Influence of Laser Shock Peening on the Bobbin Tool Friction Stir-Welded AW6060 Alloy

Friction stir welding (FSW) is a solid-state welding process that uses a rotating tool to soften and stir the base metal, thereby joining it. A special type of tool that has attracted the interest of researchers is the so-called bobbin tool (BTFSW), which, unlike conventional tools with one shoulder, features two shoulders that envelop the base metal from both the top and bottom sides. As a result, significant tensile stresses develop on both sides of the weld, caused by the action of both tool shoulders. In this paper, this issue was addressed by applying laser shock peening (LSP), aiming to introduce compressive stresses, which can be useful as a post-processing technique for BTFSW on both weld sides. It was found that this process completely alters residual stresses in the treated area, from tensile to compressive, through shock waves that impart plastic deformation in the surface layer. It was shown that the LSP effect is more pronounced as the accumulated energy is higher. As a consequence, the microhardness values were significantly increased in the surface and subsurface layers, reaching a maximum depth of 480 to 780 µm for the lowest and highest accumulated laser energy, respectively, while surface roughness increased. While increasing compressive stresses and microhardness in the surface layer is beneficial from the point of view of fatigue resistance, increased roughness has a detrimental effect. Accumulated energy was hereby shown to have a higher effect compared to the maximal energy applied to the specimens.