Semi-solid microstructure of AZ91D magnesium alloy recycled from waste chips during heat treatment

In this paper, waste chips of AZ91D magnesium alloy were recycled to prepare semi-solid billets. The chips were hot compressed into billets. And then, the billets were subjected to a series of isothermal heat treatment for various holding times. Its semi-solid microstructure evolution was investigated during heat treatment. A semi-solid microstructure with highly spheroidal and homogeneous primary particles can be obtained after partially remelting. With increasing heating temperature, the diffusion of Al element from eutectic Mg₁₇Al₁₂ phase to α-Mg first took place, resulting in the primary dendritic grains coarsening into interconnected non-dendritic grains. With heating continuously, the residual interdendritic γ-Mg₁₇Al₁₂ at the edges of the primary grains melted in succession and the primary grains separated into small polygon grains. Increasing the heating temperature can reduce the solid volume fraction. During the semi-solid isothermal treatment, with the increasing of the holding time, the solid particles became more globular and the amount of liquid increased until the solid-liquid system reached its equilibrium state. At the same time, owing to the decreasing of interfacial energy and the effect of interfacial tension, the particles gradually spheroidized and began to grow with a further increasing of the holding time.