IE 337: Materials & Manufacturing
Processes
Lectures 7&8:
Introduction to Metal
Casting
Chapters 10 & 6
Assignment
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HW 2 due today
 CH 21,22 and 24 Problems
 In Assignments folder
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This Time
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Basic Process
Casting Terminology
Heating
Pouring
Phase diagrams
Solidification
Casting
Common process attributes:
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Flow of Molten Liquid
Requires Heating
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Heat Transfer of Liquid in
Mold Cavity During and
After Pouring
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Solidification into Component
Metal Casting Processes Categories
1.
Expendable mold processes - mold is
sacrificed to remove part
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2.
Permanent mold processes - mold is made of
metal and can be used to make many castings
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Advantage: more complex shapes possible
Disadvantage: production rates often limited by
time to make mold rather than casting itself
Advantage: higher production rates
Disadvantage: geometries limited by need to open
mold
Overview of Sand Casting
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Most widely used casting process, accounting
for a significant majority of total tonnage cast
Nearly all alloys can be sand cast, including
metals with high melting temperatures, such as
steel, nickel, and titanium
Parts ranging in size from small to very large
Production quantities from one to millions
Sand Casting
Sand casting mold
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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 top of downsprue, a pouring cup is often
used to minimize splash and turbulence as the
metal flows into downsprue
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Riser
Reservoir in the mold which is a source of liquid
metal to compensate for shrinkage during
solidification
 The riser must be designed to freeze after the
main casting in order to satisfy its function
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Heating the Metal
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Heating furnaces are used to heat the metal to
molten temperature sufficient for casting
The heat required is the sum of:
1. Heat to raise temperature to melting point
2. Heat of fusion to convert from solid to liquid
3. Heat to raise molten metal to desired temperature
for pouring
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Heating the Metal
1. Heat to melting point: rVCs(Tm-To)
2. Heat of fusion to convert from solid to liquid: rVHf
3. Heat molten metal to pouring temp.: rVCl(Tp-Tm)
r = density, V = volume, Cs = specific heat,
Hf = latent heat of fusion, T = temperature
• Properties vary with temperature and phase
• Melting may occur over a temperature range
• Heat loss to the ambient
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Pouring the Molten Metal
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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
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Pouring Calculations 1
Minimum mold filling time, MFT
MFT =V/Q
Q: volumetric flow rate, cm3/s
V: mold cavity volume, cm3
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Pouring Calculations 2
Volumetric flow rate remains constant
Q =v1A1 = v2A2
Q: volume rate of flow cm3/s
v: velocity of the liquid metal
A: cross-sectional area, cm2
CONTINUITY EQUATION
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Increase in area results in decrease in velocity
Pouring Calculations 3
Flow of liquid metal through gating system & mold
v = (2gh)1/2
v: velocity of the liquid metal at base of sprue
g = gravitational acceleration, 981 cm/s2
h = height of sprue, cm
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Example 1
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The downsprue leading into the runner of a
certain mold has a length = 175 mm.
The cross-sectional area at the base of the
sprue is 400 mm2.
The mold cavity has a volume = 0.001 m3.
Determine: (a) the velocity of the molten metal
flowing through the base of the downsprue, (b)
the volumetric flow rate, and (c) the time
required to fill the mold cavity.
Example 1: Solution
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(a) Velocity v = (2gh)0.5
= (2 x 9810 x 175)0.5
= 1853 mm/s
(b) Volume flow rate Q = vA
= 1853 x 400
= 741,200 mm3/s
(c) Time to fill cavity MFT = V/Q
Solidification of Pure Metals
A pure metal solidifies at a constant temperature
equal to its freezing point (same as melting point)
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Solidification and Cooling
Figure 10.8 Shrinkage of a cylindrical casting during solidification
and cooling: (0) starting level of molten metal immediately after
pouring; (1) reduction in level caused by liquid contraction during
cooling (dimensional reductions are exaggerated for clarity).
Microstructure: I
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
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Solidification of Alloys
Most alloys freeze over a temperature range rather
than at a single temperature
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Microstructure: II
Micro-segregation
Characteristic grain structure in an alloy casting, showing
segregation of alloying components in center of casting
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Microstructure: II
Macro-segregation
Characteristic grain structure in an alloy casting, showing
segregation of alloying components in center of casting
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Phase Diagrams
A graphical means of representing the phases of
a metal alloy system as a function of
composition and temperature
 A phase diagram for an alloy system consisting
of two elements at atmospheric pressure is
called a binary phase diagram
 Other forms of phase diagrams are
discussed in texts on metallurgy and
materials science
Phase Diagrams
 Composition is plotted on the horizontal axis
and temperature on the vertical axis
 Any point in the diagram indicates the overall
composition and the phase or phases present
at the given temperature under equilibrium
conditions
Copper-Nickel Phase Diagram
Figure 6.2 Phase diagram for the copper-nickel alloy
system.
