Microstructure Evolution and Fracture Mechanisms in Electron Beam Welded Joint of Ti–6Al–4V ELI Alloy Ultra-thick Plates

It is rather difficult for titanium alloy ultra-thick plates to achieve superior weld formation and excellent mechanical properties along the weld penetration direction due to the large fluctuations of the molten pool, largely limiting their engineering application. In this study, 106-mm-thick Ti–6Al–4V ELI alloy plates were successfully butt welded via electron beam welding (EBW). The defect-free EBW joint with full penetration was obtained. The precipitated secondary α (α_s) in heat affected zone (HAZ), α lamellae in fusion line (FL) and α′ martensite in fusion zone (FZ) increased the α_s/β, α/β and α′/β interfaces, respectively, resulting in the higher microhardness and impact energy values (57 J in the HAZ, 62 J in the FL and 51.9 J in the FZ) than those in the base material (BM). The impact energy of the joint in this study was higher than that for Ti–6Al–4V ELI alloy joints as reported, which was mainly attributed to the formation of the relatively thicker α phase and finer interlamellar spacing in this study, enhancing the resistance to crack propagation. Furthermore, the average fracture toughness (90.2 MPa m^1/2) of the FZ was higher than that of the BM (74.2 MPa m^1/2). This study provides references for the welding application of titanium alloy ultra-thick plates in the manufacture of large-sized components.