A calorimetric and metallographic study of precipitation process in AA6061 and its composites

A calorimetric study carried out on AA6061 alloy and SiC particulate and δ-Al2O3 (Saffil) fibre reinforced composites has shown that the positions of the exothermic reaction peaks in DSC spectra were influenced by the vacancy concentration (V.C.) retained after quenching, in addition to the influence of the presence of ceramic reinforcement in the materials. It was also found that the influence of V.C. was more effective in the unreinforced base alloy than in the SiCp composite. Furthermore, a relatively larger exothermic reaction peak associated with precipitation of the Mg-Si needle-shaped phase and a smaller peak for the MgSi rod-shaped precipitates were observed in the DSC thermograms of the composites. These observations indicated that during the temperature scanning, a higher density of dislocations in the matrix would enhance the precipitation of the needle phase, which in turn would deplete the matrix, and suppress the formation of the rod precipitates. The results obtained from the strained AA6061 base alloy, plastically deformed after quenching, strongly support the above conclusion. It is believed that the accelerated ageing response observed in AA6061 composites was caused mainly by an enhanced precipitation of the needle-shaped precipitates in the matrix.