metal_Casting_process_Ch10_updated_Lecture

Manufacturing, Engineering & Technology, Fifth Edition, by Serope Kalpakjian and Steven R. Schmid.
ISBN 0-13-148965-8. © 2006 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.
Chapter 10
Fundamentals of Metal Casting

©2019 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 7/e
Solidification Processes
• Starting work material is either a liquid or is in a highly plastic condition, and a part
is created by solidification of the material
– Solidification processes can be classified
according to engineering material
processed:
• Metals
• Ceramics, specifically glasses
• Polymers and polymer matrix composites
(PMCs)

Classification of solidification processes

Solidification of Pure Metals
Figure 10.1 (a) Temperature as a function of time for the solidification of pure metals. Note
that the freezing takes place at a constant temperature. (b) Density as a function of time

Temperature Distribution during Metal Solidification
Figure 10.10 Temperature
distribution at the interface
of the mold wall and the
liquid metal during the
solidification of metals in
casting

Casting Design and Fluidity Test
Figure 10.8 Schematic illustration of a typical
riser-gated casting. Risers serve as reservoirs,
supplying molten metal to the casting as it
shrinks during solidification.
Figure 10.9 A test method for
fluidity using a spiral mold. The
fluidity index is the length of the
solidified metal in the spiral
passage. The greater the length
of the solidified metal, the greater
is its fluidity.

Solidification Contraction or Expansion

Casting of Metals
• 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
– The principle of casting seems simple:
1. Melt the metal
2. Pour it into a mold
3. Let it cool and solidify

Capabilities and Advantages of Casting
• Can create complex part geometries
• Can create both external and internal shapes
• Some casting processes are net shape; others are near net shape
• Can produce very large parts
• Some casting methods are suited to mass production

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

Parts Made by Casting
• Large 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

Overview of Casting Technology
• Casting is usually performed in a foundry, which is a factory equipped for
– Making molds
– Melting and handling molten metal
– Performing the casting process
– Cleaning the finished casting
• Workers who perform casting operations are called foundrymen

The Mold in Casting
• The mold contains a 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
– Molds are made of a variety of materials,
including sand, plaster, ceramic, and metal

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

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

Relative 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 and remove the casting
• Permanent mold processes are more economic in high production operations

• Due to the chilling action of the 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 out of the mold, as well as thermal
properties of the metal

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

Most Alloys Solidify Over a Temperature Range
(a) Phase diagram for a copper nickel alloy system and (b) cooling
‑
curve for a 50%Ni 50%Cu composition
‑

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

Solidification Time
• Total solidification time TTS = time required for casting to solidify after pouring
• TTS depends on size and shape of casting by a 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
 
  
 
n
TS m
V
T C
A

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 the same mold material, metal, and pouring
temperature, even though the shape of the part may be quite different

What Chvorinov's Rule Tells Us
• A 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

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

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

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

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 the pattern
shrinkage allowance
• Casting dimensions are expressed linearly, so allowances are applied linearly

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

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

Cast Structures of Solidified Metals
Figure 10.2 Schematic illustration of three cast
structures of metals solidified in a square mold: (a)
pure metals; (b) solid-solution alloys; and (c)
structure obtained by using nucleating agents.
Source: After G. W. Form, J. F. Wallace, J. L.
Walker, and A. Cibula
Figure 10.3 Development of a
preferred texture at a cool mold
wall. Note that only favorably
oriented grains grow away from the
surface of the mold

Solidification of Iron and Carbon Steels
Figure 10.5 (a) Solidification patterns for gray cast iron in a 180-mm (7-in.) square casting. Note that after
11 minutes of cooling, dendrites reach each other, but the casting is still mushy throughout. It takes about
two hours for this casting to solidify completely. (b) Solidification of carbon steels in sand and chill (metal)
molds. Note the difference in solidification patterns as the carbon content increases. Source: After H. F.
Bishop and W. S. Pellini

Cast Structures
Figure 10.7 Schematic illustration of cast structures in (a) plane front, single
phase, and (b) plane front, two phase. Source: Courtesy of D. Apelian

Solidified Skin on a Steel Casting
Figure 10.11 Solidified skin on a steel casting. The remaining molten metal is poured out
at the times indicated in the figure. Hollow ornamental and decorative objects are made by
a process called slush casting, which is based on this principle. Source: After H. F. Taylor,
J. Wulff, and M. C. Flemings

Hot Tears in Castings
Figure 10.12 Examples of hot tears in castings. These defects occur because the casting
cannot shrink freely during cooling, owing to constraints in various portions of the molds and
cores. Exothermic (heat-producing) compounds may be used (as exothermic padding) to
control cooling at critical sections to avoid hot tearing

Types of Internal and External Chills used in Casting
Figure 10.14 Various types of (a) internal and (b) external chills (dark areas at corners)
used in castings to eliminate porosity caused by shrinkage. Chills are placed in regions
where there is a larger volume of metal, as shown in (c).

Common Casting Defects
Figure 10.13 Examples of common defects in castings. These defects can be
minimized or eliminated by proper design and preparation of molds and control of
pouring procedures. Source: After J. Datsko.

Solubility of Hydrogen in Aluminum
Figure 10.15 Solubility of
hydrogen in aluminum. Note the
sharp decrease in solubility as
the molten metal begins to
solidify.

Fluid Flow and Solidification Time
Sprue design

A1
A2

h2
h1
Mass continuity

Q  A1v1  A2v2
Bernoulli’s theorem

h 
p
g

v2
2g
 constant
Reynolds number

Re 
vD

Chvorinov’s Rule

Solidification time = C
Volume
Surface Area




n

Casting of an Aluminum Piston
Figure 10.16 Aluminum piston for
an internal combustion engine: (a)
as-cast and (b) after machining.
Figure 10.17 Simulation of mold filling
and solidification. (a) 3.7 seconds after
start of pour. Note that the mushy zone
has been established before the mold is
filled completely. (b) Using a vent in the
mold for removal of entrapped air, 5
seconds after pour.

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