Metals - Orthopaedic Biomaterials in Research and Practice - page 170

technique permits casting of very complex shapes and is frequently
used to fabricate partial and total joint replacement components.
Casting alloys are designed to be clean and to solidify without
forming pores or cracks, but the resulting cast parts are rela-
tively weak since they are relatively coarse grained and contain
many defects, especially residual impurities and porosity, so they
may not be highly fatigue resistant. Casting in vacuum gener-
ally improves casting cleanliness and final properties. Heat treat-
ment, to change the phase structure, may be useful but may also
result in overall grain growth, producing further reduction in
strength.
Hot isostatic pressing. Hot isostatic pressing (HIPing) may be used
to improve casting quality. It involves placing the castings in a
suitable inert gas-filled chamber to protect the alloy and subject-
ing them to very high pressures at moderately elevated tempera-
ture, well below the alloy melting point, to increase diffusion
rates and reduce the yield point. Since the pressure (hydrostatic)
is applied uniformly through the gas surrounding the part, there
are no deforming moments and shape is retained. Internal defects
tend to become condensed, and the alloy is stronger after treat-
ment. However, HIPing tends to improve only weak parts, raising
the average properties of a production batch and thus reducing the
incidence of service failures. It cannot radically change properties
of castings.

Mechanical
forming

Cast structures, even when very well designed, well made, and impurity
free, are relatively weak since they contain fairly large essentially polyg-
onal or equiaxial grains and have not been work hardened. Grain size
may be controlled, to a degree, by controlled cooling through the S + L
region of the phase diagram, since rapid cooling produces a smaller
average grain size (Figure 7.6).
However, mechanical properties of alloys may be radically improved
by the use of a variety of mechanical postforming processes creating
noncast materials. Many of these processes also may be used to improve
the mechanical properties of devices, after a primary casting step. Some
mechanical processes (other than machining) may be applied to casting
alloys, but many require alloys specially designed to withstand the rigors
of postcasting processing.
The principal noncasting forming processes in use are as follows:

Rolling and drawing. Ingots, bar, and rod may be converted mechani-
cally into different shapes by rolling between smooth or shaped
rollers or by pulling ( drawing ) through holes in hardened plates
(to produce wire). These processes are performed at room tem-
perature ( cold rolling ) or at elevated temperatures ( hot rolling ) to
improve ductility during processing. Each produces large amounts
of work hardening, depending on the amount of plastic strain and
the temperature, and produces elongated, very strong grains in the
direction of plastic deformation.