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,Dt3-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.