Microstructural evolution during partial remelting of AM60B magnesium alloy refined by MgCO3

AM60B magnesium alloy was refined by MgCO₃ and its microstructural evolution was investigated during partial remelting. The results indicate that MgCO₃ is an effective grain refiner for AM60B alloy and can decrease the grain size from 329 μm of the unrefined alloy to 69 μm. A semisolid microstructure with small and spheroidal primary particles can be obtained after being partially remelted. The microstructure evolution can be divided into four steps: the initial rapid coarsening, structure separation, spheroidization and final coarsening. Correspondingly, these four steps result from the phase transformations of β→α, α+β→L and α→L, α→L and two reverse reactions of α→L and L→α, respectively. One spheroidal primary particle in the semisolid microstructure usually originates one dendrite in the as-cast microstructure. The variation of primary particle size with holding time does not obey the LSW law, D_t³?D₀³=Kt, after the semisolid system is in its solid-liquid equilibrium state. Longer heating duration makes the primary particles more globular, but it makes their size larger at the same time.