High Cycle Fatigue Behaviour of Friction Stir Lap Welded 6061 Aluminium Alloy to Coated Steel Sheet Joint

Multi-material fabrication such as joining of steel and aluminium has become prominence now a days in automotive industries. Friction Stir Welding (FSW), a novel solid state welding process, has already established good joint strength between steel and aluminium; but joint fatigue strength of such dissimilar material has not yet been explored. In the present study, the friction stir lap welding has been performed at each rotation speeds of 500, 1000, 1500 rpm for two different travel speed i.e. 50 and 100 mm min-1 at a constant probe depth 2.5 mm. Among the different joints, two joints: one achieved of maximum load (5 kN) and other achieved of minimum load (2 kN) have been exclusively characterized by high cycle fatigue at R=0.1. Furthermore, R ratio has been varied from +0.5, +0.3 to -0.5, -0.3 at endurance limit (106 cycles). The experimental results show that fatigue strength at 106 cycles for both the FS welded lap joint is about 20% of their respective failure load. Thus better fatigue strength is associated with the lap joint performed with the joint of maximum load. The fatigue behaviour of two FS joints has been correlated with the thickness of intermetallic compound evaluated by SEM image and accumulation of dislocations observed by TEM.