Microstructure and mechanical properties of Mg–Zn–RE–Zr alloy after thixoforming

Abstract

Magnesium is particularly challenging material, when formed from liquid phase because of high flammability risk. An alternative process for casting, which eliminates above mentioned disadvantage, is thixoforming, which involves a lower temperature of process and operation in the partially solidified state. Influence of semi-solid metal processing on EZ33A magnesium alloy (Mg–Zn–RE–Zr) microstructure and mechanical properties was studied. Ingot microstructure revealed globular grains with coarse eutectic mixture consisting of Mg7Zn3RE, T-phase – RE(Mg,Zn)11 and α(Mg). Heterogeneous nucleation of magnesium solid solution allowed obtaining structure appropriate for thixoforming. Using differential scanning calorimetry, temperature of process was determined to be 622 °C, which corresponded to about 30% of the liquid phase. Thixo-cast microstructure consisted of α(Mg) globular grains with a size of 76 ± 1.1 surrounded by fine eutectic mixture in a volume of 35%. T6 heat treatment (solution at 500 °C for 6 h and ageing at 190 °C for 33 h) caused increase of grain size to 92 μm and the precipitation of two kinds of phases within the α(Mg): β′1 and β′2 responsible for the increase of yield strength to 135 MPa, compression strength to 383 MPa and hardness to 73 HV5. Keywords: Magnesium alloys, Rare earth elements, Thixoforming, Microstructure