1. Fundamentals of Casting and Solidification
CH.PavanKumar
Asst.Professor,
Department of Mechanical Engineering
2. Acknowledgement
2
Contents of this presentation are taken from the following sources and are gratefully acknowledged:
1. Manufacturing Science – Ghosh and Mallik.
2. Manufacturing Processes for Engineering Materials - Kalpakjian and Schmid.
3. Fundamentals of Metal Casting – Richard Flinn.
4. Metal Casting – Karl Rundman.
5. Lecture Notes – Dr. M. Law (IIT Kanpur).
6. Lecture Notes – Dr.A. Kumar (IIT Kanpur).
3. Classification of Manufacturing Processes
3
Manufacturing Processes
Subtractive
Additive Joining
Mass
Containing
Manufacturing Science Lab, I.I.T.Kanpur
5. Casting
5
What is Casting ?
It is a process in which liquefied material, such as molten metal, is poured into the cavity of a specially
designed mould and allowed to harden.
After solidification, the workpiece is removed from the die to undergo various finishing treatments or for
use as a final product.
Casting is typically used to create intricate solid shapes, and cast products are found in a wide range of
applications, including automotive components, aerospace parts, electronics, mechanical devices, and
construction supplies.
The term Casting also applies to the part made in the process.
Riser and Gating system
Components of sand
casting
9. Casting
Advantages and Disadvantages of casting process
Advantages and Capabilities Disadvantages
Can create complex part geometries
Can create both external and internal shapes
Some casting processes are net shape and
near net shape
Can produce very large as well as very small
components
Limitation on mechanical properties
Poor dimensional accuracy and surface
finish for some casting processes. Ex.
sand casting
Safety hazard to workers due to hot
molten metals
Environmental problems
Some casting methods are suited for mass
production
9
13. Casting
13
Patterns
A pattern is the replica of the part to be cast and is used to prepare mold cavity. Patterns are made either of
wood or metal.
Types of Patterns
Single piece pattern Split piece pattern Match plate pattern Cope and Drag pattern
14. Casting
14
Moulds
A mould is an assembly of two or more metal blocks, or bonded refractory particles (sand) consisting of a
primary cavity.
Types of Moulds
Green sand mould Plastic mould Metal mould Shell mould
Investment mould
15. Casting
15
Green Sand Mould
Content : Sand (70-85 %)
Clay (10-20 %)
Water (3-6 %)
Additives (1-6 %) (Ex. Wood flour, dextrin, sea coal)
The success of a casting process depends upon the compressive strength, permeability, flowability,
deformability, and refractoriness of the molding sand.
Effect of WATER CONTENT on molding sand properties:
16. Casting
16
Melting
Gases in Metals (generally nitrogen and hydrogen)
In metal castings
Gases may be
mechanically trapped
Gases may be generated due to
variation in their solubility at different
temperature and phases
Gases may be produced due to
chemical reactions
Based on the solubility of hydrogen gas, metals are divided into two groups
Endothermic metals Exothermic metals
Solubility of hydrogen
increased with temperature.
Ex- Aluminum, Magnesium,
Copper, Iron, Nickel
Solubility of hydrogen
decreased with temperature.
Ex- Titanium, Zirconium
17. 17
Casting
Melting
Gases in Metals (generally nitrogen and hydrogen)
The expression for solubility in case of both endothermic and exothermic metals can be written as:
S = C exp (-Es/kθ)
where, S is the solubility of hydrogen gas in the metal
C and k are constants
Es is the heat of solution of 1 mol of hydrogen (+ for endothermic metals and – for exothermicmetals)
θ is the absolute temperature
Sievert’s law states that the amount of hydrogen dissolved in the metal varies as:
% hydrogen present = K 𝑝 𝐻2
where, 𝒑 𝑯 𝟐is the partial pressure of hydrogen in the atmosphere above the liquid metal, and K is a constant which is
different for different metals. It is solubility (solid or liquid) in terms of cc/kg.
18. 18
Casting
Melting
Furnaces
The selection of furnace depends mainly on the metal chemistry, the maximum
temperature required, and the metal delivery rate and mode.
The rate and mode of metal delivery depends mainly on the process
(batch or continuous).
Three main types of furnaces used in foundries are :
Induction furnace : 1750 º C Side blow convertor : 1700 º C Cupola : 1650 º C
19. 19
Casting
Pouring
Gating design
Mould filling time for each gating design is different
Broadly, gating designs can be classified into three categories:
TOP Gating design BOTTOM Gating design PARTING LINE Gating design
20. 20
Casting
Pouring
Gating design
To derive the expression for mould filling time (tf ) in case of vertical (top) and bottom gating design, let us consider the
following simplified case.
21. Casting
21
Gating design
Vertical gating design
Based on the principle of frictionless fluid flow.
Assuming that the mold is initially kept at an atmospheric
pressure.
The formulation is based on the integrated energy balance
equation.
Applying Bernoulli’s equation at points (1) and (3), we can
write:
+ 0 + ℎ 𝑡 =
𝑃 𝑎 𝑡𝑚 𝑃 𝑎 𝑡𝑚 𝑣2
ρ𝑔 ρ𝑔 2𝑔
+ 3
+0
or,
𝑣3 = 2𝑔ℎ 𝑡
Mould filling time (tf ) = (Volume of the casting) / ( 𝑨 𝒈 𝒗 𝟑)
where, 𝑨 𝒈 is the area of gate at point 3 (exit of sprue).
(ρ is density of liquid metal)
22. Casting
22
Gating design
Bottom gating design
Applying Bernoulli’s equation at points (1) and (3), we can
write:
𝑃 𝑎 𝑡𝑚
+ 0 + ℎ 𝑡 =
𝑃3
+ 3𝑣2
ρ𝑔 2𝑔
ℎ 𝑡 =
𝑝3 𝑣2
ρ𝑔 2𝑔
ρ𝑔
or,
+ 3 (As, 𝑝3 = 𝑃3− 𝑃 𝑎𝑡𝑚 = Gauge pressure; also
ℎ 𝑡 is assumed to be constant )
Now, applying Bernoulli’s equation at points (3) and (4), we
can write:
𝑃3
+ 0 + 0 = 𝑃 𝑎 𝑡𝑚
+ 0 +h
ρ𝑔 ρ𝑔
(Assuming that all the kinetic energy at point 3 is lost as the
liquid metal enters the mould, and pressure at point 4 is equal to
atmospheric pressure)
Then, we can write:
ρ𝑔
𝑝3
= h
23. Casting
23
Gating design
Bottom gating design
𝑝3
On substituting ρ𝑔
= h in the expression for ℎ 𝑡, we can
write:
𝑣3 = 𝑣 𝑔 = 2𝑔(ℎ 𝑡 − ℎ)
Assuming that the metal level in the mould move up through
a height dh in a time interval dt.
