Microstructure and mechanical properties of Al/Mg resistance element welded joints

In this paper, resistance element welding (REW) is used to join aluminum and magnesium to solve the problem of brittle intermetallic compounds. The effects of welding current on the nugget formation, element diffusion, and joint performance are investigated. It is found that whether the magnesium alloy sheet is melted or not has an important impact on the joint performance. Both the nugget diameter, ductility, failure load, and energy absorption of the joint increase with the increasing of the welding current when nugget diameter is smaller than the diameter of rivet shank; i.e., the Mg alloy is not melted. When nugget diameter is larger than the diameter of rivet shank, i.e., the Mg alloy starts to melt, the content of Mg element in the weld nugget increases, which gradually increases the hardness and brittleness of the weld nugget. The failure load of the joint increases first and then drops with the increases of welding current because of the tradeoff between the increase of the nugget size and the increase of the nugget brittleness. The fracture surface analysis also indicates that the high current led to the excessive melting of Mg element into the nugget, forming brittle Al-Mg intermetallic compound at the grain boundaries, resulting in the reduction of joint performance.