Evaluation of Micro Structure and Tensile Shear Load of Friction Stir Spot Welded Automotive Steel

Low alloy low carbon steels are extensively used for automotive applications mainly due to their good impact resistance, specific strength and formability. Electrical Resistance Spot Welding (ERSW) is currently used as a method of joining sheet metal together in automotive industries. Generally materials of similar thickness are blanked, preformed and then welded together to make parts like automobile door, bodies and number of similar parts. ERSW process is relatively faster, but it involves large amounts of current to make joints and cooling equipment to keep the electrode in normal working temperature. Friction stir spot welding (FSSW) is a relatively new type of welding process, which is a derivative of friction stir welding. In this process, a rotating tool is plunged in to the material under high force to create a bond. Joints are made by FSSW process and the macrostructure, microstructure and lap shear tensile strength of these joints are characterized. The metal flow during FSSW process is investigated on samples to gain better understanding of welding mechanisms. The results of the lap shear tensile test indicated that the combination of 900 rpm, 0.18mm plunge depth and 20s dwell time yielded a maximum lap shear tensile strength of 5.9 KN.