Cold-Rolling Strain Hardening Effect on the Microstructure, Serration-Flow Behaviour and Dislocation Density of Friction Stir Welded AA5083

5083 aluminium (Al) alloy materials have extensive structural applications in transportation industries because of their high strength-to-weight ratio and corrosion resistance. However, under conventional fusion weldings, these materials are limited by their porosity, hot cracking, and distortion. Herein, friction stir welding (FSW) was performed to join a similar AA5083 alloy. A post-weld cold-rolling (PWCR) process was applied on joint samples at different thickness-reduction percentages (i.e., 10%, 20%, and 40%) to identify the effect of strain hardening on the microstructure and mechanical properties of the friction-stir-welded joint of AA5083 while considering the serration-flow behaviour at stress-strain curves and dislocation density of the post-weld cold-rolled (PWCRed) samples. FSW induced a 20% reduction in the tensile strength of the joint samples relative to the base metal. PWCR also reduced the average grain size at the nugget zone and base metal because of the increase in plastic deformation imposed on the samples. Furthermore, PWCR increased the dislocation density because of the interaction among dislocation stress fields. Consequently, the tensile strength of the friction-stir-welded joint increased with the increased cold-rolling percentage and peaked at 403 MPa for PWCRed-40%, which significantly improved the serration-flow behaviour of stress-strain and welding efficiency up to 123%.