Effects of Heat Treatment on Grain-Boundary β-Mg₁₇Al₁₂ and Fracture Properties of Resistance Spot-Welded AZ80 Mg Alloy

The distribution and morphology of β-Mg₁₇Al₁₂ intermetallic phase in resistance spot-welded AZ80 Mg alloy were investigated by means of optical microscopy, scanning electron microscopy, and X-ray diffraction. The influence of intermetallic phase on mechanical strength was studied by tensile shear testing and fractography. The results showed that continuous networks of β-Mg₁₇Al₁₂ formed along grain boundaries in both the nugget and heat-affected zone of the spot-welded AZ80 Mg alloy. Those continuous grain-boundary β-Mg₁₇Al₁₂ networks acted as effective crack propagation paths, which had negative effects on the weld strength. Post-weld solution heat treatment effectively reduced the amount of β-Mg₁₇Al₁₂ and broke the grain-boundary intermetallic networks in both the nugget and heat-affected zone. This significantly increased the weld strength of AZ80 Mg alloy and changed the fracture mode from nugget pull-out in the as-welded condition to through-thickness after heat treatment.