Fatigue life improvement by friction stir processing of 5083 aluminium alloy MIG butt welds

The friction stir processing is an adaptation of the friction stir welding process. The friction stir processing technique presents several characteristics, between which can provide localized modification and control of microstructures in near-surface layers of processed metallic components in order to modify the correspondent mechanical behaviour. In this research MIG (Metal Inert Gas) welded butt joints, performed in 5083-H111 aluminium alloy plates with 6 mm thickness, were post-processed by friction stir processing. The friction stir processing effect was analysed performing tensile and fatigue tests, as well as microstructural analysis and hardness measurements. Fatigue tests were carried out under constant amplitude loading at a stress ratio R = 0 and with the load applied perpendicular to the weld bead. The hardness and mechanical strength of the welds were not significantly modified by the friction stir processing. However, an important fatigue life improvement was obtained. Friction stir processing promotes fatigue resistance improvement, mainly due to microstructure grain refinement and the removal of previous welding defects, such as porosity and lack of wetting. Additionally, the stress concentration reduction due to toe radius increase, also contributes to fatigue life enhancement.