Page 305 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

28 Chapter 11 Metal-Casting Processes and Equipment

and ejector pin. The pressure applied by the punch keeps the entrapped gases in
solution, and the contact under high pressure at the die-metal interface promotes
rapid heat transfer, thus resulting in a fine microstructure with good mechanical
properties.
The application of pressure also overcomes feeding difficulties that may arise
when casting metals with a long freezing range (Section 10.2.2). The pressures
required in squeeze casting are lower than those for hot or cold forging. Complex
parts can be made to near-net shape with fine surface detail from both nonferrous
and ferrous alloys.

Semisolid-metal Forming. Semisolid-metal forming (also called mushy-state pro-
cessing; see Fig. 10.4) was developed in the 1970s and put into commercial produc-
tion by 1981. When it enters the die, the metal (consisting of liquid and solid
components) is stirred so that all of the dendrites are crushed into fine solids, and
when cooled in the die it develops into a fine-grained structure. The alloy exhibits
thixotropic behavior, described in Section 10.2.3; hence, the process also is called
thixoforming or thixomolding, meaning its viscosity decreases when agitated. Thus,
at rest and above its solidus temperature, the alloy has the consistency of butter, but
when agitated vigorously, its consistency becomes more like motor oil. Processing
metals in their mushy state also has led to developments in mushy-state extrusion,
similar to injection molding (see Section 19.3), forging, and rolling (hence the term
semisolid metalworking). These processes also are used in making parts with spe-
cially designed casting or wrought alloys and metal-matrix composites. They also
have the capability for blending granukes of different alloys, called thixoblending,
for specific applications.
Thixotropic behavior has been utilized in developing technologies that combine
casting and forging of parts using cast billets that are forged when 30 to 40% liquid.
Parts made include control arms, brackets, and steering components. Processing
steels by thixoforming has not yet reached the same stage as with aluminum and
magnesium, largely because of the high temperatures involved which adversely affect
die life and the difficulty in making complex shapes. The advantages of semisolid
metal forming over die casting are (a) the structures developed are homogeneous,
with uniform properties, lower porosity, and high strength; (b) both thin and thick
parts can be made; (c) casting as well as wrought alloys can be used; (d) parts subse-
quently can be heat treated, and (e) the lower superheat results in shorter cycle times.
However, material and overall costs are higher than those for die casting.

Rheocasting. This technique, first investigated in the 1960s, is used for forming
metals in the semisolid state. The metal is heated to just above its solidus tempera-
ture and poured into a vessel to cool it down to the semisolid state. The slurry is
then mixed and delivered to the mold or die. This process is being used successfully
with aluminum and magnesium alloys.

I l.4.8 Composite-mold Casting Operations
Composite molds are made of two or more different materials and are used in shell
molding and other casting processes. They generally are employed in casting com-
plex shapes, such as impellers for turbines. Composite molds increase the strength of
the mold, improve the dimensional accuracy and surface finish of castings, and can
help reduce overall costs and processing time. Molding materials commonly used
are shells (made as described previously), plaster, sand with binder, metal, and
graphite. These molds also may include cores and chills to control the rate of solidi-
fication in critical areas of castings.