Fatigue life comparison of mechanically fastened and refill friction stir spot welded joints

Abstract

© 2026 The Author(s).Reducing component weight remains a central objective in aviation to improve aircraft efficiency. Additive manufacturing (AM), particularly laser powder bed fusion (L-PBF), offers near-net shape part production with integrated functionalities, but their integration into aircraft structures requires advanced joining technologies. Refill friction stir spot welding (RFSSW) has emerged as a promising alternative to conventional mechanical fasteners for high-strength aluminium joints. This study benchmarks RFSSW against established mechanical fasteners by assessing quasi-static and fatigue behaviour in AA7075-T6 similar and AA7075-T6 to L-PBF AlSi10Mg dissimilar joints. Fractography was conducted using light optical and scanning electron microscopy to investigate the crack initiation and propagation mechanisms. This work represents, to the best of the authors’ knowledge, the first fatigue investigation on dissimilar L-PBF to wrought aluminium RFSSW joints. Case studies showed that replacing mechanical fasteners with spot welds enables weight savings of 1.3% to 8.8%. The fatigue performance and fracture behaviour of the joints are dominated by strong secondary bending at high fatigue loads and, at lower fatigue loads, constrained by hook defects in RFSSW and fretting at the sheet interfaces in the mechanically fastened joints. For similar AA7075-T6 joints, pin and collar fasteners outperform RFSSW, and solid rivets exhibit superior fatigue strength above 2x105 cycles. For low fatigue loads, RFSSW proved to be a viable alternative to pin and collar dissimilar joints. Overall, the study identifies key failure mechanisms and outlines optimisation strategies for RFSSW assemblies. The systematic comparison facilitates knowledge transfer to other RFSSW applications, supporting the broader adoption of the technology.

Keywords