Effects of processing parameters on tensile properties and hardness of thixoformed ZA27 alloy

The influences of processing parameters, reheating temperature, reheating period of time and die temperature on tensile properties of thixoformed Zr modified ZA27 alloy were investigated. Simultaneously, the hardness of the alloy was also examined. The results indicated that the thixoforming process significantly improved both ultimate tensile strength and elongation of the alloy because of obvious decreasing of porosities, compared with the semi-solid die-cast no modified ZA27 alloy and permanent mould cast Zr modified ZA27 alloy. The tensile cracks initiated from the defects, such as voids, porosities and inclusions, and eutectic cell boundaries between primary particles, and then preferentially grew along eutectic structure, regardless of the processing parameters. The hardness was mainly affected by the solubility of Zn in the primary particles, the primary particle fraction and the amount of the decomposed primary particles. The hardness HB could be calculated from the micro-hardness HB_p of the primary particles, HB_e of the matrix and the primary particle fraction f_p according to an equation of HB = HB_p × f_p + HB_e(1- f_p).