𝑑ℎ
=
𝐴 𝑔
2𝑔(ℎ 𝑡−ℎ) 𝐴 𝑚
dt
At t = 0, h = 0 and at t = 𝑡 𝑓 (filling time), h = ℎ 𝑚. On
integrating between these limits, we have:
𝑡 𝑓 = 𝐴 𝑚
𝐴 𝑔 2𝑔
1
2 ( ℎ 𝑡 − ℎ 𝑡 −ℎ 𝑚
If 𝐴 𝑚 and 𝐴 𝑔 are the cross section areas of mould and gate
respectively, then we can write:
𝐴 𝑚 dh = 𝐴 𝑔 𝑣 𝑔 dt
On substituting the value of 𝑣 𝑔in the above expression, we can
write:
24. Casting
24
Pouring
Aspiration effect in mould filling
For a mould made of permeable material (e.g., sand) care should be taken to ensure that the pressure anywhere in the
liquid metal stream does not fall below the atmospheric pressure. Otherwise, gases originating from baking of the
organic compounds in the mould will enter the molten metal stream, producing porous castings. This is known as the
ASPIRATION EFFECT.
Consider the vertical gating system as shown in the figure below:
Applying Bernoulli’s equation between points 2 and 3, we get:
𝑃2
+ 2
+ ℎ2 =
𝑃3
+ 3𝑣2 𝑣2
ρ𝑔 2𝑔 ρ𝑔 2𝑔
ρ𝑔
If we subtract 𝑃 𝑎𝑡𝑚
from both left and right hand side, we get:
𝑃2−𝑃 𝑎𝑡𝑚
+ 2
+ ℎ2 =
𝑃3−𝑃 𝑎𝑡𝑚
+ 3𝑣2 𝑣2
ρ𝑔 2𝑔 ρ𝑔 2𝑔
If we assume that the pressure at point 3 is equal to atmospheric pressure
and 𝑣3 = 𝑣2 (from continuity equation),then,
𝑃2− 𝑃 𝑎𝑡𝑚 = −ρ 𝑚 𝑔 ℎ2 (It is a negative pressure and hence the design is notacceptable)
25. Casting
Pouring
Aspiration effect in mould filling
In a limiting case, if 𝑃2− 𝑃 𝑎𝑡𝑚 = 0, then we can write:
𝑣2
3
2𝑔
= ℎ2+ 2𝑣2
2𝑔
𝑣2
2𝑔
On substituting, we get: 3
= ℎ2+ 3𝑅2 𝑣2
2𝑔
Or, 𝑅2 = 1 -
2𝑔ℎ2
𝑣2
3
Now, if we applying Bernoulli’s equation between points 1 and 3 with
3𝑝1 = 𝑝3 = 0, and 𝑣1= 0, we get 𝑣2 = 2gℎ 𝑡:
ℎ 𝑡 ℎ 𝑡
Thus, 𝑅2 = 1 - ℎ2
= ℎ 𝑐
or R =
ℎ 𝑐
ℎ 𝑡
Therefore, ideally, the sprue profile should be as shown by a solid line in
the adjacent figure. But tapered profile is easy to manufacture and is more
commonly used.
25
𝐴2
Also, 𝑣2 = 𝐴3
𝑣3 = R 𝑣3 (from continuityequation)
26. Casting
Pouring
Aspiration effect in mould filling
Another situation where aspiration effect comes into the
picture is associated with a sudden change in flow
direction.
The liquid metal stream contracts around a sharp corner
due to the momentum effect.
In vertical gating, this has got nothing to do with the
acceleration due to gravity.
The constricted region at point 2 is called vena contracta.
To avoid the creation of vacuum around point 2, the
mould is made to fit the vena contracta. Thus, sharp
changes in flow direction must be avoided.
26
27. Casting
27
where, 𝑣ҧ is the average velocity, L, D are diameter and length of the
conduit, and f is friction factor.
Energy loss in friction due to sudden contraction or enlargement of flow area can be written as:
Pouring
Effects of friction and velocity distribution
The velocity distribution within the conduit depends on the shape of a conduit and nature of flow (laminar or turbulent).
The velocity of a fluid in contact with any solid surface is zero and is maximum at the axis of the conduit.
In real fluids, the frictional losses are always present, especially when there is a sudden contraction in or an enlargement
of the flow cross sections.
The energy loss due to friction in a circular conduit (per
unit mass) can be written as:
𝒇𝟏𝑬 = 4 f 𝑳
𝒗ഥ𝟐
𝑫 𝟐
𝒇𝟐𝑬 = 𝒆
𝒗ഥ
𝒇 𝟐 𝑓where, 𝑣ҧ is the average velocity of the fluid in the smaller cross section, and 𝑒 is the frictional
loss factor
28. Casting
𝒗ഥ
Pouring
Effects of friction and velocity distribution
The non-uniform velocity distribution can be accounted for by modifying the kinetic energy term in the integrated
energy balance equation by replacing the 𝑣2 term by , where 𝑣ҧ is the average velocity and β is a constant whose value
β
is 0.5 for laminar flow and 1 for turbulent flow (only for circular conduit).
where, d is the diameter of the sprue and l is the length of the sprue (= ℎ2)
28
3 β d
2ght = vത2(
1
+ ef + 4 f
l
)
If we write the integrated energy balance equation between points 1
and 3 for vertical gating design, taking into account frictional losses
due to sudden contraction of fluid at point 2, we get:
Energy loss in friction due to sudden contraction or enlargement of flow area
can be written as:
𝑝1 𝑝3
𝑣ത2
𝜌𝑔
𝜌 𝑔
2𝛽
+ 0 + ℎ 𝑡 = + 3
+ 𝐸 𝑓1 +𝐸 𝑓2
If 𝑝1= 𝑝3, then we get:
29. Casting
29
Pouring
Effects of friction and velocity distribution
The new equation for velocity 𝑣3 can be written as:
The values of
𝐿
𝐷 𝑒
for different types of fittings can be found in standard tables.
𝑓1 3
2 l
d
𝐸 = 4f vത[ +
𝐿
𝐷 𝑒
]
And the discharge coefficient 𝐶 𝐷 can be written as:
1
( 1
+ ef + 4 f [ l
+
β d
𝐿
𝐷 𝑒 𝑞
])
2𝑔ℎ 𝑡
where, 𝐶 𝐷 (discharge coefficient) can be written as:
𝑣ҧ3 =
𝐶𝐷
1( 1
+ ef + 4 f l
)
β d
Please note that in this analysis, we have neglected the fluid velocity
(and hence the loss) between point 1 and 2.
