<p>This study investigates the influence of primary friction stir welding (FSW) parameters—tool rotation speed, welding speed, and axial load—on the ultimate tensile strength (UTS) of friction stir welded dissimilar AA5083-H111 and AA7050-T7651 aluminium alloy butt joints. The central composite inscribed design of response surface methodology was employed to design the experiment and develop the predictive model. The results show that the UTS is fairly sensitive to all the three primary FSW parameters, with the tool rotation speed being the most influential parameter. The interactive effects of tool rotation speed with axial load, and welding speed with axial load on the joint UTS are significant, whereas the interactive effect of tool rotation speed with welding speed on the UTS is comparatively less pronounced. The poor plasticization and coalescence of the material, and the resulted discontinuities in the weld zone at low heat input—forging pressure conditions, and the high thermal softening and poor material coalescence at very high input conditions are major factors which limit the joint UTS. However, under moderate heat input—forging pressure conditions, due to the grain refinement, restricted thermal softening, and better material coalescence, the joints were defect free and exhibited excellent joint strength.</p> Graphical abstract <p></p>