Fracture toughness of electron-beam-welded parts of super-thick TC4-DT Ti alloy for aeronautical use

Damage-tolerance TC4-DT titanium (Ti) alloy is widely applied in the aviation field. The microstructures and mechanical properties of the base metal (BM) and weld metal (WM) in joints of 130-mm-thick TC4-DT alloy welded by electron beam welding (EBW) were compared. The nonuniformity of mechanical properties of WMs in different layers along the thickness direction of joints was mainly studied. Experimental results show that various layers of EBW joints of 130mm-thick TC4-DT alloy differ slightly in the tensile properties, while the microhardness of the weld root is higher than that in other WMs. Compared with other WMs, the weld root shows lower fracture toughness. The causes for the low toughness at the weld root were analyzed from the perspectives of microstructures in various layers of WMs of the joints and propagation behaviors of crack tips. The upper part, middle, and root of WMs of joints all contain lots of alpha ' phase, while alpha ' phase is small at the weld root. Cracks propagate in a manner that connects micro-pores in series in the upper and middle parts of the WM, which are transgranular ductile cracks; those at the weld root propagate via interfacial separation and their propagation paths are straight, which are transgranular brittle cracks.