Hot tearing characteristics of Mg–2Ca–xZn alloys

Influence of Zn content (0, 0.5, 1.5, 4 and 6 wt%) on the hot tearing characteristics of Mg–2 wt% Ca alloy was investigated. The constrained rod casting (CRC) apparatus equipped with a load cell and data acquisition system was used. The initiation of hot tearing was monitored during solidification. The effect of mould temperatures (250 and 450 °C) on the hot tearing was also investigated. The formed tears were evaluated using X-ray tomography and the tear volumes were measured. Results show that hot tearing susceptibility (HTS) of Mg–2Ca–xZn (x = 0, 0.5, 1.5, 4 and 6 wt%) alloys increases with increase in Zn content up to 1.5 wt%, then decreases with further increase in the Zn content to 6 wt%. Higher initial mould temperature (450 °C) improves the hot tearing resistance. The observations on the microstructures and the fracture surfaces suggest that the hot tear initiated at the grain boundaries and propagated along them through the thin liquid film rupture and liquid metal embrittlement of solid bridges. Tear healing by low melting point eutectic liquid is also observed in some of the alloys.