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: bRa, azbRL and aRL. The variation of the primary particles with heating time obeyed the Lifshitz, Slyozov and Wagner law, D 3 ðtÞ 2D 3 ð0Þ 5Kt, 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.
The microstructural evolution process in SiC particle (SiC p )-reinforced ZA27 composites, previously compressed by different lengths at 270 °C, was investigated during partial remelting. The results indicated that, after being compressed, the microstructure of SiC p /ZA27 composites changed gradually from developed primary ␣ dendrites, to coarsened short dendrites, and then to a structure consisting of texture cells with different orientations with increasing compression. During subsequent partial remelting, the microstructure evolution exhibited four stages: initial rapid coarsening, structural separation, spheroidization, and final coarsening. The change of the primary ␣ particle size with the heating time obeyed the law developed by Lifshitz and Slyosov and Wagner (LSW) after the semisolid system was up to the dynamic solid-liquid equilibrium state. High compression, high remelting temperature, and large size and proper content of SiC p s were beneficial to achieving a desirable structure for semisolid forming (SSF). The primary ␣ particle size distribution was significantly affected by coalescence. These phenomena were extensively discussed metallographically and theoretically.
The effects of two main processing parameters such as reheating temperature and reheating time on microstructure and tensile properties of thixoforged AZ63 magnesium alloy have been investigated. The results indicate that these two parameters all have large effects on the microstructure and tensile properties. However, in contrast, the effects of the reheating temperature on liquid fraction and liquid composition of semisolid ingot, and thus the subsequent solidification behaviour and plastic deformation during thixoforging, are larger than those of the reheating time. Therefore, the effects of the former parameter on the microstructure and tensile properties are relatively larger than those of the latter parameter. It is also just this reason that the fracture regime during tensile testing varies as the reheating temperature rises, while the regime always maintains transgranular mode with the variation of the reheating time. Stress concentration that results from the plastic deformation obviously affects the tensile properties, while the primary particle size or grain size only has small effect. The appropriate reheating technique is the heating for 60 min at 595°C when mould temperature is maintained at 350°C. The ultimate tensile strength and elongation of the resulting alloy are up to 310 MPa and 10·7 respectively.
Microstructural evolution during partial remelting of AM60B alloy refined by 0.2 wt.% SiC particles at 615 °C has been investigated. The effect of as-cast microstructure on the resulting semisolid microstructure has also been discussed. The results indicate that the evolution process includes five stages, initial rapid coarsening, structure separation, spheroidization and coarsening, partial melting and final coarsening. The finer the as-cast microstructure is, the smaller and more spheroidal the primary particles of the resulting semisolid microstructure are. There is a threshold for the grain size of the as-cast microstructure. When the grain size is about 100 μm, one dendrite frequently evolves into one primary particle after being partially remelted. But when the grain size is larger than 100 μm, one dendrite often separates into several particles, and when it is smaller than 100 μm, one primary particle always originates from several dendrites.
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