Transcript Materials - Mechanical, Industrial & Systems Engineering

Metal Casting Processes
Casting
• One of the oldest manufacturing processes – 4000 B.C. with stone
and metal molds for casting copper
• Pour molten metal into a mold cavity to produce solidified parts that
take on the shape of the cavity
• Many different casting processes, each with its own characteristics,
applications and materials, advantages, limitations, and costs
• Casting can produce complex shapes with internal cavities or hollow
sections
• Casting can produce very large parts
• Competitive with other processes
• Good net-shape manufacturing for metals
Solidification of Pure Metals
Pure metals solidify at a constant temperature. During freezing the latent
heat of solidification is given off. Most metals shrink on solidification and
shrink further as the solid cools to room temperature.
Solidification of Pure Metals
Direction of heat flow
Temperature distribution in a mold part
way through solidification.
Grain structure for pure metal
Solid Solution Alloys
Metal alloys in which one metal is soluble in the other in the solid state.
These are also called binary alloys. Copper/Nickel alloys are typical of
this type of alloy.
Solidification of Solid Solution Alloys
Nickel-Copper Alloy Phase Diagram
Mechanical Properties of Copper-Nickel and CopperZinc Alloys
Figure 4.6 Mechanical
properties of copper-nickel
and copper-zinc alloys as a
function of their
composition. The curves
for zinc are short, because
zinc has a maximum solid
solubility of 40% in
copper. Source: L. H. Van
Vlack; Materials for
Engineering. AddisonWesley Publishing Co.,
Inc., 1982.
Solidification of Solid Solution Alloys
Direction of heat flow
Temperature distribution in partial
solidified casting.
Grain structure for solid
solution alloy
Solidification of Eutectic Alloy Systems
Lead-Tin Phase Diagram
Figure 4.7 The
lead-tin phase
diagram. Note that
the composition of
the eutectic point
for this alloy is
61.9% Sn-38.1%
Pb. A composition
either lower or
higher than this
ratio will have a
higher liquidus
temperature.
The metals have very limited solubility in each other
Iron-Iron Carbide Phase Diagram
Figure 4.8 The iron-iron
carbide phase diagram.
Because of the
importance of steel as an
engineering material,
this diagram is one of
the most important of all
phase diagrams.
Solidification Patterns
Figure 10.4 (a) Solidification patterns for gray cast iron in a 180-mm (7-in.) square casting. Note
that after 11 min. 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: H. F. Bishop and W. S. Pellini.
Solidification and Cooling
• Molten metal solidifies from the mold walls inward
• At the mold walls, metal cools rapidly forming a skin, or shell, of fine
equiaxed grains
• Grains grow in a direction opposite to that of the heat transfer out
through the mold, leading to columnar grains
• Alloy solidification occurs between the liquidus (TL) and solidus (TS)
temperatures, in the freezing range
• Alloy solidification leads to dendrites and a mushy zone where both
liquid and solid phases are present
• After solidification, the casting continues to cool
• Grain shapes:
•
•
•
•
Equiaxed - approximately equal dimensions in 3 directions
Plate-like - one dimension smaller than other two
Columnar - one dimension larger than other two
Dendritic (tree-like)
Solidification Contraction for Various Cast Metals
TABLE 10.1
Metal or alloy
Aluminum
Al–4.5%Cu
Al–12%Si
Carbon steel
1% carbon steel
Copper
Source: After R. A. Flinn.
Volumetric
solidification
contraction (%)
6.6
6.3
3.8
2.5–3
4
4.9
Metal or alloy
70%Cu–30%Zn
90%Cu–10%Al
Gray iron
Magnesium
White iron
Zinc
Volumetric
solidification
contraction (%)
4.5
4
Expansion to 2.5
4.2
4–5.5
6.5
TABLE 12.1
Metal
Gray cast iron
White cast iron
Malleable cast iron
Aluminum alloys
Magnesium alloys
Yellow brass
Phosphor bronze
Aluminum bronze
High-manganese steel
Percent
0.83–1.3
2.1
0.78–1.0
1.3
1.3
1.3–1.6
1.0–1.6
2.1
2.6
Table 12.1 Normal Shrinkage Allowance for
Some Metals Cast in Sand Molds
Cooling Rates
• Slow cooling rates (102 K/s) or long local solidification times result in
coarse dendritic structures
• Fast cooling rates (104 K/s) or short local solidification times result in
finer grain structure
• Very fast cooling rates (106 to 108 K/s) lead to amorphous alloy
structures, or metallic glasses, with no grain boundaries and atoms
that are randomly and tightly packed
• Smaller grain size leads to increased strength and ductility,
decreased microporosity, and decreased tendency for cracked
castings
• Thermal gradient, G (102 to 103 K/m)
• Rate of movement for the liquid-solid interface, R (10-3 to 10-4)
• Inoculants, or nucleating agents, can be added to the alloy
Solidification Time
Figure 10.10 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: H.
