Mechanical Properties and Microstructures of Low-Cost, Friction Stir welded Ti-531C Plates for Aircraft Applications

Recently developed (α + β)-type Ti-531C (Ti-4.5Al-2.5Cr-1.2Fe-0.1C) as a low-cost titanium alloy for aircraft applications is attractive because Ti-6Al-4V, one of the most widely used alloys in the aircraft industry, contains high-cost elements such as V, and has relatively elevated processing costs because poor hot workability. Welding of titanium alloys plays an important role in the construction of aircraft because it reduces the number of process steps. Friction stir welding (FSW) is a new welding technique that is receiving considerable attention. Therefore, the mechanical properties and microstructures of FSWed Ti-531C plates were investigated in comparison with those of laser welded Ti-531C plates. The FSWed Ti-531C plates (a thickness of 2 mm) subjected to annealing exhibit a fatigue strength similar to that of the Ti-531C base material and a higher fatigue strength than that of the Ti-531C plates subjected to laser welding. However, the fatigue strength of FSWed Ti531C plates (a thickness of 4 mm) is lower than that of the Ti-531C base material because considerable microstructural coarsening occurs in the stir zone. This coarsening is the result of the exposure of the plate to FSW on both sides.