Forensic investigation into the ductility and strain localisation of an AA6005A friction stir weld: the role of temper mismatch

Abstract Friction stir welding of AA6005A plates in dissimilar temper conditions (overaged on the advancing side and T6 on the retreating side) was investigated under an intermediate heat input regime (1700 rpm, 600 mm/min; ω/v ≈ 2.83). The joint achieved a tensile strength of 193 MPa and a joint efficiency of approximately 76%, but exhibited a severely reduced elongation of 5.2%, with all specimens fracturing within the heat-affected zone of the advancing side. Hardness mapping revealed a pronounced asymmetry across the weld cross-section, with a minimum of ~ 51 HV consistently located in heat-affected zone of the advancing side, coinciding with the fracture location. Ductility loss is governed by elastic–plastic mismatch between the dissimilar temper conditions: the harder T6 side constrains deformation and concentrates nearly all plastic strain into the narrow overaged heat-affected zone, forcing premature plastic instability. By holding alloy chemistry constant and varying only the initial temper, this study demonstrates that the precipitate state alone is sufficient to produce the strain localisation and ductility loss typically attributed to alloy dissimilarity in friction stir-welded 6xxx-series joints.