Strengthening mechanism of friction stir welded alpha titanium alloy specially designed T-joints

Abstract In this study, the Ti-4Al-0.005B (TA5, Chinese brand) titanium alloy was used in a specially designed T-joint joined by friction stir welding, where two plates served as the skins and one plate served as the stringer with two separate welds. The base material (BM) has a typical rolling microstructure, consisting of α grains deformed along the rolling direction. The grains in the heat-affected zone (HAZ) are coarser than those in the BM, while the grains in the thermo-mechanically affected zone are deformed along the shear direction. When the temperature is higher than the β transus in the welding process, there are lamellar α phase structures in the stir zone (SZ). When the temperature is lower than the β transus in the welding process, there are fine equiaxed grains in the SZ due to sufficient dynamic recrystallization, while the grains in the first weld are slightly larger than those in the second weld due to their longer exposure to the thermal cycle of the second weld. The microhardness of the first weld decreases following the second weld, and the area of lowest microhardness occurs in the HAZ on the advancing side near the first weld. The microhardness of SZ at the position of 0.5 mm from the upper surface is slightly higher in the thickness direction. In the tensile tests, the initial position of the fracture occurs in the area where the skin and the stringer are joined by FSW. With the increase of tool rotation speed from 450 rpm to 850 rpm, the tensile strength increases first and then decreases along the direction of skin and stringer. In addition, a coupled Eulerian-Lagrangian (CEL) numerical model is developed to investigate the temperature distribution and material flow during FSW, showing stronger correlation with the experiments.