Identification Of Heat-Treated Aluminium Alloys

Aluminium alloys that have been subjected to heat-treatment are usually identified by markings that indicate the heat-treatments involved. Three typical identification systems are those of the British Standards Institute (BS), the Ministry of Supply (MoS), and the American systems as can be seen below,


Identification Markings of Heat Treated Aluminium Alloys

BS System Meaning
M As manufactured state
O Annealed state
OD Annealed and lightly drawn
T Solution-treated, no precipitation required
W Solution-treated, can be precipitated
WP Solution-treated and precipitation treated

MoS System Meaning
A Annealed state
N Solution-treated, no precipitation required
W Solution-treated, and requires precipitation
WP Solution-treated and precipitation treated

American System Meaning
T3 Solution-treated and cold worked
T4 Solution-treated only (naturally aged)
T6 Solution-treated and artificially aged
T8 Solution-treated, cold worked and artificially aged
T9 Solution-treated, artificially aged and cold worked

An example of one of these marking systems would be an alloy with the designation 2024-T4, which indicates an aluminium/copper alloy that has been solution-treated only, and then naturally aged

Apart from these systems, many other exist world-wide, but the British systems are broadly confined to the following for light alloys.

• British Standards for general engineering use BS 1470 -1475. In this series the prefix N is used to denote non-heat-treatable aluminium alloys and prefix H for the heat-treatable alloys.

• British Standards for aerospace use the L series such as BS 3 L72, which indicates the 3rd amendment to the basic L 72 specification whilst LM indicates a cast material. The wrought materials are commonly abbreviated to L71, L72 and L 73 et al.

Examples of some of these aircraft BS codes are:

• 159 Dural Solution-Treated and Artificially aged
• L163 Alclad Solution-Treated and Naturally aged

Dural is a Trade name for an 2017 Al/Cu/Mg/Si/Mn alloy, originally manufactured by the Duren Aluminium Company (Germany) for the Zeppelin Airships. It is often used as a generic name for similar alloys, regardless of source of manufacture.

DTD Specifications are material identification numbers issued by the Directorate of Technical Development (a Ministry Department) for specialised applications, when widespread use is not anticipated. If such a material finally becomes commonly used, a British Standards specification is compiled and issued.