Precipitation Hardening Aluminium

Precipitation Hardening Aluminium

The hardening of aluminium consists of two processes namely solution treatment and aging. 

Solution treatment is the process of heating the aluminium to temperatures of between 500 – 550oC depending on the alloy and holding it at that temperature to ensure that certain elements such as copper, magnesium, zinc, and silicon dissolve forming a single solid solution.

Once this has been achieved, the components are quenched as rapidly as possible, usually into water.  The rapidly quenched supersaturated structure shows no precipitation and is unstable.

Once the components have been solution treated, either natural or artificial aging follows.

During artificial aging, the parts are re-heated to temperatures between 100 – 200oC for a period which allows precipitates to form and grow in the lattice resulting in the lattice being stressed and causing hardness and strength to increase. Significant hardness and strength increases can be achieved through this process.

Benefits

The mechanical properties of heat-treatable aluminium alloy components can be optimised by the selection of an appropriate solution treatment 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 type of alloy, its size and shape, and cross-section at the time of solution treatment, various cooling methods can potentially be utilised to reduce distortion.

Application

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.