Age Hardening / Ageing / Precipitation hardening
The fast cooling to near room temperature upon quenching produces a supersaturated condition where the material has already dissolved in it more of the constituents than it normally can carry in solution at that temperature. Such a condition obviously is unstable. The result is that certain constituents begin to separate out or precipitate from the main mass of the aluminum alloy.
This precipitation occurs at room temperature with many of the aluminum alloys and this action then is known as natural aging. Certain other alloys must be heated slightly to bring this precipitation to completion within a reasonable length of time. This is called artificial aging. In either case, this controlled precipitation is aimed at providing the correct size, character and distribution of the precipitated particles in the aluminum to produce strength and other desired mechanical properties.
Aging treatment may be applied to castings in the as-cast or solution-heat-treated conditions to improve the mechanical properties or it may be employed to stabilize solution heat treated castings. In either case, changes in the properties result from controlled precipitation of constituents from solid solution.
It should be understood that aging treatment varies with different alloys, depending on the properties required. For instance, strength and hardness are only increased at the expense of the ductility of the alloy. The extent to which these properties are changed depends both upon the aging temperature and the duration of the aging period. By the selection of other suitable aging periods and temperatures, various combinations of these mechanical properties may be attained. however, before any nonstandard treatments are attempted, preliminary test should be performed on the alloy in question to determine the proper time and temperature.
Castings in the solution heat treated condition can be artificially aged to produce tensile strengths and hardness in excess of those obtained by solution heat treatment. if the desired properties are lower than those produced by solution heat treatment, the castings can be aged directly from the as-cast condition without the benefit of previous solution heat treatment.