Temper designations of aluminium and aluminium alloysTemper designations are used to characterise the mechanical and especially the strength properties of aluminium alloys. Heat treatment is the simplest and most important procedure for improving the strength of age-hardenable aluminium alloys. Apart from heat-treatment, cold-working represents is also applied to increase strength. In so doing, however, the form and dimensions of the work piece are changed. Both methods are often combined, for instance profiles are first quenched, then strained , i. e. cold-worked and subsequently artificially aged.
Heat Treatment:The term ‘heat treatment’ is used to describe measures taken to deliberately alter the properties of a material by exposing it to elevated temperature and thus changing its structure. With aluminium alloys, age hardening is the most important process, but heat treatment also includes thermal softening and annealing, which are described in more detail below.
Age Hardening: Age hardening is an effective method of increasing strength. It involves a non-equilibrium situation that can occur in an aluminium solid solution under certain conditions. At high temperature, the alloying constituents are normally dissolved homogenously in solid solution; however, these elements have a decreasing solubility with decreasing temperature. Thus, if the temperature is suddenly reduced, the alloying constituents present in solid solution are unable to precipitate out of solid solution and the solid solution becomes supersaturated in these elements. Long-term exposure to room temperature or a slightly elevated temperature then results in the alloying elements precipitating out of solution with the formation of fine, very uniformly distributed particles. . These particles then block the slip planes and hinder the movement of dislocations. There is a corresponding increase in strength.
(You can find more information about heat treatment, and age hardening in particular, in the specialist articles Age hardening usually takes place in three stages, namely solution treatment, quenching and then artificial or natural ageing.
Solution Treatment: Alloying elements that are in solid solution, which are often distributed non-uniformly throughout the grain structure and have precipitated out as a second phase, are dissolved to form a solid solution at a temperature of about 460–560 °C, and thus homogenised.
Quenching:The metal has to be cooled down as rapidly as possible in order to ‘freeze in’ the homogeneity obtained by solution treatment. At the same time, cooling should be carried out as slowly as possible to prevent parts distorting or warping as a result of internal stresses. Quenching also has an effect on important properties such as corrosion and toughness (so-called quench-sensitivity). The rate of cooling must therefore be defined exactly and take into account the alloy used and the type of semi-finished product.
Natural Ageing: Longer storage at ambient temperature or at around 120 – 180 °C. The foreign atoms are separated from the solid solution, resulting in the formation of particles, while hardness and strength inccrease.
Thermal Softening: Thermal softening is another method of heat treatment. This process is also referred to as recovery heat treatment. It is carried out at temperatures between 150–250 °C. The strengthening of a material by rolling or cold impact exreusion has the disadvantage that material loses its ductility: it becomes brittle. Softening by annealing at temperatures below the recrystallisation temperature results in some loss in strength and produces a so-called ‘three-quarters hard’ or ‘half hard’ temper, but there is a large increase in ductility.
Annealing: Annealing, or recrystallisation annealing, is carried out at 350–450 °C. Cold worked alloys (see above) are completely softened in order to allow them to be worked further. Annealing causes recrystallisation to occur: completely new grains grow from the nuclei (the smallest possible ideal grain) ) in the grains that have been severely deformed by cold working. The whole crystal structure is thus formed anew and creates favourable conditions for further forming.
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