Effect of solid-solution treatment on microstructure and hardness of die-casting Mg-9Al-1Zn-0.2Mn alloy

Mg-9Al-1Zn-0.2Mn alloy (mass fraction, %) prepared by cold chamber die casting was heat treated by different solution treatment technologies and then aged at 150 /5 h. The effect of solution treatment on the microstructure and hardness of the die-casting alloys was investigated using X-ray diffraction, optical microscope, scanning electron microscopy and Vickers hardness testing. The results show that the β-Mg17Al12 phases distributed along the grain boundary can dissolve into α-Mg matrix through the solid-solution treatment.With increasing temperature and time of solid-solution treatment,the β-Mg17Al12 phase existed continuously as netlike in the grain boundaries of α-Mg phase decreases and precipitates at the inner of α-Mg crystal instead of in grain boundaries. However, the crystal grain of die-casting alloy will be coarsed and over sintering if the solution temperature is too high or the solution time is too long. The aging treatment after solid-solution treatment make β-Mg17Al12 phases occur with discontinuous and continuous precipitation from oversaturated α-Mg solid solution, and thus increase the hardness of the die-casting alloys.