Microstructural evolution of equal channel angular pressed AZ91D magnesium alloy during partial remelting

In the present work, the authors investigated the microstructural evolution process of equal channel angular pressed (ECAPed) AZ91D magnesium alloy during partial remelting, and the effects of pressing pass, pressing route and heating temperature on the semisolid microstructures. The results indicated that the microstructure evolution could be divided into four steps: the initial coarsening due to the dissolution of interdendritic eutectics, structure separation resulting from the melting of the residual eutectic and the penetration of the firstly formed liquid into the recrystallised boundaries, spheroidisation due to the partial melting of primary particles and final coarsening attributed to Ostwald ripening. Correspondingly, a series of phase transformations occurred in turn: β→a, a→β→L and α→L. The variation of the primary particles with heating time obeyed the Lifshitz, Slyozov and Wagner law, D³_(t)–D³₍₀₎=Kt, after the semisolid system was up to liquid–solid equilibrium state. Increasing the heating temperature was beneficial for obtaining an ideal semisolid microstructure because of the decreased tendency of the primary particles to merge. With the increase in pressing pass, the size of the primary particles decreased and their morphology tended to be more spheroidal. Simultaneously, the amount of liquid phase slightly increased because an increased amount of structure melted due to the increased energy stored in the alloy. At a given pressing pass, the semisolid microstructure of the alloy processed by route B_C was quite ideal for thixoforming while that of the alloy processed by route A was not completely suitable. In addition, because of the difference in the stored energy, the liquid amount of the former alloy was obviously larger than that of the latter alloy.