Global mechanical tensioning for the management of residual stresses in welds

The general principles behind the global mechanical tensioning technique for controlling weld residual stresses are examined using a finite element model to follow their evolution throughout the welding process. While we focus specifically on friction stir welding, the tool is represented simply as a heat source. As a result, the findings have relevance to a wide range of welding processes. For aluminium alloy friction stir welds, the maximum longitudinal weld stresses have been reported to fall approximately linearly to zero under mechanical tensioning to a level around 40% of the yield stress. Under larger tensioning levels, the weld stress becomes increasingly compressive. This behaviour is explained in terms of the reduction in compressive plastic straining ahead, and an increase in tensile plastic straining behind, the heat source as the tensioning level is increased. Finally, it is shown that tensioning during welding is much more effective than post-weld tensioning.