During the welding of AZ91 cast alloy,the presence of eutectic β-Mg17Al12 phase results in constitutional liquation in the original interdendritic regions and in the formation of a partially melted zone(PMZ). In this study,gas tungsten arc welding(GTAW) and partial melting(simulated using furnace,salt bath and Gleeble) experiments were conducted. The results show that practically there would not be a critical heating rate during the welding to prevent constitutional liquation. The gradual change of the re-solidification microstructure within PMZ from base metal side to weld metal side was characterized. A sharp transition from base metal to PMZ has been observed. It is found that the original partially divorced eutectic has become a more regular eutectic in most of the PMZ,although close to the fusion boundary the re-solidified eutectic is again a more divorced one. Proceeding the eutectic re-solidification,α-Mg re-solidified with a cellular growth resulting in a serrated interface. The morphological change affected by the peak temperature and cooling rate will be presented and explained. During this welding of AZ91 cast alloy, the presence of eutectic β-Mg17Al12 phase results in constitutional liquation in the original interdendritic regions and in the formation of partially melted zone (PMZ). In this study, gas tungsten arc welding (GTAW) and The results show that practically there would not be a critical heating rate during the welding to prevent constitutional liquation. The gradual change of the re-solidification microstructure within PMZ from base It is found that the original partially divorced eutectic has become a more regular eutectic in most of the PMZ, although close to the fusion boundary the re -solidified eutectic is again a more divorced one. Proceeding the eutectic re-solidification, alpha-Mg re-solidified with a cellular growth resulting in a serrated interface. The morphological change affected by the peak temperature and cooling rate will be presented and explained.