抄録
Semisolid injection molding is expected to be increasingly applied to high flammable magnesium alloys as a new forming process because it can be carried out under temperatures lower than that of die-cast. In this study, we investigated the effects of molding conditions on the tensile strength and internal casting defect of AZ91D magnesium alloy. Semisolid injection molding was conducted at the injection speeds of V ＝ 220, 300 and 400mm/s, and fraction solidfs ＝ 0.0, 0.3, 0.4, and 0.5. The volume of the casting defects decreased with the reduction of the injection speed, but on the other hand, the mechanical strength reached maximum value at the injection speed of 300mm/s. The results of investigations showed that the solidification microstructure, α-Mg, and β-Mg17Al12 phase which were liquid during injection were refined with increasing injection speed, suggesting that the tensile strength increases with increasing injection speed if there are no casting defects. This may be because the heat-transfer coefficient between the mold and slurry increases with rising flow rate. On the other hand, the volume of casting defects increased with increasing injection speed and deteriorating tensile strength. When the injection speed increased, the effects of decreased strength due to the increase in the number of casting defects and effects of increased strength due to the refinement of the liquid phase counterbalanced each other. For this reason, it is considered that the mechanical strength reached the maximum value at the injection speed of 300mm/s. Thus, the tensile strength of semisolid products was affected not only by the number of casting defects but also by the microstructure of the residual liquid phase which was refined by the increase in injection speed.
