Friction stir welding is a solid-state joining process with a wide range of industrial applications in the e-mobility, automotive, aerospace and energy industries. However, friction stir welding is subjected to process-specific challenges, including comparatively high process forces and friction stir welding tool wear resulting from tribological interaction between the tool and workpiece. The geometric-related friction stir welding tool wear can cause varying material flow conditions, lateral path deviations and premature tool failure, with detrimental economic and technological consequences. This study systematically analyses the wear behaviour of friction stir welding tools as a function of tool hardness. To compare and differentiate the geometric-related tool wear as a function of tool hardness, experiments were carried out with a hardness of 240 HV, 410 HV and 580 HV. Whereas 240 HV is non-hardened, 410 HV is 50% of the secondary hardness maximum and 580 HV is the secondary hardness maximum of the tools made of H13 tool steel (hot-working steel, X40CrMoV5-1). During the experimental tests, the shoulder and probe exhibited varying wear and geometrical deviations. The investigations were carried out with a force-controlled robotized welding setup in which AA-6060 T66 sheets with a thickness of 8 mm were joined by weld seams up to a total length of 80 m.

Friction stir welding is an increasingly used method to join similar and dissimilar materials with excellent mechanical weld seam properties. However, in certain cases, friction stir welding is restricted by high mechanical loads as a result of high forces and torques during the welding process. This relates in particular stiffness-reduced machine concepts which may cause path deviations, massive vibrations and insufficient tool plunging. Against this background, this investigation demonstrates a method to reduce forces and torques by tool scaling. Due to the stepwise diameter reduction of shoulder and probe and a simultaneous adjustment of the process parameters, a significant force and spindle torque reduction was achieved. Furthermore, it could be shown that tool scaling does not affect the mechanical strength properties. The experimental investigations were carried out with EN AW 5754 H11 with a sheet thickness of 8 mm. The weld seams were performed on a robotized friction stir welding setup (KUKA KR500) with a maximum axial force of 10 kN. Based on a 26-mm shoulder and a 10-mm pin diameter, it could be demonstrated that the general weldability of 8 mm EN AW 5754 H11 is restricted (incomplete tool plunge) by the maximum axial force of the robotized friction stir welding setup (10 kN). Due to the stepwise reduction of the shoulder and probe diameter from 26 mm to 20.8 mm and 10 mm to 8 mm, respectively, a general weldability and weld seams without irregularities could be achieved by the equal robotized friction stir welding setup. Furthermore, it could be shown that an axial force and spindle torque reduction from 10 kN to 4 kN and 29 Nm to 10 Nm, respectively, was obtained due to further reduction of the tool diameters.

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