Casting, Forming and Welding

ME31201
Ajay Sidpara, S K Pal, N Das Chakladar
Venkata Naga Vamsi Munagala, Sushanta Panda, Siddharth Tamang
Syllabus
Sl No Topic
1 Overview of metal casting process, applications, advantages, types of patterns
2 Pattern (types, allowances) Green sand and mould properties
3 Core, Chaplet and chill, and core design (a few numericals and exercise)
4 Microstructure in casting, dendrite structure and CFR
5 Heat transfer and solidification time in insulating mould (Chorinov’s rule)
6 Function of feeder, types of feeder and feeder design
7 Types of gating system, and mold filling time (numerical)
8 Different casting defects
9 Die casting, Centrifugal casting, semi centrifugal casting and centrifuging casting Investment
casting, continuous casting

2
Reference books
1. Manufacturing science by A Ghosh and A K Malik.
2. Metal casting computer-aided design and analysis by B Ravi.
3. Metal casting by Prof. Karl B. Rundman
4. Complete Casting Handbook: Metal Casting Processes, Techniques and Design by John
Campbell
5. Principles of metal casting by R W Heine, C R Loper and P C Rosenthal.
6. Manufacturing engineering and technology by S Kalpakjian and S Schmid
7. Casting technology and cast alloys by A K Chakrabarti.

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 Overview of metal casting
process, applications, advantages

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Application of casting- Design and material versatility

Pump housing (Cast iron)

Turbine blade for jet engine Dental implants
Engine Block (Al-alloy)
(Ni-alloy) (cobalt-chromium
alloy)

Ancient Piston head

Wheel of racing
statue
cars (Mg-alloy)
Rail bogie side frames (Steel) (Bronze)

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Application of casting

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https://castman.co.kr/automobile-parts-casting-methods-and-materials-used-a-review/
Material processing
• Casting  Expendable and multiple use mold
• Cutting  Mechanical machining and nontraditional machining
• Forming  Hot bulk forming and cold forming
• Joining  Welding, brazing, soldering and mechanical joining
What is casting?
Casting is a manufacturing process in which molten metal flows into a mold where it solidifies
in the shape of the mold cavity. The part produced is also called casting.
Producing a “good” casting requires a design effort to:
1. Create a gating system (pouring basin, sprue, runner) to bring molten metal into the mold
cavity free from entrapped slag, sand or gases.
• Other functions: to control shrinkage, the speed of the liquid, turbulence, and trapping
of impurities (sand, slag and gases).
• It is usually attached to the thickest part of the casting to assist in controlling shrinkage.

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Producing a “good” casting requires a design effort to:
2. Provide a riser which feeds liquid metal into the casting cavity as the liquid is cooling and
solidifying (all liquid metals will shrink as they cool and most liquid metals will shrink as
they solidify).
• The riser may have to provide up to 5 - 7% by volume for the casting as it solidifies.
3. Control heat flow out of the casting so that the last liquid to solidify is in the riser.
4. Control the rate of heat flow to control the nature of the solidified product.

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Casting- a multidisciplinary approach

Shape, size, Pattern After cast

Gating and riser
tolerance, production processing
design (fluid and
dimensional for molds and heat transfer-CAE), (inspection,
change during core (CAD). selection of casting machining &
processing. finishing), cost.
processes.

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Classification of casting processes
1. Expendable mould casting: Green sand casting, dry sand casting, cement moulding, CO2
sodium silicate moulding, Investment casting, etc.
2. Permanent mould casting: Ingot casting, continuous casting, strip casting, die casting,
centrifugal casting, squeeze casting, etc.
Based on materials to be processes
1. Ferrous (Iron and steel castings)
2. Non-ferrous (copper alloys, aluminum alloys, magnesium alloys and other alloys )
Advantages and disadvantages of casting processes
Intricate shape (Molten material can flow into very small sections)
Large varieties of metal (ferrous: iron, steel and non-ferrous: Aluminium alloy, copper
alloy, magnesium alloy, and other alloys)
Large size component (few grams to tones)
Mass production

