Page 81 - Shigley's Mechanical Engineering Design

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56 Mechanical Engineering Design
Considering the cost and strength of aluminum and its alloys, they are among the
most versatile materials from the standpoint of fabrication. Aluminum can be processed
by sand casting, die casting, hot or cold working, or extruding. Its alloys can be machined,
press-worked, soldered, brazed, or welded. Pure aluminum melts at 660°C (1215°F),
which makes it very desirable for the production of either permanent or sand-mold
castings. It is commercially available in the form of plate, bar, sheet, foil, rod, and tube
and in structural and extruded shapes. Certain precautions must be taken in joining
aluminum by soldering, brazing, or welding; these joining methods are not recommended
for all alloys.
The corrosion resistance of the aluminum alloys depends upon the formation of a
thin oxide coating. This film forms spontaneously because aluminum is inherently very
reactive. Constant erosion or abrasion removes this film and allows corrosion to take
place. An extra-heavy oxide film may be produced by the process called anodizing. In
this process the specimen is made to become the anode in an electrolyte, which may be
chromic acid, oxalic acid, or sulfuric acid. It is possible in this process to control the
color of the resulting film very accurately.
The most useful alloying elements for aluminum are copper, silicon, manganese,
magnesium, and zinc. Aluminum alloys are classified as casting alloys or wrought
alloys. The casting alloys have greater percentages of alloying elements to facilitate
casting, but this makes cold working difficult. Many of the casting alloys, and some of
the wrought alloys, cannot be hardened by heat treatment. The alloys that are heat-
treatable use an alloying element that dissolves in the aluminum. The heat treatment
consists of heating the specimen to a temperature that permits the alloying element to
pass into solution, then quenching so rapidly that the alloying element is not precipi-
tated. The aging process may be accelerated by heating slightly, which results in even
greater hardness and strength. One of the better-known heat-treatable alloys is duralu-
minum, or 2017 (4 percent Cu, 0.5 percent Mg, 0.5 percent Mn). This alloy hardens in
4 days at room temperature. Because of this rapid aging, the alloy must be stored under
refrigeration after quenching and before forming, or it must be formed immediately
after quenching. Other alloys (such as 5053) have been developed that age-harden much
more slowly, so that only mild refrigeration is required before forming. After forming,
they are artificially aged in a furnace and possess approximately the same strength and
hardness as the 2024 alloys. Those alloys of aluminum that cannot be heat-treated can
be hardened only by cold working. Both work hardening and the hardening produced
by heat treatment may be removed by an annealing process.
Magnesium
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The density of magnesium is about 1800 kg/m (0.065 lb/in ), which is two-thirds that
of aluminum and one-fourth that of steel. Since it is the lightest of all commercial met-
als, its greatest use is in the aircraft and automotive industries, but other uses are now
being found for it. Although the magnesium alloys do not have great strength, because
of their light weight the strength-weight ratio compares favorably with the stronger
aluminum and steel alloys. Even so, magnesium alloys find their greatest use in appli-
cations where strength is not an important consideration. Magnesium will not withstand
elevated temperatures; the yield point is definitely reduced when the temperature is
raised to that of boiling water.
Magnesium and its alloys have a modulus of elasticity of 45 GPa (6.5 Mpsi) in ten-
sion and in compression, although some alloys are not as strong in compression as in
tension. Curiously enough, cold working reduces the modulus of elasticity. A range of
cast magnesium alloys are also available.