Effects of different processing parameters on the semisolid microstructure of the AZ91D alloy during partial remelting

The effects of different processing parameters, such as heating rate, isothermal temperature and isothermal holding time, on the semisolid microstructures of the AZ91D alloy during partial remelting had been investigated in the present paper. The experimental result showed that the high heating rate shortened the growth process of renascent recrystal grains in the deformed alloys, which ultimately attributed to refine the globular α-Mg grains in the evolved semisolid microstructure. The liquid volume fraction fL of the semisolid slurry gradually enlarged with the elevating of isothermal temperature during partial remelting. The isothermal temperature between 560 °C and 580 °C for 30 min was adequate for the AZ91D to produce an ideal semisolid slurry with liquid fraction range between 32% and 52%. During the partial remelting, it would experience a relatively long holding time that the liquid and solid phases reached a state of dynamic equilibrium in the semisolid slurry. For the AZ91D alloy, the isothermal holding time of 30 min was relatively adequate for the present experiment. During the partial remelting, the primary irregular grains gradually spherodized under a function of interface curvature. Oswald ripening mechanism, a dissolution and reprecipitation diffusion-controlled growth, could account for the coarsening of globular grains in the present experiment. The spheroidization and coarsening evolution processes of solid particles in the semisolid slurry were deduced and elucidated by the corresponding simulant formulae in the present paper.