Effect of coating type on microstructure and mechanical behavior of resistance spot welds of thin X626 aluminum sheet to low carbon steel

Resistance spot welds were produced using GM's patented welding process. The stack-ups consisted of 0.8 mm thick X626 aluminum and 0.9 mm thick low carbon steel (LCS) with three different coating types: hot dipped galvanized (HDG), Zn-Ni coating, and Zn-Ni coating with a passive layer containing trivalent clear chromate (Zn-Ni-Cr). The addition of Ni or Cr to the Zn coating provides a unique microstructure compared to that for welding to HDG coated steel by creating an increased porosity, forming agglomerated particulates near the weld nugget periphery, eliminating the oxide film defect, and retarding the formation of the Fe2Al5 layer. The RSW-HDG samples exhibited the largest nugget size and least thinning, which increased tensile strength for tensile-shear and coach-peel specimens. The shorter fatigue life of RSW-ZnNi and RSW-ZnNiCr compared to RSW-HDG was attributed to defects observed along the weld nugget perimeter and a smaller weld nugget diameter which was demonstrated using the structural stress concept.