Biomedical Engineering Reference
In-Depth Information
enriched to provide ore suitable for further processing
into pure metal and/or various alloys.
For example, with titanium, certain mines in the
southeastern United States yield sands containing pri-
marily common quartz but also mineral deposits of
zircon, titanium, iron, and rare earth elements. The sandy
mixture can be concentrated by using water flow and
gravity to separate out the metal-containing sands into
titanium-containing compounds such as rutile (TiO
2
)
and ilmenite (FeTiO
3
). To obtain rutile, which is partic-
ularly good for making metallic titanium, further
processing typically involves electrostatic separations.
Then, to extract titanium metal from the rutile, one
method involves treating the ore with chlorine to make
titanium tetrachloride liquid, which in turn is treated
with magnesium or sodium to produce chlorides of the
latter metals and bulk titanium ''sponge'' according to the
Kroll process. At this stage, the titanium sponge is not of
controlled purity. So, depending on the purity grade
desired in the final titanium product, it is necessary to
refine it further by using vacuum furnaces, remelting, and
additional steps. All of this can be critical in producing
titanium with the appropriate properties. For example,
the four most common grades of commercially pure
(CP) titanium differ in oxygen content by only tenths of
a percent, but these small differences in oxygen content
can make major differences in mechanical properties
such as yield and tensile and fatigue strength of titanium,
as discussed later in this section. In any case, from the
preceding extraction steps, the resulting raw metal
product eventually emerges in some type of bulk form,
such as ingots, which can be supplied to raw materials
vendors or metal manufacturers.
In the case of multicomponent metallic implant alloys,
the raw metal product will usually have to be processed
further both chemically and physically. Processing steps
include remelting, the addition of alloying elements, and
controlled solidification to produce an alloy that meets
certain chemical and metallurgical specifications. For
example, to make ASTM (American Society for Testing
and Materials) F138 316L stainless steel, iron is alloyed
with specific amounts of carbon, silicon, nickel, and
chromium. To make ASTM F75 or F90 alloy, cobalt is
alloyed with specific amounts of chromium, molybde-
num, carbon, nickel, and other elements.
Table 3.2.9-1
lists the chemical compositions of some metallic alloys
for surgical implants.
(e.g., implant manufacturers) who need stock metal that
is closer to the final form of the implant. For example,
a maker of screw-shaped dental implants might want to
buy rods of the appropriate metal to simplify the ma-
chining of the screws from the rod stock.
The metal supplier might transform the metal prod-
uct into stock shapes by a variety of processes, including
remelting and continuous casting, hot rolling, forging, and
cold drawing through dies. Depending on the metal,
there may also be heat-treating steps (carefully con-
trolled heating and cooling cycles) designed to facilitate
further working or shaping of the stock; relieve the ef-
fects of prior plastic deformation (e.g., as in annealing);
or produce a specific microstructure and properties in
the stock material. Because of the high chemical re-
activity of some metals at elevated temperatures, high-
temperature processes may require vacuum conditions or
inert atmospheres to prevent unwanted uptake of oxygen
by the metal, all of which adds to cost. For instance, in
the production of fine powders of ASTM F75 Co-Cr-Mo
alloy, molten metal is often ejected through a small
nozzle to produce a fine spray of atomized droplets that
solidify while cooling in an inert argon atmosphere.
For metallic implant materials in general, stock shapes
are often chemically and metallurgically tested at this
early stage to ensure that the chemical composition and
microstructure of the metal meet industry standards for
surgical implants (ASTM Standards), as discussed later
in this section. In other words, an implant manufacturer
will want assurance that they are buying an appropriate
grade of stock metal.
Stock metal shapes to preliminary
and final metal devices
Typically, an implant manufacturer will buy stock mate-
rial and then fabricate preliminary and final forms of the
device from the stock material. Specific steps depend on
a number of factors, including the final geometry of the
implant, the forming and machining properties of the
metal, and the costs of alternative fabrication methods.
Fabrication methods include investment casting (the
''lost wax'' process), conventional and computer-based
machining (CAD/CAM), forging, powder metallurgical
processes (e.g., hot isostatic pressing (HIP)), and a range
of grinding and polishing steps. A variety of fabrication
methods are required because not all implant alloys can
be feasibly or economically made in the same way. For
instance, cobalt-based alloys are extremely difficult to
machine by conventional methods into the complicated
shapes of some implants. Therefore, many cobalt-based
alloys are frequently shaped into implant forms by in-
vestment casting (e.g.,
Fig. 3.2.9-1B
) or powder metal-
lurgy. On the other hand, titanium is relatively difficult to
Raw metal product to stock metal shapes
A metal supplier further processes the bulk raw metal
product (metal or alloy) into ''stock'' bulk shapes, such as
bars, wire, sheet, rods, plates, tubes, or powders. These
stock shapes may then be sold to specialty companies
