The properties of aluminum alloys depend on a combination of chemical composition and response to thermal and/or mechanical treatments (i.e. cold work), so it is important to have a designation system that identifies the exact state or condition of the material at any point in its manufacture. Temper designations for aluminum alloys do just that but are often the subject of angst among heat treaters. It’s time to clear up any confusion. Let’s learn more.

In order to better understand temper designations, it is important to recall that there are two categories of aluminum: heat-treatable and non-heat-treatable alloys. For example, the 1xxx, 3xxx and 5xxx series wrought aluminum alloys cannot be hardened by heat treatment. These non-heat-treatable alloys produce optimum mechanical properties through strain hardening, that is, through the application of cold working.

By contrast, the 2xxx, 6xxx and 7xxx series wrought aluminum alloys are heat treatable, while the 4xxx series consist of both heat-treatable and non-heat-treatable alloys. For cast alloys, the 2xx.x, 3xx.x, 4xx.x and 7xx.x series cast alloys are heat treatable (note: mechanical-hardening methods are not generally applied to castings). Recall that in previous Heat Treat Doctor columns we have talked about how to achieve optimum mechanical properties through heat treatment (c.f. “Understanding Aluminum Heat Treatment,” Industrial Heating, February 2006 and “Heating Treating of Aluminum Castings,” Industrial Heating, February 2010).

The temper designation system is used for all forms of wrought and cast aluminum and aluminum alloys except ingots and is simply an extension of their alloy numbering system, which consists of a series of letters and numbers (Fig. 1) that follow the alloy designation number being separated by a hyphen (e.g., 6061-T6). Should some other variation of the same sequence of basic operations be applied to the same alloy, which results in different characteristics, then additional digits are added to the designation (see below).

Basic Temper Designations

Table 1 contains a list of the most common temper designations.

Common Subdivisions

Table 2 contains a list of some of the most common subdivisions of the basic tempers shown in Table 1.

Temper Variations (O)

A digit following the O, when used, indicates a product in the annealed condition has special characteristics (Table 3). It should be noted that the O temper is not part of the strain-hardened (H) series. Variations of O tempers do not apply to products that are strain hardened after annealing and in which the effect of strain hardening is recognized in the mechanical properties or other characteristics.

Unregistered Tempers

The letter P has been assigned to denote H, T and O temper variations that are negotiated between manufacturer and purchaser. The letter P immediately follows the temper designation that most nearly applies.

Stress-Relieved Tempers

The following specific additional digits have been assigned for stress-relieved tempers of wrought products.

Stress Relief by Stretching
Table 4 contains this information.

Stress Relief by Compression or Combination of Stretching and Compression
See Table 5 for this data.

Strain-Hardened Tempers

The digit following the designations H1, H2 and H3 indicates the degree of strain hardening (Table 6). Table 7 provides additional strain-hardened temper designations.