Refined microstructure and enhanced mechanical properties of AZ31 magnesium alloy welds by reduced pressure laser beam welding

Laser welding of magnesium alloys remains a significant challenge due to the tendency for microstructural coarsening and the frequent occurrence of welding defects, both of which negatively impact the mechanical performance of the joints. In this work, refined microstructure and reduced defects of enhanced properties of AZ31 welds were obtained by reduced pressure laser beam welding. The laser-welded joints were comprehensively analyzed in terms of microstructure, mechanical behavior, and fracture characteristics. The effect of ambient pressure on weld formation and defects was also thoroughly examined. The results showed sound deep-penetration weld without defects was obtained under the reduced ambient pressure of 10 kPa. When the ambient pressure further decreased below 0.1 kPa, the defects occurred by unstable keyhole caused by excessive evaporation. The microstructure results showed the average grain size has decreased from over 30 µm to approximately 20 µm. The precipitated phase changed from large-sized island-like distribution to dispersed distribution of small-sized particles. The strength of the joint welded in low vacuum environment was over 10 % higher than that in atmosphere. The crack propagation of joints in both atmospheric and vacuum environments was studied by in-situ tensile test. The result showed that the crack initiation can be found in the weld with large amounts of oxide in it and gradually expands when further displacement was applied at 101 kPa. During the process of crack propagation, it passed through the area where the other oxide inclusions are located, which accelerated the crack propagation process. The rapid propagation of cracks along the oxide inclusions accelerated the failure of the weld joint. In contrast, enhanced mechanical properties of the joint welded under the ambient pressure of 10 kPa were attributed to finer microstructures and nearly no oxide inclusions.