Aluminium metallurgy

Tempers of aluminium alloys

Alloy and Temper

All aluminium products are identified by:

  • Alloy.
    Alloy designations are four-digit numbers. These identify the alloy chemistry.
  • Temper.
    Temper designation consists of a single letter followed by 1, 2 or 3 digits. Temper designations refer to variations of the physical properties that are achievable within an alloy. They are added to the alloy designations, after the four-digit alloy designation.

An example is 6060-T6.

Here 6060 is the designation of the alloy, and T6 is the designation of the heat-hardened state of this alloy, that is, T6 temperature. Depending on the heat treatment conditions, 6060 alloy can receive other heat hardening options, such as T4 temper or T5 temper and some others.

The concept of “temper” applies to both wrought and cast alloys. However, the most numerous and varied temperas are applied to wrought alloys.

Wrought aluminium alloys

Wrought aluminium alloys are either heat treatable and non-heat treatable:

  • 1xxx, 3xxx, and 5xxx series alloys are non-heat treatable
  • 2xxx, 6xxx, and 7xxx series alloys are heat treatable
  • 4xxx series alloys contain both heat-treatable and non-heat treatable varieties.

Alloys in the non-heat-treatable group cannot be strengthened significantly by heat treatment, and their properties depend upon the degree of cold work.

Heat-treatable alloys can be strengthened by heat treatment.

The differences in the chemical and metallurgical structures of the alloy groups have an impact on how the alloys react during the welding process, as well as the other fabrications processes such as forming, bending or punching.

End users should understand these designations in detail so that, in any subsequent processes, they do not destroy key capabilities provided by the producer [1]. For example, the same 6060 aluminium alloy in different tempers can get different properties, including the following indicators:

  • maximum strength (T66)
  • maximum workability (O)
  • good bending ability (T4)
  • high corrosion resistance (T7)
  • good weldability (T4)
  • good anodisability (T6)

Tempers of wrought aluminium alloys

Temper designations tell both a producer and a user how the alloy has been mechanically and/or thermally treated to achieve the properties desired [1]. The first character in the temper designation (a capital letter, F, O, H, W, or T) indicates the general class of treatment.

Temper Designations for Aluminum and Aluminum Alloys

  • F, as fabricated.
    Most F-temper products are “semi-finished” products. They will be used in shaping, finishing or thermal processes to achieve other finished forms or tempers.
  • O, annealed.
    Annealing treatments are used to achieve the lowest-strength condition for the alloy. The main reason is to maximize workability or increase toughness and ductility.
  • H, strain-hardened. This is for non-heat-treatable alloys that have had their strength increased by strain hardening, usually at room temperature.
  • W, solution heat-treated. This designation applies only to alloys that age naturally and spontaneously after solution heat treating. It is rarely a finished temper.
  • T, thermally treated. This applies to any product form of any heat-treatable alloy that has been given a solution heat treatment followed by quenching and aging.

Preface

The temper designation system is based on sequences of basic treatments used to produce different tempers and their variations. Subdivisions
of the basic tempers are indicated by one or more digits following the letter.

Subdivisions of the Basic H Tempers

  • The first number(s) following the letter designation indicates the specific combination of basic operations:
    H1, strain hardened only:
    Applies to products that have been strain hardened to obtain a desired level of strength without a supplementary thermal treatment. The number following H1 indicates degree of strain
    hardening.
  • H2, strain hardened and partially annealed:
    Applies to products that have been strain hardened more than the desired final amount, and their strength is reduced to the desired level by partial annealing. The
    number added to H2 indicates the degree of strain hardening remaining after partial annealing.
  • H3, strain hardened and stabilized: Applies to products that have been strain hardened and then stabilized either by a low temperature thermal treatment, or as a result of heat introduced during fabrication of the product. The H3 temper is used only for those alloys that will gradually age soften at room temperature if they are not stabilized. The number added to H3
    indicates the degree of strain hardening remaining after stabilization.
  • H4, strain hardened and lacquered or painted: Applies to products that are strain hardened and that have been subjected to heat during subsequent painting or lacquering operations. The number added to H4 indicates the amount of strain hardening left after painting or lacquering.

Subdivisions of the Basic T Temper

The first number(s) following the letter T designation indicates the specific combination of basic operations:

  • T1, cooled from elevated temperature shaping process and naturally aged to a substantially stable condition
  • T2, cooled from an elevated temperature shaping process, cold worked, and naturally aged to a substantially stable condition
  • T3, solution heat treated, cold worked, and naturally aged to a substantially stable condition
  • T4, solution heat treated and naturally aged to a substantially stable condition
  • T5, cooled from an elevated temperature shaping process, then artificially aged
  • T6, solution treated, then artificially aged
  • T7, solution heat treated and overaged/stabilized
  • Q8, solution heat treated, cold worked, then artificially aged
  • T9, solution heat treated, artificially aged, then cold worked
  • T10, cooled from an elevated temperature shaping process, cold worked, then artificially aged

Relationship between different tempers of the same aluminum alloy

Fig. 1 [3]

Fig. 2 [3]

The examples of typical tempers for heat-treatable aluminium alloys

Tempers T3 and T4

Fig. 3 [4]

Fig. 4 [4]

Temper T6

Fig. 5 [4]

Fig. 6 [4]

Temper T7

Fig. 7 [4]

Sources:

  1. A quick guide to understanding aluminium temper designations /Suzanne Koch, Hydro
  2. Introduction to Aluminum Alloys and Tempers – J. Gilbert Kaufman
  3. Design of aluminium structures Introduction to Eurocode 9 with worked examples – European Aluminium – 2020
  4. Elements of Metallurgy and Engineering Alloys /ed. by F.C. Campbell – ASM International