Pressure dependence of age-hardenability of aluminum cast alloys and coarsening of precipitates during hot isostatic pressing

The application of hot isostatic pressing is often mandatory to improve the fatigue resistance of aluminum cast alloys in order to meet the high quality requirements defined by automotive and aircraft industries. A pressure difference of 75 MPa between heat treatment and hot isostatic pressing affects the diffusivity and the precipitation kinetics. It is shown that the coarsening of precipitates during hot isostatic pressing is slower compared to the coarsening of precipitates during heat treatment. It is supposed that the high pressure within this densification process reduces the density of vacancies and therefore decreases the diffusivity of silicon and magnesium atoms, resulting in a lower critical cooling rate at which an oversaturated condition of dissolved silicon and magnesium atoms within the aluminum matrix can be achieved. Thus, quenching following hot isostatic pressing can be performed in slightly modified standard hot isostatic presses at lower cooling rates than quenching during standard heat treatment and even then does result in high age-hardenability of the alloy. It becomes possible to combine hot isostatic pressing and solution annealing within a single process step.