If the sprue also has a bend or a fitting, then 𝐸 𝑓1can be written as:
30. 1.The development and introduction of new engineering materials which offer a higher degree of
strength and improved volume to weight ratio has applied a significant thrust on manufacturing industry
to develop new processes for making parts out of them. Products made from such materials are light
weight, durable, economical, and multi functional.
2.These materials are usually metal alloys such as titanium alloys (VH: 830- 3420 MPa), inconel 718
(VH: 2170-4120 MPa), and tungsten alloys (VH: 3430-4600MPa).
3.An alloy is a combination of a metal with at least one other metal or non-metal. Alloys are used because
they typically have enhanced mechanical or chemical properties.
3
Arrangement of atoms in a
lattice of a pure metal
Arrangement of atoms in a
lattice of an alloy
Why do we need Casting process ? Grain Engineering
31. Grain Engineering
What is a structure of a material ?
It is the organization of atoms of that material relative to each other.
A Crystal is an arrangement of atoms in a periodic repeatedarray.
In Crystalline materials, atoms are arranged in periodically repeated array(Silicon).
Crystal structures in metals/alloys are usually of four types : BCC, FCC, HCP, and Tetragonal.
A Crystalline material consisting of many grains of different orientation is called a polycrystalline material
(Ceramics).
At microscopic level, large groups of these atomic arrangements are considered as components of a
microstructure and determines properties of that material. A typical Microstructure is determined by grain size,
3 types of phases present, description of their shape, and sizedistribution.
BCC FCC HCP TETRAGONAL
32. 3
Metals have a crystalline structure - this is not usually visible but can be seen on galvanized lamp postsfor
example.
When a metal solidifies from the molten state, millions of tiny crystals start to grow.
The longer the metal takes to cool the larger the crystals grow. These crystals form the grains in the solid
metal. Each grain is a distinct crystal with its own orientation. The areas between the grains are known
as grain boundaries.
Within each grain, the individual atoms form a crystallinelattice.
When the metal is cold worked by forging, stamping or rolling its shape is permanently changed
(DEFORMED).
This is only possible because of defects (DISLOCATIONS) in the grain structure which move through the
crystal structure. These dislocations or slips in the grain structure allow the overall change in shape of the
metal.
Strong materials are those that can stop the
movement of the dislocations
Grain Engineering
33. Alloys
33 Strength VS Density Strength VSTemperature
Metal alloys exhibits superior thermal (melting point, thermal conductivity, specific heat capacity)
and mechanical (strength, hardness, toughness) properties as compared to puremetals.
Why ?
Alloying makes the other metal (solute) interacts with the crystal lattice of solvent material
and thereby, blocking the movement of dislocations.
34. 34
Control of the atomic arrangement and microstructure is possible through processes such as Casting,
Powder metallurgy, Working, and Heat treatment.
Microstructure
Schematic representation of a
polycrystalline multiphase metallic material
Role of microstructural constituents in
engineering metallic materials
Constituent Responsible for
Vacancies
Hardening,
diffusion processes
Dislocations
Plastic deformation,
recrystallization
Grains
Strengthening,
super plasticity
Grain boundaries Work hardening
Phase
arrangement
Controlling mechanical and
thermo-physical properties
Precipitates/
dispersoids
Increase in strength by the
interaction of moving
dislocations
35. 35
Grain Engineering
Processing Controls Properties
Processing
Microstructure
Properties
Temperature, Time
Size and distribution
of grains and phases
Strength and Ductility
37. Temperature VS Time profile of Pure Metals
Freezing takes place at a constant temperature
37
Enthalpy VS Temperature profile of PureMetals
Casting solidification
Freezing of Pure Metals
38. Casting solidification
Freezing of Pure Metals (Concept of undercooling)
Solidification curve shown in the previous slide is an idealization.
Actual solidification curve for pure metals looks as shown below:
The transformation from liquid to solid state begins only after it has cooled below its melting point. Once the
process initiates, the latent heat that is released by the metal raises the temperature back to its melting point.
Thereafter the temperature remains constant till the solidification is complete.
When a solid forms in a pool of liquid a new surface is created. This has a finite energy. This difference in free
energy acts as the driving force for solidification. Once this is large enough for a stable nucleus of solid to form the
process of solidification begins.
39. Freezing ofAlloys
Temperature VS Time profile of Ni-CuAlloy Enthalpy VS Temperature profile of Ni-CuAlloy
Freezing takes place at a range of temperature
39
40. Why does a liquid metal solidify?
• Essentially because the arrangement of atoms in a solid crystal is at a lower
energy* than that of the same atoms in a liquid state.
• Above the freezing point however, the liquid is more stable
