Aluminum alloys

(metallurgy) An alloy of aluminum and relatively small amounts of other metals, such as copper, magnesium, or manganese.

Substances formed by the addition of one or more elements, usually metals, to aluminum. The principal alloying elements in aluminum-base alloys are magnesium, silicon, copper, zinc, and manganese. In wrought products, which constitute the greatest use of aluminum, the alloys are identified by four-digit numbers of the form NXXX, where the value of N denotes the alloy type and the principal alloying element(s) as follows: 1 (Al; at least 99% aluminum by weight), 2 (Cu), 3 (Mn), 4 (Si), 5 (Mg), 6 (Mg + Si), 7 (Zn), 8 (other). See also Aluminum.

Iron and silicon are commonly present as impurities in aluminum alloys, although the amounts may be controlled to achieve specific mechanical or physical properties. Minor amounts of other elements, such as Cr, Zr, V, Pb, and Bi, are added to specific alloys for special purposes. Titanium additions are frequently employed to produce a refined cast structure.

Aluminum-base alloys are generally prepared by making the alloying additions to molten aluminum, forming a liquid solution. As the alloy freezes, phase separation occurs to satisfy phase equilibria requirements and the decrease in solubility as the temperature is lowered. The resultant solidified structure consists of grains of aluminum-rich solid solution and crystals of intermetallic compounds. See also Eutectics.

A decrease in solubility with falling temperature also provides the basis for heat treatment of solid aluminum alloys. In this operation, the alloy is held for some time at a high temperature to promote dissolution of soluble phases and homogenization of the alloy by diffusion processes. The limiting temperature is the melting point of the lowest melting phase present. The time required depends both on temperature and on the distances over which diffusion must occur to achieve the desired degree of homogenization. The solution heat treatment is followed by a quenching operation in which the article is rapidly cooled, for example, by plunging it into cold or hot water or by the use of an air blast.

Casting alloys are significant users of secondary metal (recovered from scrap for reuse). Thus, casting alloys usually contain minor amounts of a variety of elements; these do no harm as long as they are kept within certain limits. The use of secondary metal is also of increasing importance in wrought alloy manufacturing as producers take steps to reduce the energy required in producing fabricated aluminum products. See also Alloy.

Since aluminum comprises 70–80% of the weight of an airframe, metallurgists have been pursuing aluminum alloy development programs directed toward producing materials which would be characterized by stronger, stiffer, and lighter-weight properties. In addition, the titanium alloys of aircraft gas turbines are prime targets for replacement by lighter-weight alloys. Researchers have improved aluminum alloys by adding lithium and by blending aluminum alloy powders and silicon carbide fibers to form a composite. See also Composite material.