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Metal Casting Processes

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Title: Metal Casting Processes


1
Metal Casting Processes
2
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

3
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.
4
Solidification of Pure Metals
Direction of heat flow
Temperature distribution in a mold part way
through solidification.
Grain structure for pure metal
5
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.
6
Solidification of Solid Solution
AlloysNickel-Copper Alloy Phase Diagram
7
Mechanical Properties of Copper-Nickel and
Copper-Zinc 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. Addison-Wesley
Publishing Co., Inc., 1982.
8
Solidification of Solid Solution Alloys
Direction of heat flow
Temperature distribution in partial solidified
casting.
Grain structure for solid solution alloy
9
Solidification of Eutectic Alloy SystemsLead-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
10
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.
11
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.
12
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)

13
Solidification Contraction for Various Cast Metals
Table 12.1 Normal Shrinkage Allowance for Some
Metals Cast in Sand Molds
14
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

15
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.
where C is a constant that reflects mold
material, metal properties, and temperature
Chvorinovs rule empirical law for estimating
solidification times. Allows comparisons between
different shaped castings in the same material
and mold types to be made.
16
Classification of Casting Processes
17
Sand CastingExpendable 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

18
Sequence of Operations for Sand Casting
19
Sand Molding Patterns
  • Pattern Materials
  • Wood
  • Plastic
  • Aluminum
  • Steel
  • Cast iron

20
Components of a Typical Sand Mold
21
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 (heat-producing) compounds may
be used (as exothermic padding) to control
cooling at critical sections to avoid hot tearing.
22
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.
23
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).
24
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.
25
Lost Foam or Evaporative Pattern Casting
Expandable pattern/ expendable mold process
Mold metal evaporates pattern
26
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

27
Investment Casting
Figure 11.18 Schematic illustration of
investment casting, (lost-wax process). Castings
by this method can be made with very fine detail
and from a variety of metals. Source Steel
Founders' Society of America.
Expendable mold/ expendable pattern process
28
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.
29
Investment and Conventionally Cast Rotors
Figure 11.20 Cross-section and microstructure of
two rotors (top) investment-cast (bottom)
conventionally cast. Source Advanced Materials
and Processes, October 1990, p. 25 ASM
International
30
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.
31
Permanent Mold Casting Processes
32
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.
33
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.
34
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
35
Hot- and Cold-Chamber Die-Casting
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.
36
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.

37
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

38
Squeeze-Casting
Figure 11.29 Sequence of operations in the
squeeze-casting process. This process combines
the advantages of casting and forging.
39
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.
40
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.
41
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.
42
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)

43
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.
44
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.
45
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)

46
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 Normal Shrinkage Allowance for Some
Metals Cast in Sand Molds
47
General Characteristics of Casting Processes
48
Comparative Performance of Casting Processes
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