Novel hybrid bobbin tool friction stir welding of AA2219 with comparison to existing bobbin tool variants

Bobbin tool friction stir welding (BT-FSW) is a solid-state joining technique that produces high-strength welds without the need for backing plates. This study introduces a new variant, termed hybrid bobbin tool friction stir welding (HBT-FSW), which combines the advantages of conventional BT-FSW and semi-stationary bobbin tool friction stir welding (SSBT-FSW). The HBT-FSW configuration features a probe with a rotating section positioned at the same level as the upper stationary shoulder. This design enhances weld surface quality, while preserving the mechanical performance required for demanding aerospace applications. The process was applied to aluminium alloy AA2219, a common material used in space structures, such as pressurised modules and propellant tanks for orbital systems. A comprehensive microstructural and mechanical characterisation was performed to assess the effectiveness of HBT-FSW. Electron backscatter diffraction (EBSD), hardness mapping, and synchrotron wide-angle X-ray scattering (WAXS) were used to analyse the weld microstructure. Deformation and failure behaviour were investigated through tensile testing with digital image correlation (DIC) and fractographic analysis using scanning electron microscopy (SEM). The results show that HBT-FSW produced a microstructure similar to BT-FSW, achieving comparable tensile strength and ductility while maintaining the superior surface quality typical of SSBT-FSW and enabling welding speeds approximately 30% higher than SSBT-FSW. HBT-FSW therefore represents a promising advancement for the reliable and efficient joining of aluminium alloys in high-performance aerospace structures.