Study of microstructural formation mechanism of AZ91D magnesium alloy melt during semi-solid isothermal holding

The semi-solid slurry of AZ91D alloy, produced by a novel self-inoculation method, is isothermal treated. Microstructure evolution and mechanism of microstructure formation during isothermal holding are investigated. The results show that primary α-Mg grains are in small size and present polygon morphology in the slurry. During isothermal holding, the size of particles increases with increasing holding time, while the roundness turns better. But the excessive long holding time is bad for the size and roundness of particles. Deformation of dendrite arm is the primary cause of microstructure formation during pouring, meanwhile, ripening and coarsening during isothermal holding. The nucleus generated from the procedure of melt treatment by self-inoculation, and competitive nucleation in undercooled melt which is near the liquidus during isothermal holding is the main source of primary α-Mg phase.