• At the freezing point there is no driving force in either direction, i.e. we
have equilibrium, and no change of energy
• Hence, metals (pure) will always begin to freeze at a temperature lower
than its freezing temperature
• Because of this, it is possible to maintain liquids far below its freezing temperature under
controlled conditions (melts of Ni were shown to be stable when supercooled to -296 °C)
• Lets now quantify the forces affecting crystal nucleation (solidification)
• Two types of nucleation: homogenous and heterogeneous
40
Data taken from the slides of Dr. M. Law, IIT Kanpur
43. Homogenous nucleation of a new phase
47
3 𝑣𝑊 = −
4
𝜋𝑟3∆𝐺 + 4𝜋𝑟2 𝜎
• Consider the driving force for transformation, ∆𝐺
• Define a Gibbs energy of the old phase minus the new, ∆𝐺 𝑣, that is positive for a
natural transformation
• Consider the case of nucleation of a solid in a liquid with radius, 𝑟
• The work 𝑊 of forming the nucleus is
the balance of energy gained from the
transformation (a volume term) and the
energy required to form the surface
wherein ∆𝐺 𝑣 is the energy per unit volume,
and 𝜎 is the surface tension
Data taken from the slides of Dr. M. Law, IIT Kanpur
44. Homogenous nucleation of a new phase
48
3 𝑣𝑊 = −
4
𝜋𝑟3∆𝐺 + 4𝜋𝑟2 𝜎
𝑊∗ = −
3
𝜋
4 2𝜎
∆𝐺𝑣
3
∆𝐺 𝑣 + 4𝜋
2𝜎
∆𝐺𝑣
2
𝜎 =
16𝜋
3
𝜎3
∆𝐺 𝑣
2
𝑟∗ → ∞ and 𝑊∗ → ∞
There exists a critical radius of the nucleus, 𝑟∗
for 𝑟 > 𝑟∗ - the nucleus will grow;
for 𝑟 < 𝑟∗ - the nucleus will shrink
Can evaluate, 𝑟∗, by evaluating the maxima:
𝑑𝑊
= 0 → 𝑟∗ =
2𝜎
𝑑𝑟 ∆𝐺 𝑣
At 𝑟∗, can evaluate 𝑊∗
Recalling: 𝑇 → 𝑇 𝑒; ∆𝐺 𝑣→ 0;
i.e. nucleation becomes
impossible
Data taken from the slides of Dr. M. Law, IIT Kanpur
45. Calculating phase fractions (lever rule)
49
𝑉𝑆= 1 − 𝑉𝐿
𝐶 = 𝐶 𝑆(1 − 𝑉 𝐿) + 𝐶 𝐿 𝑉 𝐿
𝐶 = 𝐶 𝑆+ 𝑉 𝐿(𝐶 𝐿 − 𝐶 𝑆)
𝑉 =
𝐶 − 𝐶𝑆
𝐶 𝐿 − 𝐶 𝑆
Volume fractions: 𝑉𝑆+ 𝑉𝐿= 1
For a composition 𝐶: 𝐶 = 𝐶 𝑆 𝑉 𝑆+ 𝐶 𝐿 𝑉 𝐿
Volume
fraction of a 𝐿
liquid:
𝑆𝑉 =
𝐿𝐶 − 𝐶
𝐶 𝐿 − 𝐶 𝑆
Can similarly, show the volume
fraction of solid to be:
Data taken from the slides of Dr. M. Law, IIT Kanpur
46. Calculating phase fractions (lever rule)
50
𝑉𝑆
=
𝐶 𝐿 − 𝐶
𝑉𝐿 𝐶 − 𝐶 𝑆
𝐿𝑉 =
𝐶 − 𝐶𝑆
𝐶 𝐿 − 𝐶 𝑆
Volume fraction of a liquid:
𝑆
=
𝐶 𝐿 − 𝐶
𝐶 𝐿 − 𝐶 𝑆
Volume fraction of solid: 𝑉
𝑉𝑆 𝐶 − 𝐶 𝑆 = 𝑉 𝐿(𝐶 𝐿 − 𝐶 𝑆)
Lever rule
Data taken from the slides of Dr. M. Law, IIT Kanpur
47. Freezing of Alloys
Temperature distribution in the solidifying metal alloy (when thermal properties
(thermal conductivity and specific heat capacity) are not a function of temperature)
51
49. 53
Temperature distribution during casting solidification
Type of cast structure formed depends on the rate of heat extraction from the liquid metal
Temperature distribution in the solidifying metal alloy (when thermal properties
(thermal conductivity and specific heat capacity) are a function of temperature)
51. 55
Freezing of Pure Metals and Alloys
Columnar dendritic to Equiaxed grain transition
52. Solidification patters for gray cast iron and steel
56
KalpakjianandSchmid
Data taken from the slides of Dr. M. Law, IIT Kanpur
53. Observations on an actual sand casting
57
22 in
~7 in
Metal
~15 in
Green sand
Casting size: 7 in x 7 in x 21 in
Only one-half section shown
with no risers, gates etc.
Adapted fromFlinn
Distancefrominterface,in
0 10 20 30
Time, min
40 50
1
2
3
Centerline St art of freeze End of freeze
• Steel does not freeze with a smooth advancing front, but rather a start-of-freeze
wave passes from the surface to the center, followed much later, by an end-of-
freeze wave
• Between these waves, there is a mushy zone, i.e. solid + liquid
Data taken from the slides of Dr. M. Law, IIT Kanpur
54. Influence of chiller on solidification
58
Time, min
Distancefrominterface,in
0 10 20 30 40 50
1
2
3
Centerline
Sand
start
Sand
end
Chill
start
Chill
end
Data taken from the slides of Dr. M. Law, IIT Kanpur
55. Centerline feeding resistance (CFR)
59
𝐶𝐹𝑅𝑠 𝑎 𝑛
48
=
48 − 24
= 50%
𝐶𝐹𝑅𝑐ℎ𝑖𝑙𝑙𝑙
10
=
10 − 8
= 20%
𝐶𝐹𝑅 =
time during which crystals are forming at the centerline
× 100
total solidification time of casting
or
𝐶𝐹𝑅 =
end of freeze time at centerline − start of freeze time at centerline
× 100
total solidification time of casting
Data taken from the slides of Dr. M. Law, IIT Kanpur
56. Casting
Different scales in Casting Solidification
Micro scale : Nucleation, Grain growth, Microstructure formation
Meso scale : Two phase solid-liquid mushy zone
Macro scale : Transport phenomena ( heat, mass, fluid flow), Motion of solid grains (multiphase flow), Shrinkage
60
58. Rate of Solidification
Flux laws: Diffusive transport of energy, mass, and momentum can be described through flux laws whose
fundamental forms are as follows:
𝐴𝑟𝑒𝑎
Flux =
𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒
= 𝑡𝑟𝑎𝑛𝑠𝑝𝑜𝑟𝑡 𝑝𝑟𝑜𝑝𝑒𝑟𝑡𝑦 𝑋 𝑝𝑜𝑡𝑒𝑛𝑡𝑖𝑎𝑙 𝑔𝑟𝑎𝑑𝑖𝑒𝑛𝑡
The three laws describing diffusive transport are:
Energy:
Conduction: 𝑞 = 𝑘∇𝑇 𝑓𝑜𝑢𝑟𝑖𝑒𝑟′ 𝑠 𝑙𝑎𝑤
Convection: 𝑞 = ℎ 𝑇 𝑡 − 𝑇 0 𝑁𝑒𝑤𝑡𝑜𝑛′ 𝑠 𝑙𝑎𝑤 𝑜𝑓 𝑐𝑜𝑜𝑙𝑖𝑛𝑔
𝑥 𝜕
Momentum: 𝜏 = −𝜇 𝜕𝑉 𝑌
(𝑁𝑒𝑤𝑡𝑜𝑛′ 𝑠 𝑙𝑎𝑤 𝑜𝑓 𝑣𝑖𝑠𝑐𝑜𝑠𝑖𝑡𝑦)
ρ𝑉𝐷
μ
Flow:
Reynold’s number: Re =
Laminar flow: Re < 2000
Laminar/turbulent transition: 2000 < Re < 20000
Radiation: 𝑞 =∊ σ 𝑇4 − 𝑇4 (𝑆𝑡𝑒𝑓𝑎𝑛 − 𝐵𝑜𝑙𝑡𝑧𝑚𝑎𝑛′ 𝑠 𝐿𝑎𝑤)
1 2
Severe turbulence: Re > 20000
Mass (Species): 𝐽 𝐴 = −𝐷 𝐴𝐵 𝛻𝐶 𝐴 (𝐹𝑖𝑐𝑘′ 𝑠 𝐿𝑎𝑤)
Turbulence is harmful to gating system as it
causes air entrapment.