F. Taylor, J. Wulff, and M. C.
Flemings.
 volume 

Solidifica tionTime  C 
 SurfaceAre a 
2
where C is a constant that reflects mold material, metal properties, and temperature
Chvorinov’s rule – empirical law for estimating solidification times. Allows
comparisons between different shaped castings in the same material and mold
types to be made.
Classification of Casting Processes
Sand Casting
Expendable Mold-Permanent Pattern Process
•Versatile casting process which can be used for a wide range of shapes
•Castings can be produced in all metals
•Castings can be made of almost any size
•Molds made from sand mixed with a binder – clay, oils, sodium silicate,
etc.
•Fairly labor intensive process, but relatively economic for small
quantities of parts as mold costs are low
Sequence of Operations for Sand Casting
Sand Molding Patterns
Pattern Materials
•Wood
•Plastic
•Aluminum
•Steel
•Cast iron
Components of a Typical Sand Mold
Shrinkage and Hot Tears
Figure 10.11 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 (heatproducing) compounds may be used (as exothermic padding) to control
cooling at critical sections to avoid hot tearing.
Casting Defects
Figure 10.12 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: J. Datsko.
Internal and External Chills
Figure 10.13
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 metals, as
shown in (c).
Ceramic Molds
Figure 11.16 Sequence of operations in
making a ceramic mold. Source: Metals
Handbook, vol. 5, 8th ed.
Figure 11.17 A typical ceramic
mold (Shaw process) for
casting steel dies used in hot
forging. Source: Metals
Handbook, vol. 5, 8th ed.
Lost Foam or Evaporative Pattern Casting
Expandable pattern/ expendable mold process
Mold metal evaporates pattern
Investment Casting
• Expendable pattern/ expendable mold process
• Patterns made from wax or thermoplastic by
injection molding
• Complex patterns can be built up from multiple
pieces or clusters of similar parts can be
assembled around a single runner system
• Surface finish and accuracy good
• Can be used for most metals including those
with higher melting points
Investment
Casting
Expendable
mold/ expendable
pattern process
Figure 11.18
Schematic
illustration of
investment
casting, (lostwax process).
Castings by this
method can be
made with very
fine detail and
from a variety of
metals. Source:
Steel Founders'
Society of
America.
Investment Casting of a
Rotor
Figure 11.19 Investment casting of an integrally cast rotor for a gas turbine. (a) Wax pattern
assembly. (b) Ceramic shell around wax pattern. (c) Wax is melted out and the mold is filled, under
a vacuum, with molten superalloy. (d) The cast rotor, produced to net or near-net shape. Source:
Howmet Corporation.
Investment and Conventionally Cast
Rotors
Figure 11.20 Crosssection and
microstructure of two
rotors: (top)
investment-cast;
(bottom)
conventionally cast.
Source: Advanced
Materials and
Processes, October
1990, p. 25 ASM
International
Centrifugal Casting
Process
Figure 11.27 Schematic
illustration of the
centrifugal casting process.
Pipes, cylinder liners, and
similarly shaped parts can
be cast with this process.
Permanent Mold Casting Processes
Vacuum-Casting
Process
Figure 11.21 Schematic illustration of the vacuum-casting process. Note that the mold has a
bottom gate. (a) Before and (b) after immersion of the mold into the molten metal. Source:
From R. Blackburn, "Vacuum Casting Goes Commercial," Advanced Materials and
Processes, February 1990, p. 18. ASM International.
Pressure Casting
Figure 11.22 (a) The bottom-pressure casting process utilizes graphite molds for the production of
steel railroad wheels. Source: The Griffin Wheel Division of Amsted Industries Incorporated. (b)
Gravity-pouring method of casting a railroad wheel. Note that the pouring basin also serves as a
riser. Railroad wheels can also be manufactured by forging.
Pressure Die Casting
Wide range of
shapes
Lower melting point
alloys
High mold costs –
large quantity product
High production rates
with short cycle times
Extra dies required
for trimming flash and
runners
Hot- and Cold-Chamber DieCasting
(a)
(b)
Figure 11.23 (a) Schematic illustration of the hot-chamber die-casting process. (b) Schematic
illustration of the cold-chamber die-casting process. Source: Courtesy of Foundry Management and
Technology.
Hot Chamber Die Casting
• Used for lower melting point alloys (zinc and magnesium)
• Mold pressures usually 1000 to 2000 p.s.i, but can be up to
5000.
Cold Chamber Die Casting
•
•
•
Used for higher melting point alloys –aluminum and copper based
Die pressures from 5,000 to 20,000 psi
Die clamping forces at least pressure * project area of part in die closing
direction
Squeeze-Casting
Figure 11.29 Sequence of operations in the squeeze-casting process. This process combines the
advantages of casting and forging.
Single Crystal Casting of Turbine
Blades
Figure 11.30 Methods of casting turbine blades: (a) directional solidification; (b) method to
produce a single-crystal blade; and (c) a single-crystal blade with the constriction portion still
attached. Source: (a) and (b) B. H. Kear, Scientific American, October 1986; (c) Advanced
Materials and Processes, October 1990, p. 29, ASM International.
