Influences of Melt Superheating Treatment on Microstructure and Properties of AZ91D Magnesium Alloy

The influences of the melt superheating temperature on the solidification structure and mechanical properties of AZ91D magnesium were studied by a home-made electrical resistance furnace. The results show that with the increase of melt superheating temperature, the as-cast structure of AZ91D alloy changes from dendrite morphology into equiaxed grain morphology, the grain size gradually decreases, while the change of the grain size is not obvious when the melt superheating temperature exceeds 850 degrees C. With the enhancement of melt superheating temperature, the tensile strength, offset yield strength and specific elongation increase firstly and then decrease. The mechanical properties of AZ91D reach the maximum at 850 degrees C. The analysis of DSC shows that increasing the melt superheating temperature reduces the beginning solidification temperature, shrinks the freezing range, decreases the critical nucleus radius, increases the degree of supercooling of the melt and enhances the heterogeneous nucleation rate, which are the primary reason of grain refinement in magnesium alloy. The higher superheating temperature increases the eutectic transformation temperature, and hence the eutectic phase becomes coarse.