59. Rate of Solidification
The heat rejected by the liquid metal is dissipated
through the mould wall.
The heat released as a result of cooling and
solidification of liquid metal pass through different
layers.
The thermal resistances which govern the entire
solidification process are those of liquid, the
solidified metal, the metal-mould interface, the
mould, and the ambient air.
Temperature distribution in different layers in casting solidification of
pure metals
60. Rate of Solidification
Solidification of a large casting in an insulating mould (sand or investment casting)
It is assumed that entire resistance is offered by the mould only.
Freezing time of the casting is calculated by considering only the
resistance of region 2.
Initial mould temperature is taken to be θ 𝑜
The mould thickness is assumed to be extended upto infinity in x
direction.
61. Rate of Solidification
θ 𝑥(t) : temperature at a distance x from the mould face at any instant t.
θ 𝑥(t) = θ 𝑜 + (θ 𝑓 − θ 𝑜)[1 – erf (
𝑥
2 α𝑡
)]
Solidification of a large casting in an insulating mould (sand or investment casting)
At time t = 0, the liquid metal at temperature θ 𝑝 is poured into the mould.
It is assumed that metal just in contact with the mould solidifies instantaneously.
This means that the temperature of the mould face is raised to θ 𝑓 (freezing
temperature of the metal) at t = 0 and is maintained at that value till the
completion of solidification.
The temperature distribution within the mould at a subsequent time t
(assuming 1 D heat conduction in x- direction) is given by:
ρ𝑐
k is the thermal conductivity, ρ is the density, and ‘c’ is the specific heat of mould material.
α = 𝑘
is the thermal diffusivity of mould material.
0𝜋
𝑑
erf is the error function such that erf (z) = 2
𝑧
𝑒−𝑥2
dx and 𝑑
erf (z) =
2
𝜋
𝑒−𝑧2
.
62. Rate of solidification
66
𝑇 𝑥, 𝑡 = 𝑇0+ (𝑇 𝑚 − 𝑇0) 1 − 𝑒𝑟𝑓
𝑥
2 𝛼 𝑚 𝑡
𝑒𝑟 𝑓 𝑢 =
2
𝜋
𝑢3 𝑢5 𝑢7
𝑢 −
3.1!
+
5.2!
−
7.3!
+ ⋯
• Temperature at any time and distance in the mold:
wherein, the error function expresses as a convergent series is:
• From (a), it is evident that the temperature at 𝑥 at time 𝑡:
Increases with an increase mold temp., interface temp, and thermal diffusivity, and
Decreases as the distance 𝑥 from the interface increases
• Todetermine the rate of heat flow at the interface, the flux becomes:
𝐽 = 𝐾
𝑑𝑇
ቤ
𝑑𝑥 𝑥=0
Data taken from the slides of Dr. M. Law, IIT Kanpur
63. Rate of Solidification
Solidification of a large casting in an insulating mould (sand or investment casting)
Now the rate of heat flow through the mould face at any time instant (t) can be written as:
Heat rejected by liquid metal to solidify is given by: 𝑄 𝑅 = ρ V [L + 𝑐 𝑚(θ 𝑝 − θ 𝑓)].
where, L is the latent of liquid metal, 𝑐 𝑚 is the specific heat, V is total volume
of the casting, and ρ is the density.
𝜕
𝑄ሶ= -k A
𝜕θ 𝑥
( at x = 0) where, A is the cross sectional area of the mould – metal interface.
Using the expression for error function, we can write:
𝑄ሶ =
𝑘𝐴(θ 𝑓−θ 𝑜)
𝜋α𝑡
The total quantity of heat flow across the face of mould upto a certain time 𝑡 𝑜 is:
0
𝑡 𝑜
ሶ𝑄 𝑡 𝑜 = න 𝑄 𝑑𝑡 =
2𝑘𝐴(θ 𝑓 − θ 𝑜)
𝜋α
𝑡 𝑜
64. Rate of Solidification
Solidification of a large casting in an insulating mould (sand or investment casting)
Total solidification time (𝑡 𝑠) can be obtained by equating the heat rejected by the liquid metal and the total
quantity of heat through the mould.
γ =
2𝑘(θ 𝑓−θ 𝑜)
ρ 𝜋α [L + 𝑐 𝑚(θ 𝑝−θ 𝑓)]
2
Or, 𝑡 𝑠 = γ
𝑉
𝑆
where, γ is mould constant and is expressed as:
2𝑘𝐴(θ 𝑓−θ 𝑜)
𝜋α 𝑠 𝑚 𝑝 𝑓
2
𝑡 = ρ V [L + 𝑐 (θ − θ )]
This is known as Chvorinov’s rule for calculating total casting solidification time
with sand mould.
Mould constant depends on mould material, thermal properties of casting
material, and pouring temperature relative to freezing point.
Mould constant does not depends on the shape and size of the casting.
65. Rate of Solidification
Case of 1D heat flow
Solidification of a large casting in a permanent mould (die casting)
In this type of casting process, the heat flow is controlled significantly by the thermal resistance of the mould-
metal interface.
Assuming no superheat, the temperature distribution in this case is shown in the figure below:
66. Rate of Solidification
ρ 𝐿
Integrating the above equation with δ = 0 at t = 0, we get : δ (t) =
ℎ 𝑓 (θ 𝑓−θ 𝑜)
t
Solidification of a large casting in a permanent mould (die casting)
The rate of heat flow through the interface is given by:
𝑄ሶ= ℎ 𝑓 A (θ 𝑓 − θ 𝑜)
where, ℎ 𝑓 is the film heat transfer coefficient of the interface
and A is the surface area of theinterface.
If the solidification front at this instant is at a distance δ from
the mould face, then, on solidification,
rate of heat released = ρAL
𝑑δ
𝑑𝑡
.
𝑑
Now, rate of heat flow through the interface = rate of heat released.
Then, 𝑑δ =
ℎ 𝑓 (θ 𝑓−θ 𝑜)
ρ L
67. Rate of Solidification
ℎ 𝑓 (θ 𝑓−θ 𝑜)
where,
ρ 𝐿
is the mould constant in case of die casting.
Solidification of a large casting in a permanent mould (die casting)
To calculate the total solidification time in this type of casting:
We equate rate of heat flow through the interface = rate of heat released
ℎ 𝑓 A (θ 𝑓 − θ 𝑜) 𝑡 𝑠 = 𝑄 𝑅 = ρ V [L + 𝑐 𝑚(θ 𝑝 −θ 𝑓)]
If we substitute θ 𝑝 = θ 𝑓 in the above expression, we get:
𝑡 𝑠 = ρ 𝐿 𝑉
ℎ 𝑓 (θ 𝑓−θ 𝑜) 𝐴
68. Rate of Solidification
Solidification with constant casting surface temperature
If a large, slab shaped casting (say, of steel) is produced in a
thin, water cooled mould made of metal (say, copper) having
much higher conductivity than the solidified casting, then the
thermal resistance provided by the solidifying metal itself is
significant. In this case, the predominant thermal resistance is
offered by region 4.
