Fatigue life improvement of mig welded aluminium T-joints by friction stir processing

Friction stir processing (FSP) is an emerging metalworking technique that can provide localized modification and control of microstructures in near-surface layers of processed metallic components. In this research the FSP was applied on Metal Inert Gas (MIG) T-fillet welds performed in 6082-T651 aluminium alloy plates of 6 mm thickness. FSP potential benefits were studied through microstructural analysis, hardness measurements, tensile strength and fatigue testing. Fatigue tests of unprocessed and processed MIG welded specimens were carried out under constant amplitude loading with stress ratios R = 0 and R = 0.4. Tensile testing results show that MIG welding reduces significantly the base material mechanical strength of the. However the friction stir post-processing does not promote an additional reduction of the MIG welds hardness and mechanical strength. In contrast, an important improvement in fatigue strength was observed due to grain refinement of the microstructure, defects removal, such as porosity and lack of wetting, and stress concentration reduction due to the increased toe radius. The Smith, Watson and Topper (SWT) mean stress equation provided good correlation of fatigue data for the two stress ratios either for unprocessed or processed MIG welds.