Poor accuracy (shrinkage and other casting defects)

Poor surface finish (depends on the roughness of cavities)
Mechanical properties (variation within the casting is not good)
Environmental effect (high temperature environment, toxic metal
gases, handling of molten metal, etc.)
 Pattern design
(types of pattern, allowances)

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Pattern
Pattern is a replica of the final product and is used for preparing the mold cavity.
It is slightly bigger than the final product to account for different allowances.
It should be easily removed without any damage done to the prepared mold.
It should be also be perfectly dimensioned and very durable for the intended use.
Pattern materials:
• Wood - common material because it is easy to work, but it warps due to absorption of
moisture
• Metal - more expensive to make, but lasts much longer
• Plastic - compromise between wood and metal
Types of Pattern

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Types of pattern
One-piece patterns / Solid pattern / Loose pattern
 It is the simplest and the least expensive pattern.
 It is used only when a limited number of castings are required.
 It requires more manual operations and a much higher degree of skill
than molding with other pattern types.
Split patterns
 Patterns are split into 2 parts along the parting line with the upper half forming cavity in the
cope and the lower half in the drag.
 It is used for molding of more complex shapes in moderate quantity.
 Care must be taken to ensure alignment of the mold cavities at the parting line
Types of pattern
Match plate patterns
 Patterns are obtained by attaching 2 halves of the split
pattern on opposite sides of the match plate.
 After preparing the cope and drag cavities, the match
plate is removed to get the complete pattern.
 Several patterns can be mounted on the match plate
(good for small castings)
Loose piece pattern
 It is used when pattern is difficult to
withdraw from the mould.
 The main pattern is removed first and then
the loose piece is withdrawal separately.
Types of pattern
Cope and drag patterns
 Essentially split patterns with 2 halves of the patterns
mounted separately on 2 match plates.
 Cope and drag molds can be prepared separately and
assembled to form a complete mold.
 Mostly preferred for heavy castings (used in large
production runs and in molding machines).
Sweep pattern
 Used for forming symmetric and large circular
mould by revolving a sweep attached to a spindle.
 A sweep is a template of wood or metal.
Types of pattern
Skeleton pattern
 It is not economical to make a solid pattern when only a small number of large and heavy
castings are to be made.
 It is a ribbed construction of wood which forms an outline of the pattern to be made.
 This frame work is filled with loam sand and rammed.
 The surplus sand is removed by strickle board.

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Pattern allowances

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Shrinkage or contraction allowance
 Most metals undergo noticeable volumetric contraction when cooled.
 Shrinkage of liquid as it cools from the solidification temperature
 Solidification shrinkage as the liquid turns into solid
 Solid metal contraction as the solidified metal cools to room temperature.
 Liquid and solidification shrinkage is taken care of by riser in the feeder design.
 Solid shrinkage is taken care of by the shrinkage allowance by providing excess dimensions to the
pattern.
Pure metal
Tl

Alloy
Ts

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Shrinkage or contraction allowance
 A pure metal solidifies at a constant temperature equal to its freezing point (same as melting point)

Greater the superheat the

more time there is for the
liquid material to flow into
intricate details

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Shrinkage or contraction allowance
 In alloys  No thermal arrest, instead there is a freezing range.
 The freezing range corresponds directly to the liquidus and solidus found on the phase diagram for
the specific alloy

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Problem on shrinkage allowance
 The casting shown is to be made in cast iron using a wooden pattern. Assuming only shrinkage
allowance, calculate the dimension of the pattern. All dimensions are in inches.

