to room temperature. The solidification shrinkage for the cast metal is 5% and the solid contraction is 7.5%


• Side of the cube = 90 mm• Solidification shrinkage = 5%• Solid contraction = 7.5%• Dimension of each side of the cube after cooling:• Volume of casting (V) = (90)3 = 7,29,000 mm3

• Volume after solidification shrinkage = 7,29,000 ((1) - (5/100))

= 6,92,550 mm3

• Volume at room temperature = 6,92,550 ((1) – (7.5/100))

= 6,40,609 mm3

• Dimension of each side of the cube = (6,40,609)1/3

= 86.2 mm.

Assuming uniform cooling in all directions, determine the dimensions of a 90 mm cube casting after it cools down to room temperature. The solidification shrinkage for the cast metal is 5% and the solid contraction is 7.5%


•What will be the solidification time for a 1100 mm diameter and 33 mm thick casting of aluminium if the mould constant is 2.2 sec/mm2?




Types of risers

• Top riser:– It is located on top of the casting and has the

advantage of additional pressure head and small feeding distance over the side riser which is placed adjacent to the casting.


• Side riser:– It is filled last and contains the hot molten metal.– It receives the molten metal directly from the

runner before it enters the mould cavity and is more effective than the top riser.

Types of risers


Types of risers


• Open riser:– These risers are open to the atmosphere at the top

surface of the mould.

Types of risers


• Blind riser– The riser which does not break to the top of the

cope and is entirely surrounded by moulding sand is known as blind riser.

– These risers are often made rounded at the top to effect metal savings, because the hemispherical shape has the smallest surface area to volume ratio.

Types of risers
















• Two castings of the same metal have the same surface area. One casting is in the form of sphere and other is a cube. What is the solidification time for the sphere to that of a cube.

• Two solid workpieces (i) Sphere with radius R, (ii) A cylinder with diameter equal to its height, have to be sand cast. Both workpieces have the same volume. Show that the cylindrical workpieces will solidify faster than the spherical workpieces.


• Which one of the following casting shapes would have least solidification time?– A sphere of diameter D = 25mm;– A cylinder with both diameter d and height h = 25 mm– A cube with a length of side l = 25 mm

• Compare the solidification times for castings of three different shapes of same volume: cubic, cylindrical (with height equal to its diameter) and spherical.



DIRECTIONAL SOLIDIFICATION103Prepared for S5ME (B) by R. Ramesh, MED, NITC


Basic Components of a Gating System• The basic components of a gating system are: Pouring Basin, Sprue, Runners and Gates that feed the casting.

The metal flows through the system in this order.

Some simple diagrams to be familiar with are:


The demarcation point is

Re < 2000 is considered a Laminar Flow Re > 2000 is considered a Turbulent Flow

Objective is to maintain Re below 2000.


LAMINAR FLOW- REFERENCE107Prepared for S5ME (B) by R. Ramesh, MED, NITC

TURBULENT FLOW- REFERENCE108Prepared for S5ME (B) by R. Ramesh, MED, NITC

SEVERELY TURBULENT FLOW109Prepared for S5ME (B) by R. Ramesh, MED, NITC

Gating / Runner Design• Now a look at the flow characteristics of the metal

as it enters the mold and how it fills the casting.

Of the flow characteristicsfluidity/viscosity plays a role. Also,

velocity, gravitational acceleration & vortex, pressure zones, molten alloy aspiration from the mold and the momentum or kinetic energy of a fluid.



"Crucible-Mold Interface" is where the metal from the crucible first contacts the mold surface. This area is lower than where the Mouth of the Sprue is located, by having a pool of metal from where the flow will be less chaotic than pouring from the crucible down into the sprue."Dross-Dam" - to skim or hold back any dross from the crucible or what accumulated through the act of pouring. As the lower portion fills and the metal is skimmed, the clean(er) metal will rise up to meet the opening of the sprue in a more controlled fashion.

Pouring Basin - This is the "Crucible -Mold Interface", A pouring cup and pouring basin are not equivalents, The pouring cup is simply a larger target when pouring out of the crucible, a Pouring Basin has several components that aid in creating a laminar flow of clean metal into the sprue.The basin acts as a point for the liquid metal to enter the gating system in a laminar fashion.


Sprue Placement and Parts

The sprue is the extension of the sprue mouth into the mold

The choke or narrowest point in the taper is the point that would sustain a "Head" or pressure of molten metal.To reduce turbulence and promote Laminar Flow, from the Pouring Basin, the flow begins a near vertical incline that is acted upon by gravity

Fluids in free fall tend to distort from a columnar shape at their start into an intertwined series of flow lines that have a rotational vector or vortex effect (Clockwise in the northern hemi-sphere, and counter clockwise in the southern hemi-sphere)...



• The rotational effect, though not a strong force, is causing the cork-screwing effect of the falling fluid. If allowed to act on the fluid over a great enough duration or free fall the centrifugal force will separate the flow into droplets.

• None of the above promotes Laminar flow, plus it aids the formation of dross and gas pick-up in the stream that is going to feed the casting.


Some dimensioning ratio's from Chastain's Foundry Manual (no.2)

• 1- Choke or sprue base area is 1/5th the area of the well.• 2- The well depth is twice the runner depth.• 3- the Runner is positioned above the midpoint of the

well's depth

•By creating a sprue with a taper, the fluid is constrained to retain it's shape, reducing excessive surface area development (dross-forming property) and gas pick-up.

