3.7.3 Heating Treating Aluminum Alloys

Aluminum alloys are heat treated for the same purpose as steel: to relieve stresses, improve mechanical properties or facilitate cold working or machining. The processes generally produce nearly uniform properties throughout the forging, and are not used for localized effects such as surface hardening.

The system for designating heat treatment for forgings is the standard temper designation system for cast and wrought alloys. The designation is an alpha-numeric sequence, which follows the alloy designation and is separated from it by a hyphen. For example:

Where necessary, additional digits are used to designate variations in treatment conditions within the major subdivisions. Table 3-7 gives an overview of tempering treatments for aluminum alloys.

Table 3-7 Overview of Temper Designations for Aluminum Alloys

Condition	Designation	Comments
No special control over thermal conditions or strain hardening	F: As Fabricated	No property limits for forgings.
Lowest strength temper.	O: Annealed	May be followed by a digit other than zero.
Strengthened by strain hardening, with or without supplementary thermal treatment to produce some strength reduction.	H: Strain Hardened	Always followed by two or more digits.
Unstable temper applicable only to alloys whose strengths change spontaneously at room temperature over time.	W: Solution Heat Treated	Always followed by one or more digits, e.g. T4...solution treated T6...aged
Applicable to alloys whose strength is stable within a few weeks of solution heat treatment.