Characterization of AZ31B magnesium alloy joints welded with a Nd:YAG laser

摘要

In the present study, an AZ31B magnesium alloy was welded using a low-power pulsed Nd:YAG laser attached to a six-axis welding robot. The welding metallurgical characteristics, including microstructure, porosity formation, texture evolution and microhardness distribution were systematically investigated using a scanning electron microscope (SEM), electron backscatter diffraction (EBSD) and a microhardness tester. The transverse cross-sectional microstructure of the AZ31B-alloy joint was composed of the base metal (BM), columnar-grain zone (CGZ) and equiaxed-grain zone (EGZ) in the center of the fusion zone (FZ). Some pores were observed in the FZ and believed to be related to the pre-existing pores in the BM. Especially, it was noted that many columnar grains observed in the transverse cross-section of the FZ were almost perpendicular to the solid-fusion boundary. Nevertheless, a large number of fine equiaxed grains formed on the top surface of the FZ and exhibited obvious random orientations. Different grain-growth mechanisms found on the transverse cross-section and the top surface of the weld bead were empirically investigated. Owing to apparent grain coarsening, the lowest microhardness value (~43 HV) was detected in the center of the FZ, which was evidently lower than that of the BM (~68 HV).