Material Dimension Shrinkage allowance

(inch/ft)
Grey Cast Up to 2 feet 0.125
Iron 2 feet to 4 feet 0.105
over 4 feet 0.083
Cast Steel Up to 2 feet 0.251
2 feet to 6 feet 0.191
over 6 feet 0.155
Aluminum Up to 4 feet 0.155
4 feet to 6 feet 0.143
over 6 feet 0.125
Magnesium Up to 4 feet 0.173
Over 4 feet 0.155

8 x (0.125/12) = 0.0833 ~ 0.09 inch

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NPTEL course
Draft allowance
 Draft is the taper provided by the pattern maker on all vertical surfaces of the pattern so that it can be
removed from the sand without tearing away the sides of the sand mold.

Height of Metallic patern Wooden pattern

pattern (mm) (machined (manual mould)
mould)
Up to 20 10 30’ 30
20-50 10 10 30’
50-100 00 45’ 10 15’
100-200 00 30’ 00 45’
200-300 00 30’ 00 30’
300-800 00 20’ 00 30’
800-2000 - 00 30’
More than - 00 30’
2000

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Machining / finishing allowance
 Thickness of metallic layer to be removed from the casting to get desired surface finish, dimensional
accuracy and intricate shape.
 The amount of machining allowance is also affected by the size and shape of the casting; the casting
orientation; the metal; and the degree of accuracy and finish required.

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Distortion or camber allowance
 The distortion in casting may occur due to internal stresses.
 These internal stresses are caused on account of unequal cooling of different section of the casting.
 It depends on types of metal, shape, thickness.

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Rapping allowance
 When pattern is withdrawn from the mould, it distorts the sides and shape of the cavity.
 To avoid this, the pattern is shaked to create a small void or gap between the mould and pattern
surface for easy removal.
 This increases the size of cavity  Keep the size of the pattern is slightly smaller than castings.
 Shake allowance is considered as negative allowance
 There is no sure way of quantifying this allowance.

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Size of pattern including allowances:
Pattern allowances considering the position of the critical surface and sharp corners/abrupt change in
thickness

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Green sand and mould properties

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Molding sand [Base sand + Binder + Moisture (water)]
 Base sand
 Most common is silica sand (cheap and easily available)
 Other sands: zircon sand, chromite sand, and olivine sand.
 Particle size and its distribution  Surface roughness, permeability
 Must be able to resist the temperature of the molten metal.

 Binder  mixed with the molding sands to provide the strength

 Clay, coal dust, portland cement, organic oil, resin, etc.

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Molding sand [Base sand + Binder + Moisture (water)]
 Moisture (water)
 Clay acquires its bonding action only in the presence of the moisture.
 Water penetrates the mixture and forms a microfilm, which coats the surface of each flake of the
clay.
 High content  increase the plasticity and thus reduces the strength of the molding sand.
 Low content  less flowability of the molding sand  poor packing of the molding aggregate
around the pattern.

Typical composition (Wt %):

Sand – 80 to 90
Clay – 6 to 10
Water – 4

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Molding sand properties
 Refractoriness  ability of the molding material to resist the temperature of the liquid metal to be
poured so that it does not get fused with the metal  higher fusion point (silica sand – 1700 °C)
 Permeability  property of the molding sand which helps to escape the gases.
 Gases due to moisture and liquid metal absorbs gases from the atmosphere.
 Green strength  ability of sand particles to adhere to each other to impart sufficient strength to
the mold.
 Dry Strength  ability of the sand to retain the shape of the mold cavity and able to withstand the
metallostatic pressure of the liquid material.
 Hot strength  Strength of the sand that is required to retain the shape of the cavity when the
metal in the mold is still in liquid state after the moisture is eliminated.

