ALUMINUM - Materials for Civil and Construction Engineers - page 142

separate the aluminum and oxygen molecules. The molten aluminum is
collected at the cathode at the bottom of the bath. The molten aluminum,
with better than 99% purity, is siphoned off to a crucible. It is then
processed in a holding furnace. Hot gases are passed through the molten
material to further remove any remaining impurities. Alloying elements
are then added.
The molten aluminum is either shipped to a foundry for casting into
finished products or is cast into ingots. The ingots are formed by a direct-
chill process that produces huge sheets for rolling mills, round loglike bil-
lets for extrusion presses, or square billets for production of wire, rod, and
bar stock.
Final products are made by either casting, which is the oldest process,
or deforming solid aluminum stock. Three forms of casting are used: die
casting, permanent mold casting, and sand casting. The basic deformation
processes are forging, impact extrusion, stamping, drawing, and drawing
plus ironing. Many structural shapes are made with the extrusion process.
Either cast or deformed products can be machined to produce the final
shape and surface texture, and they can be heat treated to alter the mechan-
ical behavior of the aluminum. Casting and forming methods are summa-
rized in Table 4.1.
When recycling aluminum, the scrap stock is melted in a furnace. The
molten aluminum is purified and alloys are added. This process takes only
about 5% of the electricity that is needed to produce aluminum from bauxite.
In addition to these conventional processes, very high strength alu-
minum parts can be produced using powder metallurgy methods. A pow-
dered aluminum alloy is compacted in a mold. The material is heated to a
temperature that fuses the particles into a unified solid.

4.2 Aluminum Metallurgy

Aluminum has a face center cubic (FCC) lattice structure. It is very mal-
leable, with a typical elongation over a 50-mm (2-in.) gauge length of over
40%. It has limited tensile strength, on the order of 28 MPa (4000 psi). The
modulus of elasticity of aluminum is about 69 GPa (10,000 ksi). Commer-
cially pure aluminum (i.e., more than 99% aluminum content) is limited to
nonstructural applications, such as electrical conductors, chemical equip-
ment, and sheet metal work.
Although the strength of pure aluminum is relatively low, aluminum al-
loys can be as much as 15 times stronger than pure aluminum, through the
addition of small amounts of alloying element, strain hardening by cold
working, and heat treatment. The common alloying elements are copper,
manganese, silicon, magnesium, and zinc. Cold working increases strength
by causing a disruption of the slip planes in the material that resulted from
the production process.