Friction stir spot welding of dissimilar quality galvanized steel sheets meant for automotive applications using a consumable sheet

Abstract

Friction stir spot welding (FSSW) is a solid-state spot-joining method used to join aluminum alloys to other materials for fabricating lightweight components. However, the FSSW tool experiences wear and tear during the spot welding of steel-grade sheets. To overcome this problem, in the present work, friction stir spot welding using a consumable sheet (FSSW-C) is proposed, in which the tool does not contact the steel sheet during the entire welding cycle. The main aim of this study is to investigate the application of FSSW-C to dissimilar galvanized steel sheets at various rotational speeds via lap shear tests and examination of the joint macro/microstructures. The lap shear performance was also compared to that of the Self Pierce Rivet (SPR) joint. The results revealed that galvanized steel sheets were successfully spot welded using FSSW-C with a lap shear fracture load of 1.5 kN–2.2 kN, which is approximately 50% of that of SPR joints. The microstructures revealed important features, including microstructural zones such as the stir zone, thermomechanically-affected zone, and heat-affected zone. Intermetallic compound formation was not observed at the joint interface. As the rotational speed increased, the axial force and torque decreased, whereas the temperature increased during FSSW-C. The energy generated increased with an increase in the rotational speed and was significantly different when compared to the data for conventional FSSW.