The area below the sprue is the "Well". The well reduces the velocity of the fluid flow and acts as a reservoir for the runners and gates as they fill.


• The runner system is fed by the well and is the path that the gates are fed from.

• This path should be "Balanced" with the model of heating or AC ductwork serving as a good illustration. The Runner path should promote smooth laminar flow by a balanced volumetric flow, and avoiding sharp or abrupt changes in direction.

• The "Runner Extension" is a "Dead-End" that is placed after the last gate. The R-Ext acts as a cushion to absorb the forward momentum or kinetic energy of the fluid flow. The R-Ext also acts as a "Dross/Gas Trap" for any materials generated and picked-up along the flow of the runner.

• An Ideal Runner is also proportioned such that it maintains a constant volumetric flow through virtually any cross-sectional area. In the illustration, notice that the runner becomes proportionally shallower at the point where an in-gate creates an alternate path for the liquid flow.

The Runner System



The Gating System

• The Gates (in this case) accommodate a directional change in the fluid flow and deliver the metal to the Casting cavity.

• Again, the design objective is to promote laminar flow, the primary causes of turbulence are sharp corners, or un-proportioned gate/runner sizes.

• The 2 (two) dashed blue areas when added together form a relationship to the dashed blue area of the Runner, which forms a relationship to the Choke or base of the Sprue Area.



• The issue of sharp corners (both inner and outer) create turbulence, low & high pressure zones that promote aspiration of mold gases into the flow, and can draw mold material (sand) into the flow. None of this is good... By providing curved radius changes in direction the above effects are still at play but at a reduced level. Sharp angles impact the solidification process and may inhibit "Directional Solidification" with cross-sectional freezing...

• The image to the right is just too good a representation to pass-up..

• By proportioning the gating system, a more uniform flow is promoted with near equal volumes of metal entering the mold from all points. In an un-proportioned system the furthest gates would feed the most metal, while the gates closest to the sprue would feed the least.

(this is counter to what one initially thinks).


DIRECTIONAL SOLIDIFICATION-


Formulae, Ratios and Design Equations

• What is covered so far is comprehensive, and intuitive on a conceptual level, but the math below hopefully offers some insight into quick approximations for simple designs, and more in-depth calculations for complex systems.

• Computerized Flow Analysis programs are used extensively in large Foundry operations.

• From basic concepts, designing on a state of the art system shall be attempted:

• Continuity Equation –

• This formula allows calculation of cross-sectional areas, relative to flow Velocity and Volumetric flow over unit time. This is with the assumption that the fluid flow is a liquid that does NOT compress (that applies to all molten metals).


Here, a flow passes through A1 (1" by 1", 1 sq")

The passage narrows to a cross-sectional area A2 (.75" by .75", 0.5625 sq")

The passage expands to a cross-sectional area A3 (1" by 1", 1 sq").

Q= Rate of Flow (Constant - uncompressible)V=Velocity of flowA=Area (Cross-section)

If A1 and A2 are considered, the Area A2 is almost half of A1, thus the velocity at A2 has to be almost double of A1.124Prepared for S5ME (B) by R. Ramesh, MED, NITC


GATING RATIO is-

Areas of Choke : Runner : Gate(s)

• The base of the Sprue and Choke are the same.

• The ratios between the cross-sectional Area can be grouped into either Pressurized or Unpressurized.

•Pressurized: A system where the gate and runner cross-sectional areas are either equal or less than the choke cross-sectional area.


• Areas A2 & A3 do not get added as they are positioned in line with each other and flow is successive between the points and not simultaneous.

• While Areas A4 & A5 are added together as flow does pass through these points simultaneously.

• This example would resolve to a pressurized flow of 1 : 0.75 : 0.66

A1= Choke = 1 Sq InchA2 = 1st Runner c/s Area = 0.75 Sq InchA3 = 2nd Runner c/s Area = 0.66 Sq InchA4 = 1st Gate = 0.33 Sq inchA5 = 2nd Gate = 0.33 Sq Inch


Unpressurized:

• The key distinction is that the Runner must have a cross sectional area greater than the Choke, and it would appear that the Gate(s) would equal or be larger than the Runner(s).

• Common Ratio's noted in Chastian's Vol 2 are:• 1 : 2 : 4• 1 : 3 : 3• 1 : 4 : 4• 1 : 4 : 6


• An exception is noted in Chastain with a 1 : 8 : 6 ratio to promote dross capture in the runner system of Aero-Space castings.

• The Continuity Equation is simplified with the use of ratios as the velocity is inversely proportional between any 2 adjacent ratio values. ie H : L equates to an increase in velocity while a L : H equates to a drop in velocity.

• Laminar Flow is harder to control at a high velocity than a relatively lower velocity.

• Chastain's Vol 2 has much more mathematical expressions and calculations.


Pressurized - is a system where the gate and runner cross-sectional areas are either equal or less than the choke cross-sectional area;A1= Choke = 1 unitA2 = 1st Runner c/s Area = 0.75 unitA3 = 2nd Runner c/s Area = 0.66 unitA4 = 1st Gate = 0.33 unitA5 = 2nd Gate = 0.33 unit

Unpressurized - The key distinction is that the Runner must have a c/s area greater than the Choke, and it would appear that the Gate(s) would equal or be larger than the Runner(s).

Common Ratio's noted are;1 : 2 : 4; 1 : 3 : 31 : 4 : 4; 1 : 4 : 6


Documents

ME 3102 Manufacturing Science Module 1 2 Prepared for S5ME (B) by R. Ramesh, MED, NITC