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Solution and age: Aluminium alloys

There are a number of wrought and cast aluminium alloys that can be strengthened by solution treating and aging to a variety of different tempers.

The mechanical properties of heat treatable alloy components can be optimised by the selection of an appropriate solution and age process sequence. For certain alloys, corrosion resistance can, for example, be improved at the expense of strength and vice versa.

Depending on the alloy and cross section at the time of solution treatment, various cooling methods can potentially be utilised to reduce distortion.

Wrought alloys in the 2XXX (Al-Cu), 6XXX (Al-Mg-Si), 7XXX (Al-Zn-Mg-Cr) and 8XXX (Al-Li) series and cast alloys in the 2XX (Al-Cu), 3XX (Al-Mg-Si-Cu) and 7XX (Al-Zn) series can be solution treated and aged.

Final tempers of the T4X, T5X, T6X and T7X types are achievable as a function of alloy by thermal processing only. T3X and T8X tempers are achievable utilising a combination of thermal and thermomechanical processing, such as stretching or compressing simple shapes between solution treating and ageing.

Common wrought alloys and tempers are 2014-T4, 2014-T6, 2024-T3, 2024-T4, 2024-T6, 2024-T8, 2219-T3, 2219-T4, 2219-T6, 2219-T8, 2618-T6, 2618-T61, 6061-T4, 6061-T6, 7050-T74, 7075-T6, 7075-T73, 7075-T74, 7075-T76, 7175-T74. Common cast alloys are A201-T7, A206-T7, C355-T6, A356-T6, A357-T6.

Solution treating is typically performed in the 450 to 575°C (842 to 1067°F) range in air, followed by rapid quenching into cold water, hot water, boiling water (-T61 temper), water-polymer (glycol) solution, water spray or forced air. Natural ageing to the T4 temper will occur at ambient temperature for 2XXX, 6XXX, 2XX and 3XX alloys, with most reaching a stable temper after 96 hours. Artificial ageing in the 93 to 245°C (199 to 473°F) range is utilised to meet the T6 and T7X tempers.

Immediately after quenching from solution treating, all alloys are relatively soft and can be moderately formed or straightened if performed within a couple of hours. These alloys will naturally age harden at ambient temperature, with their hardness gradually increasing with time following quenching. This can be suppressed by refrigeration below about 0°C (32°F) if it is desired to form or straighten the material more than a couple of hours after quench. Cooling to below about minus 23°C (minus 10°F) or colder is required to retain the As-Quenched (AQ) temper for prolonged storage times beyond a few days.

For maximum formability prior to solution treating and ageing, these alloys must be fully annealed to produce a stable dead soft O temper by heating in the 400 to 425°C (752 to 797°F) range, then slow cooling (28°C or 50°F per hour or slower) to below 235°C (455°F), formed, then solution is treated and aged.