Grain refining mechanism and casting structure of Mg–Zr alloy

Abstract

The investigation was performed to examine the influence of undissolved Zr particles in molten magnesium on the grain refinement of Mg–Zr alloy. A Mg–30%Zr master alloy was added to the molten pure magnesium at 993 K. All samples revealed that 0.4%Zr was dissolved in the pure magnesium. Sedimentation of Zr particles occured for holding the molten magnesium up to 7.2 ks at the temperature of 993 K and the grain size of the solidified structure became larger with prolonging holding time. However, when the molten metal was agitated just before pouring, grain refinement always occured. It is suggested that the undissolved Zr particles in the melt are nucleation sites of the solidifying magnesium or primary zirconium crystals. Zr content was the lowest around the undissolved Zr particles observed in Zr–enriched cored structure, because the undercooling occured for growth of the predendritic region.