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Molding sand properties
 Plasticity / flowability  property of sand due to which it flows to all portions of the moulding box or
flask.
 Collapsibility  allowing free contraction of the metal to avoid tearing or cracking of the contracting
metal.
 This property is highly required in cores.
 Thermal stability  Sand adjacent to the metal is suddenly heated and undergoes expansion.
 Rapid heating  Cracks, buckling and flacking of sand may occur.
 Adhesiveness  property of molding sand to get the stick or adhere to the inner wall of molding
box.
 Cohesiveness  Sand grain particles interact and attract each other within the molding sand.
Increases binding capability of the sand.
 Reusable

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Grain size and shape
 Small grain size  high mold
strength
 Large grain size  more
permeable.
Bigger grain results worse surface finish
 Irregular grains  strong casting
due to interlocking
 Round grains  better surface
finish.
 A sand casting mold mixture with
more collapsibility has less strength Bigger grain ensures better permeability
 A sand casting mixture with more
strength has less collapsibility.
Irregular grain produces stronger mold

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Permeability
 Permeability is determined by calculating time taken by the 2000 cc (2000 ml) air to pass through a
standard specimen and the back pressure generated is measured.
v = Volume of air in ml (cc) passing through the specimen
h = Height of the specimen
p = Pressure of water in cm of water
a = Area of cross section of the specimen
t = Time in minutes
 Higher P  high openness of the sand.

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Permeability
 Permeability is controlled by
 Fineness of the sand grains
 Shape of the sand grains
 Amount and type of binder
 Moisture content
 Degree of compaction

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Foundry Manual - https://maritime.org/doc/foundry/index.htm
Variation of mold properties with water content
 Ideal water content at which all of the water is polarized and active in the bonding process.
 Water added to activate the clay bond is called temper water, this is known as the temper point.
 At the temper point, green strength of the sand is at its maximum.
 Above the temper point  some of the water will exist as liquid (not involved in bonding)
 Additions of water beyond this point
decrease the strength of the
sand/clay/water mixture.
 Below the temper point  insufficient
water to develop the bond fully.

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Effect of moisture on permeability
Permeability increases in a nearly linear Increasing the number of rams required to
manner due to the swelling action of the clay attain a 2” high specimen results in dramatic
particles, thereby pushing the sand particles decrease in permeability due to closing off of
further apart and making more room for air some of the continuous air passages.
passage.

40
MetalCasting by Prof. K. B. Rundman
Lifting properties of pattern
Sand sticking to pattern while lifting from the mold are caused by
 inadequate draft
 faulty draw mechanism
 improper ramming
 poor sand mixings which changes number of adhesive bonds.

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Core, chaplet and chill

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Core
 It is a separate entity placed in a mould to produce a hole,
recess, undercut or internal cavity.
 Cavity in the core box is a negative replica of the
corresponding part feature.
 Core should have Compressor casing and core

 high hot strength

 refractoriness
 permeability and
 collapsible
 Dispensable core  sand casting
 Permanent core  die casting

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Types of core
Horizontal core: Vertical core:
 Positioned horizontally at the  Placed vertically, with some of their
parting surface of the mould. portion lying in the sand.
 Can withstand the turbulence effect
of the molten metal poured.
Types of core
Balanced core:
 It is extended only one side of the mold.
 Only one core print is available on the pattern for balance core.
 Suitable for the casting having only one side opening or producing
blind holes or recesses in the casting.
 Core print should be large enough to support the weight of the
core.
 Should withstand the force of buoyancy of the melt.

Cover core:
 It is used when the entire pattern is rammed in the drag and the core
is required to be suspended from the top of the mold.
 It is extended horizontally in the mold cavity.
Types of core
Hanging core:
 It hangs from the cope and does not have any support at the bottom
in the drag.
 It is fastened with a wire or rod.
Wing core or Stop-off core:
 It is used when a hole or recess is required in the casting either above or
below the parting line.
 It is used when it is not possible to place the pattern in the mold such a way
that the recess can cored directly or using other types of cores.
 A part of the core placed in seat becomes a stop-off and forms a surface of
casting  Stop-off
 Other names  tail core, chair core, saddle core
depending on its shape and position in the mold.
The core print design
 The print must balance the body weight so that core does not fall during mold assembly.
 The print must not crush the mold with the bouncy force of the metal.
 The print should allow the internal gas to escape from the mold.
 The print should not hinder the heat transfer from the core to the mould.
 The print should minimize the shift and deflection of the core with use of chaplets if necessary.

