Effect of stirring probe geometry on gap-bridging mechanisms and joint strength in wire-based friction stir welding of EN AW-2024 T351 Alclad

This study investigates the influence of stirring probe geometry on gap-bridging mechanisms, microstructure, and mechanical properties in wire-based friction stir welding (W-FSW) of EN AW-2024 T351 Alclad aluminium alloy. A multi-pin tool concept with continuous wire feeding was applied to enable the filling of large joint gaps during welding. Ten different probe geometries were systematically evaluated with respect to weld integrity, material flow, and resulting joint performance. The results demonstrate that a 4 mm joint gap, corresponding to 200% of the sheet thickness, can be successfully bridged using selected probe geometries. X-ray and metallographic analyses confirmed predominantly sound weld regions for the successful configurations. The probe geometry significantly affects material flow behaviour, grain refinement, and mechanical properties. Fine-grained, recrystallised microstructures with average grain diameter down to approximately 2.55 μ m and high fractions of high-angle grain boundaries were obtained. Hardness distributions exhibit the characteristic W-shaped profile known from conventional friction stir welding (FSW), with local minima on the retreating side. The highest mechanical performance was achieved using a probe geometry with upward-orientated triangular pins (P6), reaching 85% and 89% of the base material strength in transverse and longitudinal directions, respectively. The results demonstrate the potential of W-FSW for joining high-strength aluminium alloys with increased gap tolerance while maintaining competitive mechanical properties. The findings highlight the critical role of probe geometry in controlling material flow and joint performance, providing a basis for process optimisation in industrial applications.