1. Chapter 2-FUNDAMENTALS OF METAL CASTING
Overview of Casting Technology
Heating and Pouring
Solidification and Cooling
Prepared by:
Dr. Tan Soo Jin

2. Solidification Processes
Starting work material is either a liquid or is in a highly plastic condition, and a part is created through solidification of the material
Solidification processes can be classified according to engineering material processed:
Metals
Ceramics, specifically glasses
Polymers and polymer matrix composites (PMCs)

3. Classification of solidification processes

4. Casting of Metals Melt the metal Pour it into a mold Let it freeze
Process in which molten metal flows by gravity or other force into a mold where it solidifies in the shape of the mold cavity
The term casting also applies to the part made in the process
Steps in casting seem simple:
Melt the metal
Pour it into a mold
Let it freeze

5. Capabilities and Advantages of Casting
Can create complex part geometries
Can create both external and internal shapes
Some casting processes are net shape (no further manufacturing operations are required to achieve the required geometry and dimensions of the parts.); others are near net shape
Can produce very large parts (weighing more than a hundred tons)
Some casting methods are suited to mass production

6. Disadvantages of Casting
Different disadvantages for different casting processes:
Limitations on mechanical properties (porosity)
Poor dimensional accuracy and surface finish for some processes; e.g., sand casting
Safety hazards to workers due to hot molten metals
Environmental problems

7. Parts Made by Casting
Big parts
Engine blocks and heads for automotive vehicles, wood burning stoves, machine frames, railway wheels, pipes, church bells, big statues, pump housings
Small parts
Dental crowns, jewelry, small statues, frying pans

8. Overview of Casting Technology
Casting is usually performed in a foundry
Foundry = factory equipped for making molds, melting and handling molten metal, performing the casting process, and cleaning the finished casting
Workers who perform casting are called foundrymen

9. The Mold in Casting
Contains cavity whose geometry determines part shape
Actual size and shape of cavity must be slightly enlarged to allow for shrinkage of metal during solidification and cooling
Different metals undergo different amounts of shrinkage (so the mold cavity must be designed for particular metal to be cast if dimensional accuracy is critical.)
Molds are made of a variety of materials, including sand, plaster, ceramic, and metal

10. Open Molds and Closed Molds
Two forms of mold: (a) open mold (liquid metal simply pured) and (b) closed mold for more complex mold geometry with gating system leading into the cavity

11. Two Categories of Casting Processes
Expendable mold processes – use an expendable mold which must be destroyed to remove casting
Mold materials: sand, plaster, and similar materials, plus binders
Permanent mold processes – use a permanent mold which can be used to produce many castings
Made of metal (or, less commonly, a ceramic refractory material that can withstand the high temperature of the casting operation.)

12. Advantages and Disadvantages
More intricate geometries are possible with expendable mold processes
Part shapes in permanent mold processes are limited by the need to open the mold
Permanent mold processes are more economic in high production operations

13. Sand Casting Mold

14. Terminology for Sand Casting Mold
Mold consists of two halves:
Cope = upper half of mold
Drag = bottom half
Mold halves are contained in a box, called a flask
The two halves separate at the parting line

15. Forming the Mold Cavity in Sand Casting
Mold cavity is formed by packing sand around a pattern, which has the shape of the part
When the pattern is removed, the remaining cavity of the packed sand has desired shape of cast part
The pattern is usually oversized to allow for shrinkage of metal during solidification and cooling
Sand for the mold is moist and contains a binder to maintain its shape

16. Use of a Core in the Mold Cavity
The mold cavity provides the external surfaces of the cast part
In addition, a casting may have internal surfaces, determined by a core, placed inside the mold cavity to define the interior geometry of part
In sand casting, cores are generally made of sand

17. Gating System
Channel through which molten metal flows into cavity from outside of mold
Consists of a downsprue, through which metal enters a runner leading to the main cavity
At the top of downsprue, a pouring cup is often used to minimize splash and turbulence as the metal flows into downsprue

18. Riser
Reservoir in the mold which is a source of liquid metal to compensate for shrinkage of the part during solidification
The riser must be designed to freeze after the main casting in order to satisfy its function

