Role of zinc layer in resistance spot welding of aluminium to steel

Effects of zinc layer on microstructure and mechanical behavior of resistance spot welds of aluminum to galvanized (GS/Al joint) and low carbon steel (PS-Al joint) were explored. The results showed that although nugget volume' in PS-Al joint was larger, the nugget 'diameters' of PS-Al and GS-Al joints were almost the same in size since the melted zinc layer was pushed toward the outer regions of the nugget. Melting and evaporation of zinc coat led to reduction of Al-Fe intermetallic layer thickness. Presence of zinc also reduced the fixture-induced tensile stress. Utilizing carbon steel fixtures during welding caused a sensible vibration in the joint members. The vibration resulted in fragmentation and decrease of the intermetallic compounds at the joint interface. Moreover, while PS-Al joints showed higher strength than that of GS-Al ones at the welding currents <12 kA, fracture load of GS-Al welds exceeded PS-Al joints beyond 12 kA. Low welding current resulted in an incomplete joint at GS-Al welds. However, lower induced tensile stress, as well as the formation of intermetallic layer with the thickness smaller than the critical value (similar to 5.5 mu m) in GS-Al joints, led to superior mechanical properties at high welding currents. (C) 2016 Elsevier Ltd. All rights reserved.