Residual strain and strain evolution of dissimilar aluminium-steel friction stir lap welding during lap shear tests

• Welds of carbon steel and dual-phase steel with 5000-series aluminium were studied. • In all cases, residual tensile strains along the welding direction were measured. • Maximum strains of approximately 3000 µε were observed. • Formation of steel inclusions were detected using AISI 1010 carbon steel. • In lap shear tests, lateral contraction reduces the effective strain in the steel. Friction Stir Lap Welding (FSLW) is a technique used to join dissimilar materials, such as aluminium alloys and steel sheets, applied in the automotive industry. The residual strain distribution generated during the process, strongly affects the mechanical performance and long-term durability of the welded components. In this study, the residual strains generated during FSLW of aluminium alloy and steel sheets were investigated using Bragg edge neutron imaging (BEI). Different combinations of thin aluminium alloy and steel sheets with thicknesses between 0.8 mm and 2 mm were analysed. 5052 and 5182 alloys, in combination with AISI 1010 carbon steel and dual phase (DP) 1000 steel were employed. Additionally, the evolution of the actual strain under lap shear tests was monitored. The presence of the steel inclusions was detected by neutron transmission imaging. The BEI results showed tensile residual strain along the longitudinal direction in the steel sheets after the welding process, in all cases displaying a M-shaped strain field. During the lap shear tests, a reduction of the actual tensile strains was observed due to the lateral contraction produced in the mechanical testing.