Microstructure evolution of AZ91D magnesium alloy during semi-solid isothermal heat treatment under direct current

The influences of direct current density, heating temperature, and holding time on the microstructure evolution of magnesium alloy AZ91D during semi-solid isothermal heat treatment were studied, and the related mechanism was also discussed. When heating temperature and holding time are constant, with the increment of current density the primary phase becomes small and smooth. But if current density is too high, the primary grains become coarser. Direct current can accelerate the course of semi-solid isothermal heat treatment and improve globular effect of dendrites obviously. It is proposed that the activation energy is increased by direct current, so the diffusivity of solute atoms is increased, which accelerates the microstructure evolution. The effect of electromigration, Joule heat and Peltier heat caused by direct current will promote remelting and globuling of dendrites.