Assumptions:
1. Thermal resistance of all other regions is neglected.
2. Mould-metal interface temperature is assumed to
remain constant at its initial value θ 𝑜.
3. θ 𝑓 is freezing temperature of the metal and is taken as
pouring temperature.
4. δ (t) is depth of solidification at any time instant (t).
5. Solidification time (𝑡 𝑠) is calculated from δ (t) = h/2,
where h is the thickness of the slab being cast.
(Assumption for idealizing the problem as 1D heat flow
problem).
69. Rate of Solidification
Solidification with constant casting surface temperature
The temperature profile within the range 0 < x < δ (t) is given by:
= erf
θ − θ 𝑠 𝑥
θ∞ − θ 𝑠 2 α 𝑠 𝑡
where, α 𝑠 is the thermal diffusivity of the solidified metal and
θ∞ is a constant of integration. At x = δ (t), θ = θ 𝑓 . On
substituting these values in the above expression, we get:
δ (t)
θ∞−θ 𝑠 2 α 𝑠 𝑡
θ 𝑓−θ 𝑠
= erf = constant = λ=erf (ζ)
A
B
Hence, δ (t) = 2ζ α 𝑠 𝑡 C
Note that the depth of solidification varies as the square root of solidification of time.
Now, considering the rate of energy flow balance at the solid-liquid interface, we have:
70. Rate of Solidification
Solidification with constant casting surface temperature
𝑘 𝑠
𝜕θ
𝑎𝑡 𝑥 = δ = ρ L
𝑑δ
𝜕 𝑥
𝑑
𝜕
Now, substituting the value of 𝜕θ
in D, we get:
Substituting in the above expression, the values of (θ∞ − θ 𝑠) and δ from B and C respectively, weget:
where, 𝑘 𝑠 is the thermal conductivity of the solidified metal
and L is the latent heat of fusion of the cast metal.
From the above equation, we can write:
𝜕θ
= (θ∞ − θ 𝑠)
𝑑
[erf
𝑥
]
𝜕𝑥 𝑑𝑥 2 α 𝑠 𝑡
∞ 𝑠 2 α 𝑠 𝑡 𝜋
= (θ − θ )
1 2
𝑒𝑥𝑝 [−
𝑥
2 α 𝑠 𝑡
2
]
D
(θ∞ −θ 𝑠)
𝑘 𝑠 2
𝑒𝑥𝑝 [−
𝑥
2
] = ρ L
𝑑δ
2 α 𝑠 𝑡 𝜋 2 α 𝑠 𝑡 𝑑𝑡
71. Rate of Solidification
Solidification with constant casting surface temperature
𝑘 ρ𝑐𝑠 𝑠 erf(ζ)
𝜋
(θ 𝑓−θ 𝑠) 1
𝑒−ζ2
= ρ L ζ α 𝑠
1
𝑡
or, 𝑠𝑡 =
ℎ2
16ζ2α 𝑠
Or, ζ𝑒ζ2
erf (ζ) =
(θ 𝑓−θ 𝑠) 𝑐 𝑠
𝜋 𝐿
The solution for the above equation for ζ can be found by trial and
error method of by plotting the graph for ζ𝑒ζ2
erf (ζ) for different
values of ζ.
After calculating the value of ζ , solidification time (𝑡 𝑠) can be
calculated as follows:
𝑠 𝑠2ζ α 𝑡 =
ℎ
2
72. Rate of Solidification
Solidification with predominant resistance in mould and solidified metal
If a large, slab shaped casting (say, of steel) is produced in a
thick mould without any water cooling, then the mould-metal
interface temperature can no longer be assumed to remain at its
initial value. The significant thermal resistance is offered by
region 2 and 4 and the resulting temperature profile is shown as:
For this case, the expression for solidification time (𝑡 𝑠) is
same as that in previous case:
𝑠𝑡 =
ℎ2
16ζ2α 𝑠
However, the value of ζ is calculated from the following
expression:
ζ𝑒ζ2
erf [(ζ) + ϕ] =
(θ 𝑓−θ 𝑜) 𝑐 𝑠
𝜋 𝐿
where, ϕ = 𝑘 𝑠ρ 𝑚 𝑐 𝑠
= constant. (𝑘, ρ, 𝑐 are the properties of mouldmaterial)
𝑘ρ𝑐
73. Rate of Solidification
Practice problem: A cylindrical riser must be designed for a sand-casting mould. The casting itself is a steel
rectangular plate with dimensions 7.5 cm x 12.5 cm x 2.0 cm. Previous observations have indicated that the total
solidification time (T) for this casting = 1.6 min. The cylinder for the riser will have a diameter-to-height ratio=1.0.
Determine the dimensions of the riser so that it’s T = 2.0 min.
Solution: First determine the V/A ratio for the plate. Its volume V = 7.5 x 12.5 x 2.0= 187.5 cm3 , and its surface
area A= 2 (7.5 x 12.5 + 7.5 x 2.0 + 12.5 x 2.0) = 267.5 cm2 . Given that T = 1.6 min, we can determine the mould
constant C, using a value of n = 2 in the equation.
Next we must design the riser so that its total solidification time is 2.0 min, using the same value of mould constant.
The volume of the riser is given by:
74. Rate of Solidification
Practice problem: A cylindrical riser must be designed for a sand-casting mould. The casting itself is a steel
rectangular plate with dimensions 7.5 cm x 12.5 cm x 2.0 cm. Previous observations have indicated that the total
solidification time (T) for this casting = 1.6 min. The cylinder for the riser will have a diameter-to-height ratio=1.0.
Determine the dimensions of the riser so that it’s T = 2.0 min.
Solution:
75. Concept of Fluidity
Fluidity in casting terms is the distance to which a metal, when cast at a given temperature will flow in a given test
mould before it is solidified.
Fluidity is therefore a length measured in meters or millimeters and must not be confused with the physicists’
definition of fluidity which is a reciprocal of viscosity.
Fluidity test forAluminum
Poor Fluidity Better Fluidity Best Fluidity
76. Concept of Fluidity
𝜋𝑎2ρ 𝑚 𝐿 𝑓Δ𝐻
= 𝜋𝑎2ρ 𝑚 𝑉Δ𝐻
Consider a metal at its melting point Tm, poured in a channel of radius ‘a’ and flowing with an average velocity ‘V’.
The metal solidifies at a distance Lf in time ‘t’ by loosing the latent heat to themould.
