Microstructure and properties of semi-solid magnesium alloy billets prepared by cold rolling and SIMA process

The semi-solid billets of AZ91D magnesium alloy were prepared by the cold rolling and semi-solid heat treatment process. The effects of the cold rolling reduction on microstructure and compressive properties of the semi-solid billets were analyzed, the forming mechanism of the billets and the spheroidizing mechanism of the solid particles were discussed. The results show that with the cold rolling reduction increasing, the average size of the particles decreases and its sphericity rate increases, and the liquid fraction increases gradually; however the compressive properties of the billets increase first and decrease afterwards. Otherwise, it is very sensitive to the microstructure morphology of the billets while the cold rolling reduction is less than 8.72%. During the semi-solid heat treatment, the interface on the atomic scale of the solid particles has a transformation from smooth small crystal face to rough and non-small crystal face, which is the polygon transforming to sphericity on the macroscale. With the cold rolling reduction increasing, the transformation process speeds up obviously.