Microstructural evolution and fracture toughness mechanisms of near-full-penetration electron beam welded joints in 65 mm-thick TC4 titanium alloy

Thick-plate titanium alloys are vital for lightweight structural applications. This study systematically investigated the microstructure, mechanical properties, and anisotropic fracture toughness along the thickness direction of electron beam welded (EBW) joints in 65-mm-thick TC4 titanium alloy. The joint is single-pass, near-full-penetration. From base metal (BM) to weld zone (WZ), microstructure evolves from equiaxed primary α and lamellar α colonies, through a mixed structure in the heat-affected zone (HAZ), to fine acicular α′ martensite basket-weave in the WZ. The WZ exhibits reduced β-phase fraction, more high-angle grain boundaries, and refined grains. Microhardness follows WZ > HAZ > BM. All tensile fractures occurred in the BM, indicating welding does not compromise ultimate tensile strength. Marked anisotropy in fracture toughness was observed: higher longitudinally than transversely. Transversely, BM is tougher than WZ due to weak columnar grain boundaries. Longitudinally, WZ is tougher than BM because cracks propagate perpendicular to columnar grains, causing repeated transgranular fracture and tortuous paths for greater energy dissipation. This study elucidates the microstructure–fracture toughness relationship in EBW thick-plate titanium alloys and provides a basis for damage tolerance evaluation and process optimization.