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12 METAL ASIA | 12/
variation, together with controlled and rapid solidification,
increases properties further because it produces castings that
are almost entirely free of porosity.
Although both variations improve properties and speed
casting cycles, the added equipment complexities limit the casting
size that can be handled. Consequently, all three permanent-
mold processes are in use today, turning out aluminium
castings weighing from less than one pound to several hundred
pounds.
Aluminium Matrix Composites: Metal matrix composites
(MMCs) consist of metal alloys reinforced with fibers, whiskers,
particulates, or wires. Alloys of numerous metals (aluminium,
titanium, magnesium and copper) have been used as matrices
to date. Recent MMC developments, however, seem to thrust
aluminium into the spotlight. In the NASA space shuttle, for
example, 240 struts are made from aluminium reinforced with
boron fibers. Also, aluminium diesel-engine pistons that have
been locally reinforced with ceramic fibers are eliminating the
need for wear-resistant nickel-cast iron inserts in the automotive
environment.
Fabrication methods differ for both products. Monolayer
tapes in the space shuttle struts are wrapped around a mandrel
and hot isostatically pressed to diffusion bond the layers. For the
pistons, a squeeze-casting process infiltrates liquid metal into
a fiber preform under pressure. Other fabrication methods for
MMCs include: hot pressing a layer of parallel fibers between foils
to create a monolayer tape; creep and super plastic forming in a
die; and spraying metal plasmas on collimated fibers followed
by hot pressing.
Super Plastic Aluminium: Super plastic forming of metal, a
process similar to vacuum forming of plastic sheet, has been used
to form low-strength aluminium into nonstructural parts such
as cash-register housings, luggage compartments for passenger
trains, and nonload-bearing aircraft components. New in this
area of technology is a super plastic-formable high-strength
aluminium alloy, now available for structural applications and
designated 7475-02. Strength of alloy 7475 is in the range of
aerospace alloy 7075, which requires conventional forming
operations. Although initial cost of 7475 is higher, finished part
cost is usually lower than that of 7075 because of the savings
involved in the simplified design/assembly.
Beryllium
Among structural metals, beryllium has a unique combination
of properties. It has low density (two-thirds that of aluminium),
high modulus per weight (five times that of ultrahigh-strength
steels), high specific heat, high strength per density, excellent
dimensional stability, and transparency to X-rays. Beryllium is
expensive, however, and its impact strength is low compared
to values for most other metals. Available forms include block,
rod, sheet, plate, foil, extrusions, and wire. Machining blanks,
which are machined from large vacuum hot pressings, make
up the majority of beryllium purchases. However, shapes can
also be produced directly from powder by processes such as
cold-press/sinter/coin, CIP/HIP, CIP/sinter, CIP/hot-press
and plasma spray/sinter. CIP is cold-isostatic press, and HIP is
hot-isostatic press. Mechanical properties depend on powder
characteristics, chemistry, consolidation process, and thermal
treatment. Wrought forms, produced by hot working, have high
strength in the working direction, but properties are usually
anisotropic.
Beryllium parts can be hot formed from cross-rolled sheet
and plate as well as plate machined from hot-pressed block.
Forming rates are slower than for titanium, for example, but
tooling and forming costs for production items are comparable.
Structural assemblies of beryllium components can be joined by
most techniques such as mechanical fasteners, rivets, adhesive
bonding, brazing, and diffusion bonding. Fusion-welding
processes are generally avoided because they cause excessive
grain growth and reduced mechanical properties.
Beryllium behaves like other light metals when exposed to
air by forming a tenacious protective oxide film that provides
corrosion protection. However, the bare metal corrodes readily
when exposed for prolonged periods to tap or seawater or
to a corrosive environment that includes high humidity. The
corrosion resistance of beryllium in both aqueous and gaseous
environments can be improved by applying chemical conversion,
metallic, or nonmetallic coatings. Beryllium can be electroless
nickel plated, and flame or plasma sprayed.
All conventional machining operations are possible with
beryllium, including EDM and ECM. However, beryllium
powder is toxic if inhaled. Since airborne beryllium particles
and beryllium salts present a health hazard, the metal must be
machined in specially equipped facilities for safety. Machining
damages the surface of beryllium parts. Strength is reduced by
the formation of micro cracks and “twinning.” The depth of the
damage can be limited during finish machining by taking several
light machining cuts and sharpening cutting tools frequently
or by using nonconventional metal-removal processes. For
highly stressed structural parts, 0.002 to 0.004 in. should be
removed from each surface by chemical etching or milling
after machining. This process removes cracks and other surface
damage caused by machining, thereby preventing premature
failure. Precision parts should be machined with a sequence of
light cuts and intermediate thermal stress reliefs to provide the
greatest dimensional stability.©periodictable