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 6h and ageing at 190°C for 33h) caused increase of grain size to 92μm 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 135MPa, compression strength to 383MPa and hardness to 73HV5.
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