Improving the prediction accuracy of tool damage in friction stir welding by applying experimentally determined S–N curves

Friction stir welding (FSW) subjects the welding tool to severe thermo-mechanical loading, resulting in complex fatigue conditions that limit tool life. Current tool design approaches remain largely empirical, leading to considerable uncertainty in lifetime prediction under realistic process conditions. This study presents a fatigue-based methodology for predicting FSW tool damage by combining experimentally measured process loads with linear damage accumulation. Process forces and torque were acquired during welding of AA 6060 T66 and AA 5754 H111 and converted into probe stresses. Fatigue data for H13 tool steel were experimentally determined at 530 °C for rotating bending, torsion and axial tension/compression. Owing to the pronounced scatter of the axial data, the experimental bending and torsional S–N curves were implemented in the lifetime assessment, whereas a hybrid axial S–N curve combining the experimentally determined median endurance limit with the FKM-based finite-life slope was used for axial loading. The workflow was evaluated for precipitation-hardenable and strain-hardened aluminum alloys. For AA 6060, its application reduced the overestimation of tool life to a factor of 2.1, while AA 5754 still exhibited an overestimation factor of 5.1. The remaining deviations highlight the limitations of linear damage accumulation under non-stationary welding conditions. Overall, the results demonstrate that experimentally supported fatigue data improve the prediction accuracy of FSW tool life and provide a first-order basis for predictive tool assessment.