Effect of Gd content on hot-tearing susceptibility of Mg-6Zn-xGd casting alloys

The hot-tearing susceptibility (HTS) of Mg-6Zn-xGd (x=0.5,1,2,3,4,6) alloys was evaluated using a constrained rod casting (CRC) method.The results show that the HTS curve follows a typical "Λ" shape with the increase of Gd content.The Mg-6Zn-2Gd alloy has the highest while Mg-6Zn-6Gd alloy has the lowest HTS value.The hot-tearing behavior characteristics of Mg-6Zn-xGd alloys were further studied through a multifunctional hot-tearing test device.According to the dendrite contact point inferred from the stress curve,the Clyne-Davies criterion was modified and found to be accurate in predicting the HTS of Mg-6Zn-xGd alloys.Microstructure observation reveals that the grain size and the volume of eutectic liquid are the two key factors affecting HTS of Mg-6Zn-xGd alloys.The large grain with columnar structure can easily promote initiation and propagation of hot-tearing due to the poor feeding and coordinating deformation capability,which have a harmful effect on HTS.A higher volume fraction of eutectic phase can refill the cracking and provide continuous feeding channels by dendrite bridge and thicker liquid film,thus increase the hot-tearing resistance.