Microstructure and tensile properties of thixomolded magnesium alloys

Weight reduction is one of effective ways to reduce fuel consumption and greenhouse gas emissions in aerospace and ground vehicles. Cast magnesium alloys are increasingly used in the automotive industry because of the flexibility in fabrication and high strength-to-weight ratio. Thixomolding is an innovative and energy-saving manufacturing process which can be used to produce net-shape magnesium components. This study was aimed at evaluating tensile properties, strain hardening, strain-rate sensitivity, and fracture mechanisms of thixomolded AZ91D and AM60B alloys in relation to the microstructural features. The microstructure of both thixomolded alloys consisted mainly of globular primary α-Mg phase surrounded by network-like eutectic structure containing intermetallic β-Mg17Al12 phase. AM60B alloy had a higher volume fraction of α-Mg phase and lower amount of β-Mg17Al12 phase, exhibiting a lower yield strength and a higher ductility and strain hardening exponent than AZ91D alloy. The strain-rate sensitivity in both thixomolded alloys appeared to be weak, but it was slightly higher in AM60B alloy compared to AZ91D alloy. While AZ91D alloy displayed a gradual transition from elastic to plastic deformation, AM60B alloy showed a tendency of yield-point-like phenomenon which became more obvious with increasing strain rate. Stage III strain hardening rate decreased linearly with increasing true stress, but the slope of stage III hardening increased with increasing strain rate in both alloys. The AZ91D alloy exhibited more cleavage-like fracture characteristics together with the secondary cracking along the eutectic structure, while AM60B had more dimple-like features arising from the formation and coalescence of microvoids.