SYNOPSISAlthough Friction Stir Welding (FSW) avoids many of the problems encountered when fusion welding high strength Al-alloys, it can still result in substantial residual stresses that have a detrimental impact on service life and induce distortion. An FE model has been developed to investigate the effectives of the mechanical tensioning technique for controlling residual stresses in FSWs. The model purely considered the heat input and the mechanical effects of the tool were ignored. Variables, such as tensioning level, heat input, and plate geometry, have been studied. Good general agreement was found between modelling results and residual stress measurements, justifying the assumption that the stress development is dominated by the thermal field. The results showed a progressive decrease in the residual stresses for increasing tensioning levels and, although affected by the heat input, a relatively low sensitivity to the welding variables. At tensioning levels greater than ~ 50% of the room temperature yield stress, tensile stresses were replaced by compressive residual stresses within the weld. Subsequently the modelling has been used to examine how mechanical tensioning can be applied in practical applications.