19. Heating the Metal Heat to raise temperature to melting point
Heating furnaces are used to heat the metal to molten temperature sufficient for casting
The heat required is the sum of:
Heat to raise temperature to melting point
Heat of fusion to convert from solid to liquid
Heat to raise molten metal to desired temperature for pouring

20. Heating the Metal Equation (10
Heating the Metal Equation (10.1) H = ρV{Cs(Tm – To) + Hf + Cl(Tp –Tm)}
H = Total heat required, J or Btu
ρ = density, g/cm3 or lbm/in3
V = volume of metal being heated, cm3 or in3
Cs = weight specific heat of solid metal, J/g-oC or Btu/lbm-F
Tm = melting temperature, oC or oF
To = starting temperature, oC or oF
Tp = pouring temperature, oC or oF
Hf = heat of fusion J/g or Btu/lbm
Cl = weight specific heat of liquid metal J/g-oC or Btu/lbm-oF

21. Example 1:
One cubic meter of a certain eutectic alloy will be heated in a crucible from room temperature to 1100oC above its melting point for pouring. Density = 7.5g/cm3; melting point = 800oC; specific heat of the metal = 0.33 J/g-oC in the solid state and 0.29 J/g-oC in the liquid state; and heat of fusion = 160 J/g. How much energy must be added to accomplish the heating? Assume ambient temperature in the foundry = 25oC, and assume that the density of the liquid and solid states of metal are the same.
H = ρV{Cs(Tm – To) + Hf + Cl(Tp –Tm)} H = (7.5) (106) [ 0.33(800-25) ( )]
= 5368 (106) J

22. Pouring the Molten Metal
For this step to be successful, metal must flow into all regions of the mold, most importantly the main cavity, before solidifying
Factors that determine success
Pouring temperature
Pouring rate
Turbulence (flow is agitated and irregular-tends to accelerate the formation of metal oxides)

23. Pouring the Molten Metal: Bernoulli’s theorem
Bernoulli’s theorem states that sum of the energies (head, pressure, kinetic and friction) at any two points in a flowing liquid are equal. This can be written as:
Eq (10.2)
h = head, cm (in)
p = pressure on the liquid N/cm2 (ib/in2)
ρ = density, g/cm3 (ibm/in3)
v = flow velocity, cm/s (in/sec)
g = gravitational acceleration constant, 981 cm/s/s (386 in./sec/sec)
F = head losses due to friction, cm (in)
Subcripts 1 and 2 indicate any two locations in the liquid flow

24. Bernoulli’s theorem
Bernoulli’s equation can be simplified in several ways. If we ignore friction losses and assume that the system remains at atmospheric pressure throughout, then the equation can be reduced to:
Eq (10.3)
When the metal is poured into the pouring cup and overflows down the sprue, its initial velocity at the top is zero (v1 = 0), Hence, Eq (10.3) further simplifies to
Eq (10.4)
Which can be solved for the flow velocity:
Eq (10.5)
v = velocity of the liquid metal at the base of the sprue, cm/s (in/sec)
g = 981 cm/s/s (386 in./sec)
h = the height of the sprue, cm (in)

25. Pouring the Molten Metal: Continuity Law
Continuity law states that volume rate of the flow remains constant throughout the liquid. It can be expressed as:
Eq (10.5)
Q = volumettric flow rate, cm3/s (in3/sec)
v = velocity as before
A = cross-sectional area of the liquid, cm2 (in2)
Subscripts refer to any two points in the flow system.
Assuming that the runner from the sprue base to the mold cavity is horizontal ( and therefore the head, h is the same as at the sprue base), then the volume rate of flow through the gate and into the mold cavity remains equal to vA at the base. Accordingly, we can estimate the time required to filled a mold cavity of volume, V as Eq (10.6)
MFT = mold filling time, s (sec)
V = volume of mold cavity, cm3 (in3)
Q = volume flow rate, as before

26. Example 2
A certain mold has a sprue whose length is 20cm and the cross-sectional area at the base of the sprue is 2.5cm2. The sprue feeds a horizontal runner leading into a mold cavity whose volume is 1560cm3. Determine (a) velocity of the molten metal at the base of the sprue (b) volume rate of flow (c) time to fill the mold.

