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 grain refining technique of AZ91D magnesium alloy by MgCO 3 has been investigated. The refining mechanism and tensile properties of the resulting alloy have also been discussed. The results indicate that MgCO 3 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 MgCO 3 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 MgCO 3 belongs to heterogeneous nucleation and the nucleant substrates are believed to be the Al 4 C 3 particles formed from the reactions between the MgCO 3 and the molten alloy.

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 b→a, the structure separation is due to the a + b→L and a→L, the spheroidization is attributed to the a→L and the final coarsening is ascribed to the two reverse reactions of a→L and L→a. Rising the heating temperature during partial remelting is beneficial for obtaining small and spheroidal primary particles.

The experiment of laser alloying on the surface of P20 steel was made. Tungsten carbide (WC) powder was used as alloying material. The microstructure and property of laser alloying layer and nitriding layer are studied. The research showed that laser alloying layer had better properties such as minute crystals, deeper layer, higher hardness and good metallurgical bonding with base metal. The average hardness of alloying zone was 600HV0.2. The average hardness of phase-change hard zone was 450HV0.2. P20 steel was widely used in the field of plastic mold manufacture, especially mold core and cavity. Abrasion, corrosion and pressure resulted in change of mold size and shape which could greatly affect molding precision. Using laser alloying, the good wear layer would be made on the surface of p20 steel and would greatly increase the mold useful life.