Staggered Twin-StirTM - Lap Joining of Aluminium Alloy

This research demonstrates the feasibility of Friction Stir Welding by the simultaneous use of two tools (Twin-stir™) in staggered arrangement for lap joint. The strength of a lap joint is strongly influenced by its width and notch tip morphology at the edge of the weld region. The use of double pass welding, with the second pass partially overlapped on the previous weld region, allows wider welds to be produced but has the limit of a low productivity. The Staggered Twin-stir™ provides exceptionally wide lap joints with productivity higher than consecutive double pass welding. The mechanical and microstructure properties were investigated to evaluate the effect of the simultaneous use of two contra-rotating tools during the welding cycle. Shear test produced different results according to which side of the top sheet is loaded: the higher strength weld (joint efficiency of 70%) was obtained when the side of the follower tool was loaded. This strength difference was due to different interface morphology produced by the leading and follower tool. The microscopic analysis identified a good correlation between the hardness distribution and the different welding zones with an increase of hardness value in the stirred zones that presented an average grain dimension of 7 μm. The grain dimension in the stirred zone of the leading tool was smaller than that of follower tool.