The heat treatment improving the mechanical and fatigue property of TA15 alloy joint by friction stir welding

The paper investigates the properties and structure of titanium TA15 alloy welded by friction stir welding(FSW) and heat-treated. The advantages of friction stir welding are low energy consumption, high efficiency, and minor joint deformation. It can be used to produce high-quality, ready-to-use joints. However, it is challenging to obtain high-quality joints by friction stir welding because of the TA15 high melting point. In terms of processability, performing heat treatment after welding is a rather archaic and effective solution. Many studies have only focused on the welding process but ignored the welded heat treatment. This research selected the heat treatment mode to obtain the joints with the best quality. Two heat-treatment temperatures were used to heat the joints, namely 820 °C (HT-820) and 920 °C (HT-920). The X-ray diffraction(XRD) was utilized to systematically investigate the relationship between the heat treatment and the recrystallization in SZ and BM. As the temperature increases, the preferred orientation of BM disappears, and the (002) peak gradually shifts to the (100) peak and (101) peak. Finally, the grain orientations of BM and SZ are similar. Heat treatment increases the tensile strength due to structure homogeneous and precipitation hardening. Compared with the untreated welded TA15 joint, the HT-920 welded joint expresses an 8.3% improvement in the tensile strength. Moreover, the fatigue testing results revealed that the HT-920 welded joint showed an improved fatigue limit of 28.7% (107 cycles).