The term "heat treating" refers to the heating and cooling operations that are performed in order to change the mechanical properties, metallurgical structure or residual stress state of a metal product. For aluminum alloys, the term "heat treating" usually refers to precipitation hardening of the heat treatable aluminum alloys. Annealing, a process that reduces strength and hardness while increasing ductility, can also be used for both the non-heat treatable and heat treatable grades of wrought and cast alloys.
The importance of precipitation hardening of aluminum alloys can be appreciated by examining the data presented in Fig. 2.10 for naturally aged 2024 and the artificially aged 7075. Note the dramatic increase in strength of both due to precipitation hardening with only a moderate reduction in elongation.
For an aluminum alloy to be precipitation hardened, certain conditions must be satisfied. First, the alloy must contain at least one element or compound in a sufficient amount that has a decreasing solid solubility in aluminum with decreasing temperature. In other words, the elements or compounds must have an appreciable solubility at high temperatures and only minimal solubility at lower temperatures. Elements that have this characteristic are copper, zinc, silicon, and magnesium, with compounds such as CuAl2, Mg2Si, and MgZn2. While this is a requirement, it is not sufficient; some aluminum systems that display this behavior cannot be strengthened by heat treatment. The second
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