Fatigue Strength of Friction Stir Welded Joints in Aluminium

Abstract: Solid state Friction stir welding (FSW) is of major interest in the welding of aluminium since it improves the joint properties. Many applications where Al-alloys are used are subject to varying load conditions, making fatigue failure a critical issue. In the scope of this thesis, the fatigue performance of friction stir welded AlMgSi-alloy 6082 has been investigated. Static and dynamic properties of different joint configurations and welds produced with varying process parameters have been determined. Microstructures of fractured surfaces have been studied to evaluate the effect of weld discontinuities on fatigue. The mechanical strength of the friction stir welds was set in relation to that of conventional fusion welds, and that of other FS welded Al-alloys.The friction stir process produced aluminium butt welds with high and consistent fatigue strengths, which exceeded the strengths of similar fusion welded samples. A smooth weld geometry showed to be of great importance for the fatigue performance, favouring the friction stir welds. Welding speed in a tested range of 0.35-1.4 m/min had only a modest influence on the properties of the friction stir welds; properties were not deteriorating at the highest speed. The softening of the alloy around the weldline was modelled. A fair description of the hardness profiles across the weld was obtained. At a low and high welding speed a full and partial softening respectively was predicted, indicating that full softening is not required to obtain a flawless weld.In case of friction stir overlap welds, tool design is even more important than in butt welding to secure weld quality. A broad tool shoulder with a concave pin end gave the best performance. In particular, the minimal influence on the sheet interface when welding with such a tool was beneficial for the fatigue strength. The stress distribution in overlap and T-type test specimens has been modelled. The stress intensity factors were determined. The corresponding crack propagation rates were in fair accordance with the experimental results. It was found that a simplified approach, developed to estimate ∆K for overlap spot welds, could be used also for friction stir overlap joints.