Modelling of Precipitation Hardening in Casting Aluminium Alloys

Precipitation hardening, because it involves the hardening of the material over a prolonged time, is also called age hardening, or ageing. By the appropriate heat treatment of precipitation hardening, the strength or hardness of some heat-treatable aluminium alloys can be enhanced by the formation of nano-meter sized second-phase precipitated particles within the original phase matrix. The fine precipitates in the alloy impede dislocation movement by forcing the dislocations to either cut through the precipitated particles or go around them. By restricting dislocation movement during deformation, the alloy is strengthened. Thus precipitation hardening is the most versatile and demanding heat treatment in aluminium alloys, either for wrought or casting alloys. The precipitation-hardening process generally involves following three basic steps: a. Solution heat treatment, or homogenization, is the first step where the alloy is heated between the solvus and solidus temperatures and soaked until all of the soluble phases are dissolved and a homogeneous solid-solution structure is produced. b. Quenching is the second step where the solid solution is rapidly cooled to a lower temperature, usually room temperature. This forms the formulation of a supersaturated solid solution (SSSS) since the solubility of one or more alloying elements in aluminium decreases with decreasing temperature. c. The final step in the precipitation hardening process is aging that allows controlled decomposition of the supersaturated solid solution (SSSS) and the formation of strengthening precipitates. It is the process where the supersaturated solution is heated below the solvus temperature to produce a finely dispersed precipitates. When ageing occurs at room temperature, it is called natural ageing. Ageing above room temperature is called artificial ageing. With the age-hardening aluminium alloys having become the backbone of the automotive and aerospace industries, although a century has now elapsed since the phenomenon of age or precipitation hardening was discovered by the German metallurgist, Alfred Wilm (Wilm, 1911), new observations are still being made as the latest experimental techniques reveal more details of the actual atomic process involved. Precipitation hardening was hailed as the first nanotechnology (Polmear, 2006) and now it is possible to develop fine-scale microstructures in a much wider range of alloys through the use of novel processing methods. The optimization of material processing techniques has, however, for a long time,