Aluminium alloys are alloys in which aluminium (Al) is the predominant metal. The typical alloying elements are copper, magnesium, manganese, silicon and zinc. There are two principal classifications, namely casting alloys and wrought alloys, both of which are further subdivided into the categories heat-treatable and non-heat-treatable. About 85% of aluminium is used for wrought products, for example rolled plate, foils and extrusions. Cast aluminium alloys yield cost effective products due to the low melting point, although they generally have lower tensile strengths than wrought alloys. The most important cast aluminium alloy system is Al-Si, where the high levels of silicon (4.0% to 13%) contribute to give good casting characteristics. Aluminium alloys are widely used in engineering structures and components where light weight or corrosion resistance is required.
Alloys composed mostly of the two lightweight metals aluminium and magnesium have been very important in aerospace manufacturing since somewhat before 1940. Aluminium-magnesium alloys are both lighter than other aluminium alloys and much less flammable than alloys that contain a very high percentage of magnesium.
Aluminium alloy surfaces will keep their apparent shine in a dry environment due to the formation of a clear, protective layer of aluminium oxide. In a wet environment, galvanic corrosion can occur when an aluminium alloy is placed in electrical contact with other metals with more negative corrosion potentials than aluminium.
Aluminium alloy compositions are registered with The Aluminum Association. Many organizations publish more specific standards for the manufacture of aluminium alloy, including the Society of Automotive Engineers standards organization, specifically its aerospace standards subgroups, and ASTM International.
The temper designation follows the cast or wrought designation number with a dash, a letter, and potentially a one to three digit number, e.g. 6061-T6. The definitions for the tempers are:
Strain hardened (cold worked) with or without thermal treatment
Strain hardened without thermal treatment
Strain hardened and partially annealed
Strain hardened and stabilized by low temperature heating
A second digit denotes the degree of hardness
-HX2 = 1/4 hard
-HX4 = 1/2 hard
-HX6 = 3/4 hard
-HX8 = full hard
-HX9 = extra hard
Full soft (annealed)
Heat treated to produce stable tempers
Cooled from hot working and naturally aged (at room temperature)
Cooled from hot working, cold-worked, and naturally aged
Solution heat treated and cold worked
Solution heat treated and naturally aged
Cooled from hot working and artificially aged (at elevated temperature)
Stress relieved by stretching
No further straightening after stretching
Minor straightening after stretching
Stress relieved by thermal treatment
Solution heat treated and artificially aged
Solution heat treated and stabilized
Solution heat treated, cold worked, and artificially aged
Solution heat treated, artificially aged, and cold worked
Cooled from hot working, cold-worked, and artificially aged
Solution heat treated only.
Note: -W is a relatively soft intermediary designation that applies after heat treat and before aging is completed. The -W condition can be extended at extremely low temperatures but not indefinitely and depending on the material will typically last no longer than 15 minutes at ambient temperatures.