Effect of melting technique on the microstructure and mechanical properties of AZ91 commercial magnesium alloys

The microstructure of AZ91 magnesium alloy refined by thermal rate treatment technique (TRT), and the β-phase is more dispersive and uniform. Though the superheating microstructure is finer than microstructure with TRT technique, TRT technique can reduce the casting defects such as hot tears and shot run, more suits the practical requirement. The microstructure of AZ91 alloy keeps some characteristics of the high temperature melt after TRT. On the other hand, the microstructure of AZ91 magnesium alloy containing excess element Fe could not be refined and even be coarsened after thermal rate treatment, which brings on the decrease of mechanical properties as the results of interdendritic shrinkage microporosity and coarse β-phases. These different effects of TRT on AZ91 magnesium alloys can be related to the presence of non-equilibrium condition in the mixed melt structure through mixing the high superheating temperature and low temperature melts in TRT technique.