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Chaplets
 Chaplets are metal supports used to hold a core in place when core prints are inadequate.
 They must be clean  Free from rust, oil, grease, moisture. Otherwise, cause poor fusion or porosity.
 Copper and nickel plating is a good method of protecting chaplets from rusting.
 They should be the same composition as the casting,
if possible.
 Soft-steel chaplets are used in iron and steel casting
 Copper chaplets in brass and bronze castings
 Strong enough to carry the weight of the core until
sufficient metal is solidified to provide the required
strength.
 Over size chaplet  poor fusion and often causes
cracks in the casting.
 They should not have any sharp, internal corners 
metal will not fill a sharp internal groove.

48
Mikell P. Groover - Fundamentals of Modern Manufacturing_ Materials, Processes, and Systems-Wiley (2012)
Chaplet example
 Brass chaplets support sand core inside a sand mold cavity. The projected core print area is 13 cm2 for
each end of the cylindrical sand core, which supports both ends. The design of the chaplets and how
they are placed in the mold cavity surface allows each chaplet to sustain a force of 45 N. If the volume
of the core = 7.5 x 103 cm3, and the metal poured is brass, determine the minimum number of
chaplets that should be placed (a) beneath the core, and (b) above the core. (Green sand strength is
6.9 x 103 N/m2)
Density of Core Weight of the core = Density x Volume
Sand core = 1.6 g/cc Core print 1.6 x (7.5 x 103) = 12 kg
Brass = 8.67 g/cc Mould cavity Weight of the displaced metal
= Density x Volume
Mould
8.67 x (7.5 x 103) = 65 kg
Load sustained by core print = Projected area x Strength = 13 x 0.69 = 8.97 N = 0.91 kg  Both side = 1.82 kg
Chaplets required below the core = (Wt. of core – Load sustained by core print) / Load sustained by a chaplet =
(12 – 1.82) / 4.6 = 10.18/4.6 = 2.21 = 3 Chaplets
Buoyancy force = Wt. of the displaced liquid – (Wt. of core + load sustained by core print)
= 65 – (12+1.82) = 51.18 kg
Chaplets required above the core = Buoyancy force / load sustained by a chaplet
= 51.18 / 4.6 = 11.13 = 12 Chaplets 49
Chills
 Slow cooling has advantages and disadvantages.
 Permits better feeding in thin sections and produce intricate detail
 It slows production by lengthening the time between pouring and shake out and for most alloys it
results in castings of lower strength.
 This can usually be eliminated using chills in specific areas.
 Chills  fast heat extracting metallic materials incorporated separately along with sand mold surface
during molding.
 To produce a hard surface at a particular place in the casting.

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Chills
 A chill is commonly used to promote directional solidification.
 Chills and antichills can be used to adjust solidification rates
 Freezing proceeds rapidly from thin to intermediate sections and then into heavy sections, and
finally into the feeding system.
 This can be done by directing cooling air jets against a chill inserted in the mold or, more simply, by
using a metal insert without auxiliary cooling.

 Chills can be used

 to increase production rate
 to improve metal soundness
 to increase mechanical properties

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Nucleation and growth

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Nucleation and growth
 Nucleation  Formation of stable nuclei
 Growth of nuclei  Formation of grain structure

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Nucleation and growth
 A thin layer of metal (thin skin or shell of solid metal) is formed next to the mold wall.
 The shell gradually thickens as more and more metal is cooled, until all the metal gets solidified.
 Solidification always starts at the surface and finishes in the center of a section  follows the
direction that the metal is cooled.

