Tempers of heat-treatable aluminium alloys

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Definitions

Temper

Condition of metal of aluminium or aluminium alloy produced by mechanical and/or thermal processing. Typically characterized by a certain macro or micro metallurgical structure and specified properties. These properties may be mechanical, corrosion, electrical or xxxxx.

The temper designation always is presented immediately following the alloy designation, with a hyphen between the two (e.g., 6063-T6).
The temper designation consists of a capital letter indicating the major class of fabrication treatment(s) used, plus one or more numbers providing more specific information about how the processing was carried out.

Solution heat treatment

Heating an alloy to a suitable temperature for sufficient time to allow one or more soluble constituent to enter into solid solution, where they are retained in supersaturated state after quenching.

Furnace solution heat treatment is heating an alloy to a suitable temperature in a furnace and holding for a sufficient time to allow one or more soluble constituents to enter into solid solution, where they are retained in a supersaturated state after quenching.
Extrusion press solution heat treatment is heating an alloy to a suitable temperature then extruding, while holding for sufficient time to allow one or more soluble constituents to enter into solid solution, where they are retained in a supersaturated state after quenching

Quenching

Cooling a metal from an elevated temperature by contact with a solid, a liquid or a gas, at a rate rapid enough to retain most or all of the soluble constituents in solid solution

Press quenching is quenching of an extruded product on its exit from the extrusion press.
Critical quenching rate is minimum mean cooling rate from solution treatment temperature necessary to enable an alloy to possess certain mechanical properties in the precipitation hardened condition.

Ageing

Treatment of a metal aiming at a change in its properties by precipitation of intermetallic phases from supersaturated solid solution.

Ageing can be a treatment at room temperature (natural ageing) or a thermal treatment (artificial ageing).
Under-ageing is artificial ageing below peak ageing temperature and time conditions resulting in slightly reduced tensile strength and improved ductility, compared with peak aged metal.
Over-ageing is artificial ageing beyond peak ageing temperature and time conditions, in order to improve selected metallurgical characteristics of the metal, Alloys Used without Heat-Treatment. resistance to stress corrosion or intergranular corrosion. Over-ageing results in reduced tensile properties compared with peak aged metal.

Annealing

Thermal treatment to soften metal:

by reduction or removal of strain hardening resulting from cold working
and/or
by coalescing precipitates from the solid solution.

Non-heat Treatable Alloys

Alloys whose strength/mechanical properties are achieved by cold working (rolling, extruding, etc.). Sometimes called work hardening alloys or strain hardening alloys. Tempers are denoted by first letter H. The example is 5052-H24.

Heat Treatable Alloys

Alloys whose strength/mechanical properties are achieved by heat treatment followed by cooling and natural or artificial ageing. Tempers are denoted by first letter T. The example is 6060-T66.

Heat treatment of aluminium alloys

The complete heat-treatment consists of:

a solution heat-treatment,
a quenching process and
subsequent ageing, where the actual hardening occurs.

Unlike steel, aluminium alloys are not hard immediately after quenching. To get the highest strength values it is important to keep the material at the correct solution heat temperature for enough time and to follow the correct quenching procedure. Depending on the alloy, this may be carried out using water or air.

Heat-treatable alloys are produced in many tempers. The basic tempers for heat-treatable alloys are shown below. Each of these basic tempers can have several pieces and even dozens of options for more specific tempers for various types of aluminum products.

Basis tempers for heat-treatable aluminium alloys

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

Table 1 – Various uses of the second digit after letter T [3]

Fig. 1 – The relationships for some T7X temper proprties [3]

Fig. 2 – Relative position of the variations of T6 and T7 tempers [3]

The tempers of extrusion 6xxx series aluminium alloys

For extrusion 6xxx series aluminium alloys only a limited number of tempers is important. Below are listed the tempers that are used in the European standard EN 755-2 which specifies the requirements for the mechanical properties of extruded aluminum products.

The tempers of 6xxx series extruded aluminium alloys

O – Annealed. Product achived the required annealed properties after hot extrusion and may be designated as O temper.
H111 – Annealed and slightly strain-hardened during subsequent operations such as stretching.
T4 – Solution heat-treated and naturally aged.
T4510 – Solution heat-treated, stress-relieved by stretching a controlled amount (permanent set 1 % to 3 %) and naturally aged. The products receive no further straightening after stretching.
T4511 – Same as T4510 exept that minor straightening is allowedafter stretching to comply with dimensional tolerances.
T5 – Cooled from hot extrusion and then artifically aged.
T6 – Solution heat-treated and then artificially aged.
T61 – Solution heat-treated and then artificially aged in underageing conditions to improve formability.
T64 – Solution heat-treated and then artificially aged in underageing conditions (between T6 and T61) to improve formability.
T6510 – Solution heat-treated, stress-relieved by stretching a controlled amount (permanent set 1 % to 3 %) and naturally aged. The products receive no further straightening after stretching.
T6511 – Same as T6510 exept that minor straightening is allowed after stretching to comply with dimensional tolerances.
T66 – Solution heat-treated and then artificially aged. Mechanical property level higher than T6 achieved through special control of the process 6000 series alloys.
T7 – Solution heat-treated and then artifiacally overaged.
T78 – Solution heat-treated and then artifiacally overaged in order to achieve optimal intercrystalline corrosion resistance.
T78510 – Solution heat-treated, stress-relieved by stretching a controlled amount (permanent set 1 % to 3 %) and then articially overaged to T78 condition. The products receive no further straightening after stretching.
T78511 – Same as T78510 exept that minor straightening is allowed after stretching to comply with dimensional tolerances.