Abstract: For a long time, the large loss of the strength and toughness of fusion welded joints for thick near β titanium alloys has largely hindered their engineering application, which results from the few precipitations of the strengthening α phase during welding cooling. In this study, double annealing treatment was designed for electron beam welded joints of 30-mm-thick near β Ti-5Al-5Mo-5V-1Cr-1Fe alloy, with the aim of regulating the proportion of multi-level lamellar microstructures and enhancing the joint properties. Among various annealing temperatures (first annealing at 750-880 °C + second annealing at 580 °C), the 750 °C + 580 °C annealed joint exhibited simultaneously enhanced strength and toughness, with the increase in tensile strength and impact energy from 844 MPa and 8.8 J for the as-welded joint to 1129 MPa and 14.5 J for annealed joint, respectively, which were superior to those of the joints of Ti-5Al-5Mo-5V-1Cr-1Fe alloy as reported. The great increases in the strength and toughness were mainly attributed to the excellent proportion matching of formed multi-level lamellar microstructures (76.1 % of primary α (α_p) lamellae and 7.9 % of secondary α (α_s) lamellae), among which the α_p phase and α_s phase mainly affected the joint toughness and strength, respectively. The good coupling of α_p phase and α_s phase improved the precipitation strengthening and the resistance to crack propagation. The modified strengthening mechanism models were proposed by introducing the thickness and proportion parameters of the precipitated phase. It was indicated that the theoretical calculation values were in good agreement with the experimental ones, and the solution strengthening and precipitation strengthening provided a large contribution (a sum of about 75 %) to the yield strength of the annealed joints. This study provides a novel method via designing proper multi-level lamellar microstructures to simultaneously improve the strength and toughness of near β titanium alloy joints.

Key words: Titanium alloys, Electron beam welding, Double annealing, Microstructure, Mechanical properties