Comparison of residual stress distributions in conventional and stationary shoulder high-strength aluminum alloy friction stir welds

The residual stresses generated in stationary shoulder friction stir welds (SSFSWs) produced in a typical high strength aluminum alloy (AA7010) in 6.3mm thick plate has been mapped over full weld cross sections, using the contour method, and compared to those introduced by conventional friction stir welding (FSW) for welding speeds ranging from 100 to 400mm/min. Compared to in conventional FSW, as a consequence of the material flow being affected by only a rotating probe, the SSFSW process produced a narrower and more uniform weld nugget and heat affected zone profile through the plate thickness. For both processes, ‘M’ shaped residual stress distributions were determined. However, the peak stresses measured in the SSFSWs were slightly lower than those found in the conventional FSWs and the width of the tensile region was appreciably reduced when using a stationary shoulder welding tool. This is shown to be resulting from a more focused temperature distribution obtained from using only a rotating probe to generate heat in the SSFSW process. In both processes, increasing the welding speed led to a narrower residual stress profile, but higher peak tensile residual stresses.