27. Solidification of Metals
Transformation of molten metal back into solid state
Solidification differs depending on whether the metal is
A pure element or
An alloy

28. Solidification of Pure Metals
Due to chilling action of mold wall, a thin skin of solid metal is formed at the interface immediately after pouring
Skin thickness increases to form a shell around the molten metal as solidification progresses
Rate of freezing depends on heat transfer into mold, as well as thermal properties of the metal

29. Cooling Curve for a Pure Metal
A pure metal solidifies at a constant temperature equal to its freezing point (same as melting point)

30. Solidification of Pure Metals
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 growth)

31. Solidification of Alloys
Most alloys freeze over a temperature range
Phase diagram for a copper‑nickel alloy system and cooling curve for a 50%Ni‑50%Cu composition

32. Solidification of Alloys
Characteristic grain structure in an alloy casting, showing segregation of alloying components in center of casting

33. Solidification Time
Total solidification time TTS = time required for casting to solidify after pouring
TTS depends on size and shape of casting by relationship known as Chvorinov's Rule
where TTS = total solidification time; V = volume of the casting; A = surface area of casting; n = exponent with typical value = 2; and Cm is mold constant.

34. Mold Constant in Chvorinov's Rule
Mold constant Cm depends on:
Mold material
Thermal properties of casting metal
Pouring temperature relative to melting point
Value of Cm for a given casting operation can be based on experimental data from previous operations carried out using same mold material, metal, and pouring temperature, even though the shape of the part may be quite different

35. What Chvorinov's Rule Tells Us
Casting with a higher volume‑to‑surface area ratio cools and solidifies more slowly than one with a lower ratio
To feed molten metal to the main cavity, TTS for riser must be greater than TTS for main casting
Since mold constants of riser and casting will be equal, design the riser to have a larger volume‑to‑area ratio so that the main casting solidifies first
This minimizes the effects of shrinkage

36. Shrinkage during Solidification and Cooling
(0) starting level of molten metal immediately after pouring; (1) reduction in level caused by liquid contraction during cooling

37. Shrinkage during Solidification and Cooling
(2) reduction in height and formation of shrinkage cavity caused by solidification; (3) further reduction in volume due to thermal contraction during cooling of solid metal

38. Solidification Shrinkage
Occurs in nearly all metals because the solid phase has a higher density than the liquid phase
Thus, solidification causes a reduction in volume per unit weight of metal
Exception: cast iron with high C content
Graphitization during final stages of freezing causes expansion that counteracts volumetric decrease associated with phase change

39. Shrinkage Allowance
Patternmakers correct for solidification shrinkage and thermal contraction by making the mold cavity oversized
Amount by which mold is made larger relative to final casting size is called pattern shrinkage allowance
Casting dimensions are expressed linearly, so allowances are applied accordingly

40. Directional Solidification
To minimize effects of shrinkage, it is desirable for regions of the casting most distant from the liquid metal supply to freeze first and for solidification to progress from these regions toward the riser(s)
Thus, molten metal is continually available from risers to prevent shrinkage voids
The term directional solidification describes this aspect of freezing and methods by which it is controlled

41. Achieving Directional Solidification
Directional solidification is achieved using Chvorinov's Rule to design the casting, its orientation in the mold, and the riser system that feeds it
Locate sections of the casting with lower V/A ratios away from riser, so freezing occurs first in these regions, and the liquid metal supply for the rest of the casting remains open
Chills ‑ internal or external heat sinks that cause rapid freezing in certain regions of the casting

42. External Chills
Metal inserts in the walls of mold cavity that can remove heat from the molten metal more rapidly than the surrounding sand in order to promote solidification
They are used effectively in the sections of casting that are difficult to feed with liquid metal, thus encouraging rapid freezing in these sections while the connection to liquid metal is still open.

43. Riser Design
Riser is waste metal that is separated from the casting and remelted to make more castings
To minimize waste in the unit operation, it is desirable for the volume of metal in the riser to be a minimum
Since the shape of the riser is normally designed to maximize the V/A ratio, this allows riser volume to be reduced to the minimum possible value