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©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.™ Four typical casting processes: (a) and (b) Green sand molding where clay-bonded sand is packed around a pattern. Sand cores can produce internal cavities in the casting. (c) The permanent mold process where metal is poured into an iron or steel mold. (d) Die casting where metal is injected at high pressure into a steel die. (e) Investment casting where a wax pattern is surrounded by a ceramic; after the wax is melted and
Casting Methods
• Casting a fabrication process
whereby a totally molten metal is
poured into a mold cavity having the
desired shape; upon solidification, the
metal assumes the shape of the mold
but experiences some shrinkage.
• Casting techniques are used when
1. The finished shape is so large or complicated that
any other method would be impractical
2. A particular alloy is so low in ductility that forming
by either hot or cold working would be difficult
3. In comparison to other fabrication processes,
casting is the most economical.
Casting
Die Casting
(^) The liquid metal is forced into a mold (die) under pressure at a relatively high velocity, then allowed to solidify with the pressure maintained. (^) A two-piece permanent steel mold is used; when clamped together, the two pieces form the desired shape. (^) When complete solidification has been achieved, the mold pieces are opened and the cast piece is ejected. (^) Rapid casting rates are possible, making this an inexpensive method; a single set of molds may be used for thousands of castings. (^) This technique lends itself only to relatively small pieces and to alloys of low melting points such as Zn, Al and Mg
5
- (^) Stage I — Mold formed by
pouring
plaster of paris around wax
pattern.
Plaster allowed to harden.
- Stage II — Wax is melted and
then
poured from mold—hollow
mold
cavity remains.
- Stage III — Molten metal is
poured
into mold and allowed to
solidify.
• Investment Casting
(low volume, complex shapes like jewelry, turbine blades,
jewelry and dental crowns and inlays, and jet engine impellers) wax I II III
Investment Casting (lost-wax
casting)
Plaster die formed around wax prototyp e
• Blowholes, pinholes, shrinkage cavities, & porosity
– Blowholes and pinholes^ are holes formed by gas
entrapped during solidification.
– Shrinkage cavities^ are cavities that have a rougher
shape and sometimes penetrate deep into the casting.
Shrinkage cavities are caused by lack of proper
feeding or non-progressive solidification.
– Porosity^ is pockets of gas inside the metal caused by
micro-shrinkage, e.g. dendritic shrinkage during
solidification.
Casting Defects — Cavities
• Cracks in casting and are caused by^ hot tearing,
hot cracking, and lack of fusion (cold shut)
– A^ hot tear^ is a fracture formed during solidification
because of hindered contraction.
– A^ hot crack^ is a crack formed during cooling after
solidification because of internal stresses developed in the casting.
– Lack of fusion^ is a discontinuity caused when two streams
of liquid in the solidifying casting meet but fail to unite.
– Rounded edges indicate poor contact between various
metal streams during filling of the mold.
Casting Defects — Discontinuities
• The distinctive metallurgical characteristics of
castings are acquired during solidification,
whereas with wrought materials, they are
acquired during mechanical deformation.
• The principal metallurgical difference between
castings and wrought materials is that castings
lack homogeneity.
– They do not have the benefit of hot work to
accelerate the diffusion of the chemical elements to
achieve homogenization.
– Cast alloys require significantly longer soaking
times to achieve homogenization.
– Cast alloys frequently contain more silicon to
improve the fluidity of the molten metal.
– Solidified castings contain high residual stresses
from solid shrinkage, unless they are removed by a
stress relief annealing process.
Cast and Wrought Alloys
How Metals are Made
Cool Stuff Being Made: How Steel Is Made From ore to sheet, watch how US Steel makes steel products. Learn more. Watch how Superior Tube rolls its product. A flat strip is rolled into a tubular shape and the seam is welded, without the use of flux or filler metal. Watch the video. Watch aluminum foil being made Learn more. This 1300pound bronze bell was cast by the Meneely Foundry in West Troy, New York, in
- The video shows how a bronze bell is cast. Learn more.
( Source: www.steel.org. Used with permission of the American Iron and Steel Institute .) Secondary processing steps in processing of steel and alloys. Animation shows blast furnace operation in a training video from Corus Steel (now part of Tata Steel). Watch video.
