Effect of initial as-cast microstructure of AZ91D magnesium alloy on its semisolid microstructure

The effect of initial as-cast microstructure of AZ91D alloy on its semisolid microstructure has been investigated. The uniformities of semisolid microstructures of cast ingots with different diameters have also been examined. The results indicate that the initial as-cast microstructure has large effect on the semisolid microstructure: the finer the as-cast microstructure, the smaller and more spheroidal the primary particles in the resulting semisolid microstructure. For the alloys with grain sizes of 70-100 μm, one dendrite in the as-cast microstructure frequently evolves into one spheroidal primary particle in the semisolid microstructure during partial remelting. But for the alloys having grain sizes of larger than 100 μm, one dendrite always separates into two or more particles with large-size difference and irregular morphologies. The microstructure inhomogeneity of cast ingots must result in the inhomogeneity of their semisolid microstructures and these two inhomogeneities are enhanced as the ingot diameter increases. The microstructure inhomogeneity can be decreased to some extent after being partially remelted. But to obtain a uniform semisolid microstructure, it is necessary to achieve a uniform as-cast microstructure.