Effect of thermal treatments on microstructure and impact toughness of die cast Mg–Al–Mn alloys

Abstract

A study of the effects of heat treatment on an Mg–Al–Mn alloy was carried out. Die cast AM60B alloy (Mg–6.0Al–>0.25Mn–<0.010Cu–<0.002Ni–<0.005Fe (wt-%)) specimens for microstructural investigation, tensile testing, and impact toughness testing, were produced using a multispecimen die in a high pressure, cold chamber apparatus. As cast specimens were studied either in their original condition or after they had been subjected to a direct aging treatment at 175°C. Solution treatment was also carried out, producing a T4 temper and a T6 temper by subsequent additional aging, and the resulting specimens examined. The investigation allowed evaluation of modifications to microstructural and mechanical properties produced by thermal treatments. In particular, the analysis of structure and solidification defect evolution showed that, despite an increase in void volume fraction and size induced by thermal treatments, a significant improvement of toughness during the crack growth process could be achieved with the appropriate tempers. Marked modifications to high strain rate loading conditions were detected, with improvements of total absorbed impact energy of up to 40% with respect to the as cast condition. This was associated with changes in fracture mechanisms, promoting a transition to a completely ductile mode in solution treated specimens.