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Foundry Manual - https://maritime.org/doc/foundry/index.htm
Why to study solidification mechanism?
 Solidification is an important industrial process since most metals are melted and then cast into a
semi-finished or finished shape.
 80% of ALL industry involves a casting and solidification process of materials in various ways.
 The initial microstructure of the material forms during solidification process  microstructures
influenced the material properties

55
The Science and Engineering of Materials - Askeland phule wright
Cooling curves of pure metal
 Liquid cools as specific heat is removed (A to B).
 Undercooling (B to C).
 Nucleation begins (C), latent heat of solidification is released 
temperature of the liquid increases (recalescence, C - D).
 Metal continues to solidify at a constant temperature (D - E) 
latent heat of solidification balances the heat being lost due to
cooling. Pure metal without
inoculated (nucleating
 Solidification is complete (E). Casting continues to cool.
agents)

 Undercooling is negligible.
 Recalescence is not observed (very negligible).
 Solidification begins at the melting temperature

Well inoculated pure metal

(containing nucleating agents)
56
The Science and Engineering of Materials - Askeland phule wright
Solidification mechanism
 At the melting point, both phases have the same free energy  co-exist.
 Above the melting point  liquid is in the state of lower free energy  liquid is stable.
 Below the melting point solid is at low free energy  solid is more stable.
 The system can release energy when it solidifies  Driving force for phase transformation.
 Energy difference between the liquid and the solid is the driving force for solidification.

Undercooling  The temperature to which the liquid

metal cools below the equilibrium freezing temperature
before nucleation occurs.

57
Solidification mechanism
 If the liquid is undercooled to To  there will be a driving force
(∆Gv = Gso - Glo) which favors the nucleation and growth of the solid.
 Undercooling provides a driving force for the nucleation event.
 Recallescence resulting from input of latent heat of solidification

Gl = Hl – TSl and Gs = Hs – TSs

At Tm, Gs = Gl  Hs – Hl = Tm (Ss – Sl)  latent heat of solidification.

At Tm  Solid begins to form releasing the latent heat

of solidification.

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MetalCasting by Prof. K. B. Rundman
Solidification mechanism
 Homogeneous nucleation:
 Metal itself provides atoms to form nuclei.
 Metal cools (below freezing T)  Slow moving atoms bond each other to form nuclei.
 Cluster of atoms below critical size is called embryo (continuously being formed or/and re-dissolved in
a molten metal) .
 Clusters of atoms reach critical size  they grow into crystals.
 Cluster of atoms greater than critical size are called nucleus.

59
The Science and Engineering of Materials - Askeland phule wright
Solidification mechanism
Formation of solid phase from the liquid is subjected to a change in the free energy.
Total free energy change comprises of 2 components in homogeneous nucleation.
 ∆Gv: change in free energy per unit volume between liquid and solid  Volume free-energy change.
 ∆Gs: energy required to form new solid surface  surface free-energy change

∆G = (4/3)πr3 ∆Gv + 4πr2γ

γ - surface free energy of the solid-liquid interface

Energy opposing to the formation of embryos, the energy to

form the surface of these particles ~specific surface free energy

Energy is released by the

liquid to solid transformation

60
The Science and Engineering of Materials - Askeland phule wright
Solidification mechanism
The critical radius (r*) is the minimum size of a crystal that must be formed by atoms clustering together
in the liquid before the solid particle is stable and begins to grow  where ∆G reaches the maximum
(∆GT).
∆G = (4/3)πr3 ∆Gv + 4πr2γ

For Critical size of nucleus  d(ΔGT)/dr = 0 when r = r*

d ( G T )
 4r 2 Gv  8r  0
dr
2
r  r*  
Gv

61
The Science and Engineering of Materials - Askeland phule wright
 Microstructure in casting,
dendrite structure and CFR

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Possible casting microstructure
 During the formation of the thin skin 
 Fine equi-axed grains near the under-cooled mold wall
due to high rate of nucleation  extraction of heat is
greatest at the mold wall.
 Latent heat of solidification is released from the
interface through the solid and
 The remaining liquid metal soon loses most of the
undercooling.
 Nucleation gets restricted and grain growth starts in
the direction opposite of heat flow.
 In pure metal, columnar grains extend to the center of the
casting.
 Grains that have favorable orientation grow
preferentially and are called columnar grains