(c)
Continuous Casting
Figure 5.4 The
continuous-casting
process for steel.
Typically, the solidified
metal descends at a
speed of 25 mm/s (1
in./s). Note that the
platform is about 20 m
(65 ft) above ground
level. Source:
Metalcaster's Reference
and Guide, American
Foundrymen's Society.
Casting Design Considerations
• Sharp corners, angles, and fillets should be avoided because they
act as stress raisers and may cause cracking and tearing of the
metal or dies during solidification
• Fillet radii should be between 3 mm and 25 mm (1/8 inch to 1 inch)
to reduce stress concentrations and ensure proper liquid-metal flow
• Larger fillet radii leads to larger local volumes of material that cool
too slowly and may lead to shrinkage cavities
Figure 12.1 Suggested
design modifications to
avoid defects in castings.
Note that sharp corners
are avoided to reduce
stress concentrations.
Casting Design Considerations
• Avoid casting designs that will have hot spots, leading to shrinkage
cavities and porosity
• Maintain uniform cross sections and wall thicknesses when possible
• Reduce cross sections when possible to reduce solidification time
and save raw materials
• Smoothly transition between sections with different cross sectional
areas
• Consider adding a cored hole if necessary (figure e)
Casting Design Considerations
• Use external chills to reduce hot spots (or internal chills if needed)
• Avoid large flat areas that may warp during cooling due to
temperature gradients or have poor surface finish due to uneven
metal flow – use ribs or serrations to break up the flat surface
Figure 12.3, 12.4 Source: Steel Castings Handbook, 5th ed. Steel
Founders' Society of America, 1980. Used with permission.
Casting Design Considerations
• The parting line separating the top and bottom halves of the mold
should be along a flat plane and at corners or edges when possible
• Parting line location influences ease of molding, cores, support,
gating system, etc.
Figure 12.5 Source: Steel Casting Handbook, 5th ed. Steel
Founders' Society of America, 1980. Used with permission.
Casting Process Economics
•
•
•
•
Casting costs include labor, materials, machinery, tooling and dies
Preparation time for molds and dies varies, as well as skill required
Furnace and machinery costs depend on the level of automation
Post processing, heat treating, cleaning, and inspecting castings
also costs money
• Ultimately, per unit costs must be balanced with functional
requirements of the cast product
• Safety considerations in casting are important! (see page 295)
TABLE 12.6
Cost*
Process
Die
Sand
L
Shell-mold
L–M
Plaster
L–M
Investment
M–H
Permanent mold
M
Die
H
Centrifugal
M
* L, low; M, medium; H, high.
Equipment
L
M-H
M
L-M
M
H
H
Labor
L–M
L–M
M–H
H
L–M
L–M
L–M
Production
rate (Pc/hr)
<20
<50
<10
<1000
<60
<200
<50
Casting Design Considerations
• Adjust mold dimensions to:
• avoid cracking the casting and
to account for shrinkage during
solidification (typically 1-2%)
• account for machining
allowances when finishing
operations are needed
• Set dimensional tolerances as wide
as possible while still meeting
performance requirements to avoid
extra casting costs
• Provide draft angles of 0.5 to 2
degrees for outer surfaces of sand
castings to allow for removal of the
pattern without damaging the mold
(0.25 or 0.5 degrees for permanent
mold casting)
TABLE 12.1
Metal
Gray cast iron
White cast iron
Malleable cast iron
Aluminum alloys
Magnesium alloys
Yellow brass
Phosphor bronze
Aluminum bronze
High-manganese steel
Percent
0.83–1.3
2.1
0.78–1.0
1.3
1.3
1.3–1.6
1.0–1.6
2.1
2.6
Table 12.1 Normal Shrinkage
Allowance for Some Metals
Cast in Sand Molds
General Characteristics of Casting
Processes
TABLE 11.2
Process
Sand
Shell
Expendable
mold
pattern
Typical
materials
cast
Minimum
Maximum
All
All
0.05
0.05
No limit
100+
5-25
1-3
4
4
1-2
2-3
0.05
No limit
5-20
4
0.05
50+
1-2
0.005
100+
0.5
<0.05
--
All
Nonferrous
Plaster
(Al, Mg, Zn,
mold
Cu)
All
(High melting
Investment
pt.)
Permanent
mold
All
Nonferrous
(Al, Mg, Zn,
Die
Cu)
Centrifugal
All
Weig ht (kg)
Typical
surface
finish
(mm, Ra)
Section thic kness (mm)
Shape
Dimensional
Porosity* complexity* accuracy*
Minimum
Maximum
3
2
3
2
No limit
--
1
2
2
No limit
3
1-2
2
1
--
1-3
3
1
1
1
75
300
2-3
2-3
3-4
1
2
50
50
5000+
1-2
2-10
1-2
1-2
3-4
3-4
1
3
0.5
2
12
100
*Relative rating: 1 best, 5 worst.
Note : These ratings are only general; significant variations can occur, depending on the methods used.
Comparative Performance of Casting Processes