Effect of Cooling Rate on Mg₁₇Al₁₂ Volume Fraction and Compositional Inhomogeneity in a Sand-Cast AZ91D Magnesium Plate

抄録

Mg₁₇Al₁₂ compound is a hardening phase in AZ91D alloy. Moreover, the concentration of Al and Zn has been found to be closely related to the corrosion performance of the magnesium alloy. The aim of this work was to study the effect of cooling rate on distribution of Mg₁₇Al₁₂ compound and compositional inhomogeneity in an AZ91D magnesium sand-cast plate (200×140×20 mm³). A copper chill block, which was placed at the end of mold cavity, was used to increase the cooling rate during solidification. A sand-cast plate was also produced, where no chill block being mounted at the end of mold cavity. The “with chill block” plate showed a rapid increasing in cooling rate with respect to distance from riser, as compared to the “without chill block” plate where almost no cooling rate fluctuation occurred. The volume fraction of Mg₁₇Al₁₂ (β phase) in the “without chill block” plate was higher than that in “with chill block” plate. In the “without chill block” plate, volume fraction of Mg₁₇Al₁₂ was about 13.9 vol% (near the riser) to about 19.3 vol% (close to the end of the plate). However, the “with chill block” plate was solidified in a higher cooling rate, leading to low volume fraction of the β phase (13.4 vol%). Higher cooling rate also resulted in more severe compositional inhomogeneity in the sand-cast plate. The Al and Zn concentration in the “with chill block” plate showed a concave downward dependence against the distance to riser. Moreover, in the “with chill block” plate, concentrations of Al and Zn did not enrich at the position near chill face. Instead, the Al and Zn contents near the chill surface were well below the average value. This finding is in disagreement with previous studies.