Bridgeability of large gaps in EN AW-6063 aluminium alloy by wire-based friction stir welding with a multi-pin tool

Manufacturing deviations, assembly inaccuracies, and tolerances can create joint gaps that challenge conventional friction stir welding (FSW), which typically requires a tight fit-up to avoid issues such as tunnel defects, surface grooves, or insufficient bonding. In this study, a wire-based friction stir welding (W-FSW) approach using a stationary-shoulder multi-pin tool with an integrated extrusion-screw pin is presented to continuously feed EN AW-6063 aluminium filler wire into the joint. The filler wire is mechanically segmented, pre-plasticised within the tool, and intensively stirred into the substrate, enabling effective gap filling and a uniform material distribution in the weld zone. Gap widths of up to 4 mm, corresponding to 200 % of the base material thickness, were successfully bridged, representing an extreme gap condition for thin-sheet aluminium joining, resulting in fully filled joints with a smooth and well-consolidated weld surface across all investigated gap widths. No weld undercuts or surface-related imperfections were observed. Electron backscatter diffraction (EBSD) analysis revealed a refined and stable microstructure. The weld nugget consistently exhibited grain sizes of 12–15 μ m, independent of gap width, while the thermo-mechanically affected zone and heat-affected zone remained nearly constant at 29–30 μ m (base material: 32 μ m). Tensile testing yielded an average ultimate tensile strength of 155 MPa with ductility comparable to reference joints, with fracture consistently occurring in the heat-affected zone, indicating stable mechanical performance despite the large joint gaps. Overall, the results demonstrate that multi-pin W-FSW provides a gap-tolerant and robust solid-state joining approach, significantly extending the practical application range of friction stir welding for EN AW-6063. • A multi-pin wire-based friction stir welding approach enables the bridging of joint gaps in aluminium butt joints. • Extreme joint gaps of up to 4 mm, corresponding to 200 % of the 2 mm sheet thickness, can be successfully bridged. • Weld integrity and ultimate tensile strength are preserved across extreme gap widths within the resolution of metallographic and mechanical characterisation. • The introduction of filler material leads to hardness distributions comparable to the characteristic W-shaped profile known from conventional friction stir welding.