Control of Liquid Fraction and Microstructure of AZ91D Mg alloy Slurry

摘要

Microstructural evolution of AZ91D Mg alloy during partial remelting was evaluated as a function of isothermal holding temperature and time in the semisolid state, with special attention to the prior solidification and reheating conditions before and during partial remelting, respectively. The shape and size of the solid phase and the liquid fraction were particularly examined. The results show that the globalization of the solid phase occurs, and the total liquid fraction increases with isothermal holding time while the grain-interior liquid fraction decreases. It is also reported that the size of the solid phase in the semisolid state is independent of holding temperature, indicating that the size of the solid phase is a function of isothermal holding time with the initial size determined by the initial grain size. The grain-interior liquid is induced by the coalescence of dendrite arms with the liquation of small grain boundaries and segregated regions, and the shape of it is almost globular due to the crystallographic related surface energy consideration. The fact that the average diameter of the solid globules is reduced at a relatively high temperature is mainly due to the preferential remelting.