Serope Kalpakjian, Steven R. Schmid - Manufacturing Engineering and Technology-Prentice Hall (2009). The 63
Science and Engineering of Materials - Askeland phule wright
Possible casting microstructure
 In solid-solution alloys, columnar grain growth may be interrupted by an equi-
axed grain growth.
 Grains that have substantially different orientations are blocked from further
growth.
 Heat transfer is reduced away from the mold walls  grains become equiaxed
and coarse.
 The alloys, generally produce a partially columnar and partially equi-axed grain
structure.

64
Serope Kalpakjian, Steven R. Schmid - Manufacturing Engineering and Technology-Prentice Hall (2009)
Possible casting microstructure
 The zone of equi-axed grains can be extended throughout the casting by adding
nucleating agents or catalyst (sodium, magnesium, bismuth, etc.)

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Serope Kalpakjian, Steven R. Schmid - Manufacturing Engineering and Technology-Prentice Hall (2009)
Dendritic structure
 Alloys have freezing range where it is in a mushy state consisting of
columnar dendrites  liquid metal between the dendrite arms.
 Concentration of alloying element in liquid is higher.
 Increase in liquidus concentration level reduces the liquidus temperature 
undercooling is affected
 Combined effect of change in concentration
(constitutional undercooling) and local thermal
undercooling  columnar grains to branch and
further branch  called dendritic structure.

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Serope Kalpakjian, Steven R. Schmid - Manufacturing Engineering and Technology-Prentice Hall (2009)
Constitutional undercooling
As the thermal gradient during solidification of the alloys decreases  constitutional undercooling
increases  more instability at the solid-liquid interface  high probability of formation of an
equiaxed dendritic microstructure.
Actual melt temperature attains
equilibrium temperature only at
the interface

Actual melt temperature is lower

than the equilibrium temperature 
difference is Constitutional under-
cooling  conditions are more
favorable for freezing than at the
interface.
Planar growth vs. Dendrite growth
 When the temperature of the liquid is above the
freezing temperature  a protuberance on the solid-
liquid interface will not grow, leading to maintenance
of a planer interface.
 Latent heat is removed from the interface through
the solid

 If the liquid is undercooled, a protuberance on the

solid-liquid interface can grow rapidly as a dendrite.
 The latent heat is removed by raising the
temperature of the liquid back to the freezing
temperature.

68
The Science and Engineering of Materials - Askeland phule wright
Centerline shrinkage
 Dendritic structure makes the movement of liquid metal to compensate for solidification shrinkage
difficult.
 Increases micro porosity (voids within the casting)  affects strength and ductility.
 Compositional variations, segregation.

 If it is impossible to feed properly the last parts of the dendrites, the casting defect known as
centerline shrinkage is formed.
 Common way to reduce this risk is the use of chills  accelerate the cooling rate and directional
solidification.

69
Foundry Manual - https://maritime.org/doc/foundry/index.htm
Center-line feeding resistance (CFR)
Ordinary sand mold Chilled mold
Solidification starts at the center line Rapid heat extraction  a narrow liquid-
of the mold before the solidification solid zone quickly sweeps across the
is completed even at the mold face. molten metal.

70
Manufacturing Science by Ghosh and Mallik (East-West Press Pvt Ltd)
Center-line feeding resistance (CFR)
 Difficulty in feeding an alloy in a mold is expressed by CFR
Time interval between start and end of freezing time at center line
CFR = ------------------------------------------------------------------------------
Total solidification time of casting
AC
----- x 100% If CFR > 70%  feeding is considered to be difficult
OC

