Age-hardening behavior of the extruded Al 7075/Mg EW75 composite

Abstract

The Al 7075/Mg EW75 composite with bulk Al and bulk Mg joined by metallurgical bonding on the interface was heat treated to improve the strength of the composite. The evolution of the microstructure and hardness of the Al 7075/Mg EW75 composite was investigated by combing experiments and numerical simulations. The thickness of the interface first increased slowly and then sharply during solution heat treatment and the Mg matrix near the interface incipiently melted after prolonged treatment at 470 °C for 40 min. The small difference in the coefficient of thermal expansion between the intermetallic phases and that of the matrix was responsible for the low stress developed in the interface with a low cooling rate. The low thermal conductivity of the intermetallic phases generated high temperature gradient under a high quenching rate which plastically deformed the matrix and led to high stress magnitude and cracks in the intermetallic layer. The hardness of the Mg matrix increased slightly and that of the Al matrix decreased sharply after T5 heat treatment. The hardness of both Mg and Al matrix increased apparently after T6 heat treatment. The intermetallic phase had a negligible effect on the hardness under both T5 and T6 heat treatment. T6 treatment was preferable to strengthen the composite.