Effect of Alloyed Hot-Dip-Galvanized Zinc Plating on Optimum Welding Conditions for Three-Layer Spot Welding

In regard to spot welding of manufactured products like automotive parts, it is common practice to join three steel sheets having different grades and thicknesses and subjected to different kinds of surface processing. In such a case, it is more difficult to set the welding conditions than in the case of joining two sheets of the same grade and thickness. The present study aimed to determine by investigation the optimum welding conditions for spot-welded joints consisting of three stacked steel sheets of the sort supposed to be applied in fabricating actual auto components. Furthermore, the effect of zinc plating (i.e., hot-dip galvanizing) on the outer sheet on spot-welding characteristics—which is becoming a serious manufacturing problem—was investigated. According to the results of these investigations, as for a three-layer spot weld, the fused part of the welded joint (i.e., “nugget”) is distorted to the high-tensile side. This distortion causes a difference in the resistance of the sheets, and the nugget is originated on the high-tensile side of the joint (which has higher resistance) and grows from that point of origin. Although two fracture modes are possible, namely, fracture in the base material or fracture in the weld nugget, fracture diameter and tensile shear strength have a proportional relationship in both cases regardless of the welding conditions. In the case that a zinc-plated (i.e., hot-dip zinc galvanized) steel sheet is used in the sheet stack, tensile shear strength falls and becomes more variable at low welding current. Accordingly, it is necessary to control the welding conditions so as to reduce that tensile-shear-strength variability.