Phenomenological observations on thixoformability of a zinc alloy ZA27 and the resulting microstructures

The thixoformability to cylindrical rods of a Zr-modified zinc alloy ZA27 and the resulting microstructures were observed with different reheating temperatures, reheating durations and mould temperatures. The results were compared with those of an unmodified ZA27 alloy formed by semi-solid die-casting. It was found that the formability of the Zr-modified alloy was significantly superior to that of the unmodified one. The microstructures of the thixoformed rods were basically uniform along their axial direction and the primary particle fraction obviously decreased along radial direction. The primary particle size slightly increased and the primary particle fraction decreased with increasing reheating duration. Both the size and fraction decreased with the increase of reheating temperature, while both of them increased as the die temperature increased. The mould-filling process and subsequent secondary solidification were also discussed.