The rate of heat dissipation by solidifying metal is equal to the rate at which heat is transferred between mould-metal
interface. The thermal resistance at the mould-metal interface is specified by the heat transfer coefficient ‘h’.
Heat lost per unit time when the length Lf solidifies in time ‘t’ can be expressed as:
A
𝑡
where, Δ𝐻 is the Enthalpy of liquid metal in kJ/kg.
Heat transferred through convection across mould-metal interface in time ‘t’ considering the resistance betweenmold-
metal interface only, is given by:
2𝜋 𝑎 𝐿 𝑓h (𝑇 𝑚 − 𝑇 𝑜) B
(B) is written considering the heat from the solidified metal is convected from the curved surface area of the solidified
metal.
By equating A and B, weget:
77. Concept of Fluidity
𝑳 𝒇 =
ρ 𝑠 𝑽Δ𝐻𝒂
2h ( 𝑇 𝑚−𝑇 𝑜)
(This expression is valid for the solidification of pure metals)
𝑠where, 𝑓 𝑐𝑟 is the critical fraction of solid at which the fluidstops.
Fluidity of alloys when solid grains are moving, can be expressed as:
𝐿 𝑓 = 𝑓 𝑐𝑟V 𝑡
𝑠 𝑠
78. Risers
Production of sound castings requires risers to satisfy two independentrequirements:
1.Riser size: Riser must freeze after the casting, i.e. the modulus (V/A) of the riser must be greater
than that of the casting
2.Riser placement should be such as to minimize the CFR, i.e. the centreline feeding resistance.
Riser design using Chvorinov’s idea
For the riser to solidify after the casting: 𝑡𝑠𝑅 > 𝑡𝑠𝐶
For sand castings, we can write: 𝑘 𝑅
𝑉
𝑆
2
> 𝑘 𝐶
𝑉
𝑆
2
𝑅 𝐶
OR 𝑘 𝑅 𝑀 2
𝑅> 𝑘 𝐶 𝑀 2
𝐶
The modulus (M) for some common shapes is as follows:
a
a
a
Cube
𝑀 =
𝑎
6 L t
w
Bar
𝑀~
𝑤𝑡
2(𝑤 + 𝑡) L
w
t
Plate
𝑀~
𝑡
2
79. Risers
For a given shape of the riser, the dimensions of the riser should, however, be chosen so as to give a minimum A/V
ratio, and the minimum volume should be ensured from the shrinkage consideration.
It must be remembered that a liquid metal flows from the riser into the mould only during the early part of the
solidification process.
This necessitates the minimum volume of the riser to be approximately three times that dictated by the shrinkage
consideration alone.
To check the adequacy of the riser size for a steel casting, Caine’s relationship is normally used. The solidification time
is proportional to the square of the ratio volume/surface area.
For a given casting-riser
combination, if the point
falls to the right of the
curve, the adequacy of
the riser is ensured.
80. Risers
Plate with one central riser Maximum distance between two consecutive risers
Placement of risers
For a steel plate of up to 100 mm thickness, one central riser is satisfactory if the maximum feeding distance is less
than 4.5 times the plate thickness. The feeding distance should be measured from the edge of the riser.
It should be noted that, of the total distance 4.5t, the riser gradient prevails up to a distance 2t, whereas the end-wall
gradient prevails in the remaining distance 2.5t.
Thus, the maximum distance between the edges of two consecutive risers is 4t.
81. Risers
Placement of risers and chill blocks
Placement of risers
A bar of square cross-section with sides measuring 50-200 mm can be fed satisfactorily from a single riser, up to a
maximum distance of 30 √s, where s is the side of the square expressed in mm. The maximum distance between the
edges of two consecutive risers is found to be 1.2s
The presence of a chill in the mould increases the feeding distance of the riser. This is achieved by providing a
sharp thermal gradient with consequent decrease in the feeding resistance. It is obvious that the chill should be
placed at the ends if a single riser is used. For more than one riser, the chill should be placed midway between the
two risers.
82. Design considerations
Gating system
Top gating design- causes turbulence Bottom gating design- prevents turbulence
Pouring basin
BAD design
Conical basin
BETTER design
Offset basin
BEST design
Offset-stepped basin
87. Defects in Castings
1. Shift or Mismatch
Causes:
(i) Improper alignment of upper and lower part during mould preparation.
(ii) Misalignment of flask (a flask is type of tool which is used to contain a mould in metal casting. it may be square,
round, rectangular or of any convenient shape.)
Remedies:
(i) Proper alignment of the pattern or die part, moulding boxes.
(ii) Correct mountings of pattern on pattern plates.
(iii) Check the alignment of flask.
88. Defects in Castings
2. Swell
Causes:
(i) Defective or improper ramming of
the mould.
Remedies:
(i) The sand should be rammed properly
and evenly.
3. Blowholes
Causes:
(i) Excessive moisture in the sand.
(ii) Low Permeability of the sand.
(iii) Sand grains are too fine.
(iv) Too hard rammed sand.
(v) Insufficient venting is provided.
Remedies:
(i) The moisture content in the sand must be controlled and kept at desired level.
(ii) High permeability sand should be used.
(iii) Sand of appropriate grain size should be used.
(iv) Sufficient ramming should be done.
(v) Adequate venting facility should be provided.
89. Defects in Castings
4. Drop
Drop defect occurs when there is cracking on the upper
surface of the sand and sand pieces fall into the molten
metal.
Causes:
(i) Soft ramming and low strength of sand.
(ii)Insufficient fluxing of molten metal. Fluxing means
addition of a substance in molten metal to remove
impurities. After fluxing the impurities from the molten
metal can be easily removed.
(iii)Insufficient reinforcement of sand projections in the
cope.
Remedies:
(i) Sand of high strength should be used with proper
ramming (neither too hard nor soft).
(ii)There should be proper fluxing of molten metal, so the
impurities present in molten metal is removed easily before
pouring it into the mould.
(iii)Sufficient reinforcement of the sand projections in the
cope.
5. Metal penetration
These casting defects appear as an uneven and
rough surface of the casting. When the size of sand
grains is larges, the molten fuses into the sand and
solidifies giving us metal penetration defect.
Causes:
(i)It is caused due to low strength, large grain size,
high permeability and soft ramming of sand.
Because of this the molten metal penetrates in the
moulding sand and we get rough or uneven casting
surface.
Remedies:
(ii)This defect can be eliminated by using high
strength, small grain size, low permeability and soft
ramming of sand.
90. Defects in Castings
6. Pinholes
They are very small holes of about 2 mm in size which appears on the surface of the casting. This defect happens
because of the dissolution of the hydrogen gases in the molten metal. When the molten metal is poured in the mold
cavity and as it starts to solidify, the solubility of the hydrogen gas decreases and it starts escaping out the molten metal
leaves behind small number of holes called as pinholes.
