Two Step Preaging of an Al Mg Si Alloy

Preaging PA is an industrial routine that suppresses the deleterious effect of natural aging and enhances the final paint bake hardening of 6XXX aluminum alloys. In view of the different advantages of PA at high and low temperatures, the effects of performing PA in two steps at different temperatures, namely, 80 and 160 C, are explored. Various two step PA combinations are investigated, involving both the temperature orders and various aging times at both temperatures, while aiming at the same hardness after all PA treatments. These results are interpreted on the basis of cluster formation and vacancy evolution during two step PA. In particular, the first PA step is found to play a more important role than suggested by the durations of the two PA steps as the clusters formed in the first step can strongly influence the subsequent evolution of both clusters and vacancies in the second step. It is found that two step PA results in a compromise between the effects of one step PA at both temperatures, that is, the enhancement of natural secondary aging stability is accompanied by a reduced paint bake hardening or vice versa