Copper-Nickel (Cu-Ni) Alloy System
 Solid solution alloy throughout entire range of
compositions below the solidus
 No intermediate solid phases in this alloy
system
 However, there is a mixture of phases (solid +
liquid) in the region bounded by the solidus and
liquidus
Chemical Compositions of Phases
 The overall composition of the alloy is given by
its position along the horizontal axis
 However, the compositions of liquid and solid
phases are not the same
 These compositions can be found by
drawing a horizontal line at the temperature
of interest
 Where the line intersects the solidus and
liquidus indicates the compositions of solid
and liquid phases, respectively
Example
Determine compositions of liquid and solid phases
in the Cu-Ni system at an aggregate composition
of 50% nickel and a temperature of 1316oC
(2400oF)
Inverse Lever Rule – Step 1
 The phase diagram can be used to determine
the amounts of each phase present at a given
temperature
 Using the horizontal line that indicates
overall composition at a given temperature,
measure the distances between the
aggregate composition and the intersection
points with the liquidus and solidus,
identifying the distances as CL and CS,
respectively
Inverse Lever Rule – Step 2
 The proportion of liquid phase present is given
by
L phase proportion =
CS
(CS  CL )
 And the proportion of solid phase present is
given by
CL
S phase proportion =
CS  CL
Applications of the Inverse Lever Rule
 Methods for determining chemical composition
of phases and amounts of each phase are
applicable to the solid region of the phase
diagram as well as the liquidus-solidus region
 Wherever there are regions in which two phases
are present, these methods can be utilized
 When only one phase is present, the
composition of the phase is its aggregate
composition under equilibrium conditions, and
the inverse lever rule does not apply
Tin-Lead Phase Diagram
Figure 6.3 Phase diagram for the tin-lead alloy system.
Effect of Solidification Rate
Mechanical properties of 2
identical composition samples with
different cooling rates
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Solidification Time
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Solidification takes time
Total solidification time TST = time required for
casting to solidify after pouring
TST depends on size and shape of casting by
relationship known as Chvorinov's Rule
Chvorinov's Rule
V 
TST  Cm  
 A
n
where TST = total solidification
time;
V = volume of the casting;
A = surface area of casting;
n = exponent usually taken to
have a value = 2; and
Cm is mold constant
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Mold Constant in Chvorinov's Rule
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Cm depends on mold material, thermal
properties of casting metal, and 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
Example 1
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In casting experiments performed using a
titanium alloy and a zircon sand mold, it took
155 s for a cube-shaped casting to solidify.
The cube was 50 mm on a side.
If the same alloy and mold type were used, find
the total solidification time for a cylindrical
casting in which the diameter = 30 mm and
length = 50 mm.
Example 1: Solution
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Cube Volume V
(50)3
=
= 125,000 mm3
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Cube Area A
= 6 x (50)2
= 15,000 mm2
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= 35,343 mm3
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Cylinder Area A
= 2pD2/4 + pDL
= p(30)2/2 + p(30)(50)
= 6126 mm2
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Cylinder (V/A)
= 35,343/6126
= 5.77 mm
Cm = TST/(V/A)2
= 155/(8.33)2
= 2.23 s/mm2
Cylinder Volume V
= pD2L/4 = p(30)2(50)/4
Cube (V/A)
= 125,000/15,000
= 8.33 mm
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TST = Cm(V/A)2
= 2.23 (5.77)2 = 74.3 s
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What Chvorinov's Rule Tells Us
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A casting with a higher volume-to-surface area
ratio cools and solidifies more slowly than one
with a lower ratio
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Since riser and casting mold constants will be
equal, design the riser to have a larger
volume-to-area ratio so that the main casting
solidifies first
Shrinkage in Solidification and Cooling
Figure 10.8 Shrinkage of a cylindrical casting during solidification
and cooling: (0) starting level of molten metal immediately after
pouring; (1) reduction in level caused by liquid contraction during
cooling (dimensional reductions are exaggerated for clarity).
Shrinkage in Solidification and Cooling
Figure 10.8 (2) reduction in height and formation of shrinkage
cavity caused by solidification shrinkage; (3) further reduction in
height and diameter due to thermal contraction during cooling of
solid metal (dimensional reductions are exaggerated for clarity).
Solidification Shrinkage
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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
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Shrinkage Allowance
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Patternmakers account for solidification
shrinkage and thermal contraction by making
mold cavity oversized (See Table 10.1)
Amount by which mold is made larger relative
to final casting size is called pattern shrinkage
allowance
Example 2
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A mold cavity has the shape of a cube, 100
mm on a side. Determine the volume and
dimensions of the final cube after cooling to
room temperature if the cast metal is copper.
Assume that the mold is full at the start of
solidification and that shrinkage occurs
uniformly in all directions.
For copper, solidification shrinkage is 4.9%,
solid contraction during cooling is 7.5%.
Example 2: Solution
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Volume of cavity V
= (100)3
= 106 mm3
Volume of casting V
= 106(1-0.049)(1-0.075)
= 879,675 mm3
Dimension on each side of cube
= (879,675)0.333
= 95.82 mm
You should have learned
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Basic Process
Sand Casting Terminology
Heating
Pouring
Phase diagrams
Solidification
Next Class
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Casting Continued
 Chapter 11
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