Microstructures and mechanical properties of resistance spot welded magnesium alloy joints

The resistance spot welded magnesium alloy joints consist mainly of weld nugget and heat-affected zone (HAZ). The nugget contains two different structures, the cellular-dendritic structure at the edge of the nugget and the equiaxed dendritic structure in the center of the nugget. The structure transition is attributed to the changes of solidification conditions. In HAZ, the grain boundary melting occurred and grain boundaries became coarse. The magnesium alloy nugget has a high hot cracking susceptibility. The cracks appeared in the nuggets when the welding current was higher than 15 kA. With an increase of welding currents from 15 kA to 23 kA, the joint tensile shear load rises from 1460 N to 3000 N and the nugget diameter increases from 4.2 mm to 6.5 mm. The joint strength rises are mainly related to increasing the nugget diameter. It is favorable to select relatively high welding current for improving mechanical properties of spot welded magnesium alloy joint.