Causes:
(i) Use of high moisture content sand.
(ii) Absorption of hydrogen or carbon monoxide gas by molten metal.
(iii) Pouring of steel from wet ladles or not sufficiently gasified.
Remedies:
(i) By reducing the moisture content of the moulding sand.
(ii) Good fluxing and melting practices should be used.
(iii) Increasing permeability of the sand.
(iv) By doing rapid rate of solidification.
91. Defects in Castings
7. Shrinkage cavity
The formation of cavity in the casting due to volumetric
contraction is called as shrinkage cavity.
Causes:
(i) Uneven or uncontrolled solidification of molten metal.
(ii) Pouring temperature is too high.
Remedies:
(i)This defect can be removed by applying principle of
directional solidification in mold design.
(ii)Wise use of chills (a chill is an object which is used to
promote solidification in a specific portion of a metal
casting) and padding.
8. Cold shut
It is a type of surface defects and a line on the surface can be
seen. When the molten metal enters into the mold from two
gates and when these two streams of molten metal meet at a
junction with low temperatures than they do not fuse with
each other and solidifies creating a cold shut (appear as line
on the casting). It looks like a crack with round edge.
Causes:
(i) Poor gating system
(ii) Low melting temperature
(iii) Lack of fluidity
Remedies:
(i) Improved gating system.
(ii)Proper pouring temperature.
soft ramming of sand.
92. Defects in Castings
9. Misrun
When the molten metal solidifies before completely filling
the mold cavity and leaves a space in the mold called as
misrun.
Causes:
(i) Low fluidity of the molten metal.
(ii)Low temperature of the molten metal which decreases
its fluidity.
(iii) Too thin section and improper gating system.
Remedies:
(i)Increasing the pouring temperature of the molten metal
increases the fluidity.
(ii) Proper gating system
(iii) Too thin section is avoided.
10. Slag inclusion
This defect is caused when the molten metal containing slag
particles is poured in the mold cavity and it gets solidifies.
Causes:
(i) The presence of slag in the molten metal
Remedies:
(i) Remove slag particles form the molten metal before
pouring it into the mold cavity.
93. Defects in Castings
11. Hot tears or hot cracks
When the metal is hot it is weak and the residual stress (tensile) in the material cause the casting fails as the molten
metal cools down. The failure of casting in this case is looks like cracks and called as hot tears or hot cracking.
Causes
(i) Improper mold design.
Remedies
(i) Proper mold design can easily eliminate these types of casting defects.
(ii) Elimination of residual stress from the material of the casting.
96. Dry sand mould casting
Dry sand casting is a sophisticated form of green sand process, in which the sand mold is baked at a given temperature
to make it stronger.
This process in mostly used in large foundries to produce big ferrous and non-ferrous castings like engine blocks,
construction parts, etc.
Dry sand casting ensures precise size and perfect dimensions of the metal casting products.
Advantages :
•Complicate designs required for engines and automobiles can
be designed with relative ease
•Accuracy is terms of dimensions, size, designs, is the main
benefit
• Though an expensive process, here accuracy is maintained in
every respect
• A process is favoured by large foundries.
97. Shell mould casting
mass• Shell moulding can be completely automated for
production.
• Low tooling cost, Little scrap generated.
•Very large parts and complex shapes can be produced.
•Scrap can be recycled.
•Short lead time possible.
It is an expendable mold casting process that uses a resin covered sand to form the mould.
As compared to sand casting, this process has better dimensional accuracy, a higher productivity rate, and lower labour
requirements.
It is used for small to medium parts that require high precision.
In shell mold casting, the mold is a thin-walled shell created from
applying a sand-resin mixture around a pattern. The pattern, a metal
piece in the shape of the desired part, is reused to form multiple
shell moulds.
Advantages :
99. Investment casting
Investment casting is a manufacturing process in which a wax pattern is coated with a refractory ceramic material.
Once the ceramic material is hardened its internal geometry takes the shape of the casting.
The wax is melted out and molten metal is poured into the cavity where the wax pattern was.
The metal solidifies within the ceramic mold and then the metal
casting is broken out. This manufacturing technique is also
known as the lost wax process.
Advantages :
• Can form complex shapes and fine details
•Many material options
•High strength parts
•Very good surface finish and accuracy
•Little need for secondary machining
101. Continuous casting process
Continuous casting, also known as strand casting, is
the process where a metal is heated until it liquefies.
The molten metal is then allowed to solidify until it
becomes a semi-finished slab that is later rolled in
the finishing mill. It is used to cast metals of
uninterrupted lengths.
In this process, the molten metal is continuously
supplied to the mould. The mould has an
indeterminate length. When the molten metal is cast
through a mould, it keeps travelling downward
increasing in its length as the time passes by.
The molten metal is continuously passed through the
mould, at the same rate to match the solidifying
casting. This results in casting of long strands of
metal.
The whole process of continuous casting is a
precisely deliberated process that can produce
astounding results.
102. Gravity die casting
Gravity Die Casting is a permanent mould casting process, where the molten metal is poured from a vessel or ladle into
the mould. The mould cavity fills with no force other than gravity, filling can be controlled by tilting the die.
The heated mould is coated with a die release agent. The release agent spray also has a secondary function in that it aids
cooling of the mould face after the previous part has been removed from the die.
Molten alloy is poured into channels of the tool to allow the material to fill all the extremities of the mould cavity. The
alloy is either hand poured using steel ladles or dosed using mechanical methods.
Advantages :
• Good dimensional accuracy.
• Smoother cast surface finish than sand casting.
• Improved mechanical properties compared to sand casting.
• Thinner walls can be cast compared to sand casting.
• Reverse draft internal pockets and forms can be cast in using
preformed sand core inserts.
• Steel pins and inserts can be cast into the part.
104. Centrifugal casting
In centrifugal casting, a permanent mold is rotated continuously at high speeds (300 to 3000 rpm) as the molten metal is
poured.
The molten metal spreads along the inside mold wall, where it solidifies after cooling. The casting is usually a fine-
grained casting with an especially fine-grained outer diameter, due to the rapid cooling at the surface of the mold.
• It provides dense metal and high mechanical properties.
• Unidirectional solidification can obtain up to a certain
thickness.
• It can use for mass production.
• No cores are required for cast hollow shapes like tubes etc.
• Gating system and runner are totally eliminated.
• All the impurity like oxide or other slag particles, segregated
at centre from where it can easily remove.
• It required lower pouring temperature thus save energy.
• Lower casting defects due to uniform solidification.
Lighter impurities and inclusions move towards the inside diameter and can be machined away following the casting.
Advantages :
