Non-uniformity of fracture toughness of ultra-thick titanium alloy double-sided electron beam welding joint

The Ti6Al4V (TC4) alloy has been extensively utilized in the aviation industry because of its favorable comprehensive mechanical properties. In this study, we examined the microstructures and fracture toughness of three layers of double-pass double-sided joints with unequal penetrations of vacuum electron beam welded joint (VEBW) with a thickness of 140 mm. It was found that the overlapped zone of two passes at the roots exhibits poor fracture toughness. Microhardness of this overlapped zone exhibits a significantly elevated level in comparison to other areas, which contributes to its low fracture toughness. We analyzed the cause for this higher microhardness from a perspective of microstructures. The microstructure across the joint thickness is non-uniform. It was observed that α′ phase accounts for a larger fraction and has a smaller size in the overlapped zone compared to other WMs. Our research results indicate that changes in the size and fraction of α′ phase in EBW joint of TC4 alloy have significant influences on both microhardness and fracture toughness.