The first step in the heat treatment for precipitation hardening is to equilibrate the alloy in the α region; that is, to solution heat treat the alloy. This alloy must be heated between 500 and 570 °C (930 and 1060 °F) for sufficient time to homogenize the structure. After solution heat treatment, the alloy must be cooled to 20 °C at a rate sufficient to suppress any precipitation. The precipitation process begins immediately, and the rate depends on the temperature at which the alloy is held after quenching from the solution treatment temperature. In this Al-Cu-Mg alloy, the rate at 20 °C is sufficiently great to cause prominent mechanical property changes. Precipitation hardening at room temperature is referred to as age hardening, or natural hardening.
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