Previous studies have shown that the wear of FSW tools is influenced by process parameters as well as the relation between tool material and workpiece material. However, the effect of tool geometry and the associated material flow on geometrically induced tool wear has not yet been addressed in the existing literature. Against this background, the study systematically investigates the wear behaviour of various shoulder (cylindrical, scroll) and probe geometries (cylindrical, conical, threaded, threaded with flats) as a function of material flow during friction stir welding of the aluminium alloy AA-6060 T66 with a thickness of 5 mm. The wear behaviour was investigated over weld lengths of up to 500 m at a rotational speed of 2000 rpm, a welding speed of 0.5 m/min, and an axial force of 5.5 kN. The study revealed that both horizontal and vertical material flow contribute to wear in the transition zone between the shoulder and the probe. Furthermore, the results showed that reducing tool complexity can significantly reduce tool wear. The experiments were carried out using a force-controlled robotic welding system from Grenzebach Maschinenbau GmbH.

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.