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The most common heat treatments used in the processing of aluminum products are solution heat treating and aging (tempering). Unlike ferrous alloys, which harden after quenching from an austenitizing temperature and then soften during tempering, nonferrous alloys, such as aluminum, soften when quenched from a solutionizing temperature and then harden when tempered or aged.
Solution heat treatment is achieved by heating cast or wrought products to a suitable temperature, holding at that temperature long enough to allow alloying elements to enter into solid solution, and cooling rapidly enough to hold the element constituents in solution. An aging process that occurs naturally at room temperature (natural aging) or may be induced by exposing the products to an elevated temperature (artificial aging) then follows the solution heat treatment process.
The strength of an alloy that exhibits natural aging will increase either continuously at room temperature or stabilize. Artificial aging is characterized by an increase in strength and hardness to a maximum and subsequently decrease. Softening effects observed as more complete precipitation, occur during extended aging at elevated temperatures and are referred to as "overaging." They are as significant as the strengthening effects associated with the preceding stages of precipitation.
Softening results from dilution of the solid solution and changes in both the type and size of precipitates. The softest, lowest strength condition of a heat treatable alloy is obtained by annealing treatments that precipitate the maximum amount of solute as relatively large, widely spaced, incoherent particles.
Temper Designations
F As fabricated: Applies to the products of shaping processes in which no special control over thermal conditions or strain-hardening is used. There are no mechanical property limits for wrought products,
O Annealed: Applies to wrought products that are annealed to obtain the lowest strength temper, and to cast products that are annealed to improve ductility and dimensional stability. The 0 may be followed by a digit other than zero.
W Solution heat treated: An unstable heat treatment...