Microstructural variation through weld thickness and mechanical properties of peened friction stir welded 6061 aluminum alloy joints

The current study examined the effect of microstructure variation on the development of mechanical properties in friction stir welded joints of 6061-T6 aluminum alloy, which were subsequently processed by shot peening (SP). Following to FSW, fatigue specimens were extracted perpendicularly to the welding direction. Surface Skimming to 0.5mm from crown and root sides of the joint was made and SP was later applied on the two sides using ceramic shots of two different Almen intensities of 0.18mmA and 0.24mmA. Microstructural examination by electron back scattered diffraction (EBSD) indicated variation in the grain refinement of the weld zone, with coarsest grains (5μm) at the crown side and finest grains (2μm) at the root side. Reduction of microhardness to 60HV occurred in the weld zone for samples in FSW condition. Application of SP promoted significant strain hardening at the crown side, with Almen intensities of 0.24mmA providing maximum increase in microhardness to 120HV. On the contrary, only a maximum microhardness of 75HV was obtained at the root side. The difference in strain hardening capability at the two sides was strongly dependent on grain size. The two Almen intensities produced similar distribution of compressive residual stresses in the subsurface regions that led to enhance the fatigue strength to the level of base metal for N≥105cycles. Yet, the increase in fatigue strength was more pronounced with increasing Almen intensity to 0.24mmA, demonstrating further enhancement by strain hardening. •Grain refinement was observed after friction stir welding of AA 6061-T6.•Reduction in microhardness and fatigue strength were obtained after welding.•Variation in grain refinement led to different hardening behavior after peening.•Shot peening induced beneficial compressive residual stresses.•Shot peening and surface skimming markedly improved the fatigue performance.