71
Manufacturing Science by Ghosh and Mallik (East-West Press Pvt Ltd)
Example: Center-line feeding resistance (CFR)
0.6 % carbon steel has following data for sand and chilled mold casting. Which will be difficult to feed?
Start and end of freezing time at the centre line= 24, 48 min (for sand mold) & 8, 10 min (for chill
mold). Assume liquid metal starts freezing at the mold wall as soon as it is filled.
Time interval between start and end of freezing time at center line
---------------------------------------------------
Total solidification time of casting

For sand mold: 48 – 24 / 48 = 0.5 = 50 %

For chill mold: 10 – 8 / 10 = 0.2 = 20 %

72
Mold filling time and
Solidification time

73
Teeming of metal melt out of ladle
Using Bernoulli equation
2 2
V1 P1 V2 P2
h1    h2  
2 g g 2 g g

h2 = 0, h1 = h
P1 = P2 = P = atm pressure
V1 is neglected compared to V2

V2  2 gh

74
Example:
A ladle with a circular cross-section (diameter 2.0 m) contains 70 x 103 kg molten steel. The steel is
teemed through a circular hole in the bottom of ladle. The diameter of the hole is 3.0 cm. Calculate
the time required to empty the ladle.
V1 can be neglected
Density of steel is 7.8x103 kg/m3 compared to V2 in
Bernoulli’s equation
 A2    .(0.015) 2  4
V1  V2    V2  2 
  2. 25  10 V2
 A1    .(1.0)  m
h0   2.86m
A1
 A1  dh
V2  2 gh   .V1 and V1   h=0
 A2  dt Ans t = 3391 sec
= 56.5 min

h   dt t  A1  2 h0  h 
 A1   dh   A1   dh  t  
 .    2 gh  .  
 A2   dt   A2  h0  2 gh   A2  2g 
  0
75
Heat transfer and solidification time for an insulating mold
Heat transfer by mold
(Fick’s 2nd law) T K   2T  2T  2T  K
  2  2  2     2T
t C  x y z  C

αM=K/(ρ.C) for mold material=thermal

T  2T diffusivity for mold
 M . 2
t x
K – thermal conductivity ρ–
density, C – heat capacity

76
Metal Casting by Prof. Karl B. Rundman
 Total heat Q through the total surface area (A) of the casting in time t
t ts t ts
KA(Tm  T0 ) 2 KA(Tm  T0 )
Q   qAdt   dt  ts
t 0 t 0  M t  M
Total heat liberated before solidification  Latent heat of solidification (QF)
+ Superheat (Qs)
Latent heat (L) Super heat

2 K M A(Tm  T0 )
Heat balance  
t s   metalV H F  Cmetal (Tp  Tm ) 
 M
Mold constant (Bs) for a given metal and mold poured at a temperature (TP)
2 2

Bs 
 M  2
metal H F  Cmetal (Tp  Tm ) 2
V 
t s  Bs  
V 
ts   
4 K M2 (Tm  T0 ) 2  A  A
Chvorinov’s Rule

Metal Casting by Prof. Karl B. Rundman 77

Principles of Metal Manufacturing Processes by J. Beddoes and M. J. Bibby
Example:
A company currently is producing a disk-shaped brass casting 2 in. thick and 18 in. in diameter. You
believe that by making the casting solidify 25% faster, the improvement in the tensile properties of
the casting will permit the casting to be made lighter in weight. Design the casting (maintaining the
same diameter) using the same casting process to permit this. Assume that the mould constant is 22
min/in2 for this process.
d = 18 in.
  2
Surface area of disc (A) = 2 d 2   dh Volume of disc (V) = d h
4 4
2
h = 2 in.
2   2  2
V
  d h 11  dh 
ts1  Bs    22  4 
   ts1 = 14.7 mins
 A  
2

 2 d   dh  2  d  2h 
 4 
For 25% faster solidification  ts2 = 0.75 ts1 = 11 mins
2 2 2
V  11  dh   18h 
ts 2  Bs   11   2 
 A 2  d  2h  18  2h 

316h 2  72h  648  0  h = 1.55 in. 78