The microstructure of the thixoformed ZA27 alloy naturally aged for 5 years has been observed and analyzed. The results indicate that most of the original β + η laminar eutectics have changed into the structures that small α particles uniformly distribute in η phase matrix. Both the β phase and α’ phase have also decomposed. It is suggested that a continuous or dot-line α phase first precipitates in the β phase close to the eutectic η phase layer. Then eutectoid η phase nucleates or directly grows up from the eutectic η phase, which promotes the nucleation of eutectoid α phase or the direct growth of the α phase from the existing α phase. Subsequently, the resulting α + η phases grow towards the β phase inside, forming the regular laminar α + η structures. The laminar structures always continuously grow across the center α’ phase, which leads the α’ phase also to transform into regular α + η laminar structures. The laminar structures may crack possibly due to internal stress. The eutectoid η phase lamellae have a tendency of changing from laminar shape to particle form, resulting in the formation of non-laminar structures.

The effects of processing parameters, such as the punch speed, reheating duration and reheating temperature, on microstructure and ultimate tensile strength (UTS) of thixoformed AM60B magnesium alloy have been investigated. The results indicate that low punch speed, short reheating duration or low reheating temperature often produces the defect of cold shuts or shrinkage porosities. On the contrary, gas pores can easily form. In addition, the reheating duration or temperature also has obvious effects on the primary particle size, fraction and morphology. The three parameters can significantly affect the UTS due to their effects on the formation of pores (cold shuts, shrinkage porosities and gas pores). But they can not alter the fracture path during tensile testing and the path is always along the secondarily solidified structures between the primary particles. The effect of pore amount on the UTS is lager than that of the primary particle size, fraction or morphology. In view of their effects on the microstructure compactness and the resultant UTS, the optimized parameters are reheating for 130 minutes at 610 °C and punch speed of 3 m/s.

The microstructural evolution and corresponding phase transformations have been investigated during partial remelting of AZ91D magnesium alloy refined by Al-Ti-B master alloy. The effect of heating temperature on semisolid microstructure has also been discussed. The results indicate that the microstructural evolution process includes four stages, the initial rapid coarsening, structure separation, spheroidization accompanied by coarsening and the final coarsening. Two or more equiaxed dendrites in the as-cast microstructure evolve into one spheroidal primary particle in the semisolid microstructure through the former three stages. The initial rapid coarsening results from the reaction of β→α, the structure separation is due to the α + β→L and α→L, the spheroidization is attributed to the α→L and the final coarsening is ascribed to the two reverse reactions of α→L and L→α. Rising the heating temperature during partial remelting is beneficial for obtaining small and spheroidal primary particles.

The grain refining technique of AZ91D magnesium alloy by MgCO3 has been investigated. The refining mechanism and tensile properties of the resulting alloy have also been discussed. The results indicate that MgCO3 can decrease its grain size from 311 to 53µm. Correspondingly, the tensile properties are obviously improved. The higher the cooling rate from addition temperature to pouring temperature or the higher the addition temperature, the finer the grains. The melt treated by MgCO3 should be poured as soon as possible because the inoculation fading is quite quick. The microstructure sensitivity to the diameter of a cast rod is relatively high and the microstructures of the rods with large diameters are quite inhomogeneous. The refining mechanism of MgCO3 belongs to heterogeneous nucleation and the nucleant substrates are believed to be the Al4C3 particles formed from the reactions between the MgCO3 and the molten alloy.