Effect of Ultrasonic Impact Treatment on the Residual Stress Distribution of Electron Beam Welded TA15/BTi-6431S Dissimilar Titanium Alloys

Ti-6.5Al-2Zr-1Mo-1V (TA15) and Ti-6.5Al-3Sn-3Zr-3Mo-3Nb-1W-0.2Si (BTi-6431S) dissimilar titanium alloy plates were welded through electron beam welding (EBW) method and different ultrasonic impact treatment (UIT) technologies were performed on the weldment. The effects of UIT on the microstructure and residual stress distribution and mechanical properties for the welding joint were investigated. A significant microstructural change occurs after welding. The structure of welding joint is composed of fusion zone (FZ), heat-affected zone (HAZ) and base metal. The microstructure of FZ is composed of martensite structure and a clear interface is observed because of the fusion of dissimilar alloys. HAZ near fusion zone is comprised of martensite structure, while the grain size is much smaller than that in FZ. The HAZ microstructure near TA15 alloy is composed of coarse equiaxed α phases and HAZ microstructure near BTi-6431S alloy is lamellar α phases. Through ultrasonic impact treatment for three times, the residual stress near the welding joint shows a uniform distribution and the maximum tensile stress changes to compressive stress. A characteristic asymmetrical microhardness profile is observed in TA15/BTi-6431S dissimilar alloy welded joint. The HAZ in BTi-6431S alloy side possesses higher hardness compared with that in TA15 alloy side. The microhardness and tensile properties change slightly after ultrasonic impact treatment.