High-cycle fatigue properties of an easily hot-workable (α+β)-type titanium alloy butt joint prepared by friction stir welding below β transus temperature

Abstract

Friction stir welding (FSW) was applied to a butt joint between the 2-mm-thick plates of an (α+β)-type titanium alloy with high hot workability, Ti-4.5Al-2.5Cr-1.2Fe-0.1C alloy (Ti531C), to obtain defect-free welds. Refined equiaxed grains (equiaxed structure) were obtained and anisotropic mechanical properties of the parent material disappeared by grain subdivision of the α phase during FSW. However, grain diameter and crystal texture of the equiaxed structure remained unchanged during annealing. The tensile strengths of the welded joints with and without annealing were comparable to that of the Ti531C parent plate. However, the elongations of the welded joints were lower than that of the Ti531C parent plate because the stir zones are harder than the base metal, which results in deformation preferentially in the base metal region of the specimens. Tensile testing caused a failure in the base metal region in all welded joints. Annealing improved the high-cycle fatigue strength of the welded joints greatly. However, the high-cycle fatigue strength of the welded joints subjected to annealing was still slightly lower than that of the Ti531C parent plate. Fatigue failure point was in the stir zone or at the boundary between the stir zone and the base metal, which are different from the tensile test. These results imply that failure factors are different between tensile and fatigue tests. The small portion of the isothermal ω phase in the β phase in the stir zone and discontinuous microstructure at the boundary between the stir zone and the base metal were potential stress concentration sites, inducing high-cycle fatigue failure.