This study aimed to systematically investigate the microstructure evolution of the nail-shaped weld on electron beam welded joint of Ti–6Al–4V alloy. The microstructure evolution demonstrated that the needle martensitic α′ phases constituted the microstructure in the fusion zone, in which the grain size decreased in the depth direction, and the Widmannstätten structure lamellar α phases covered the base metal. The fusion zone got the strongest tensile strength that reached 980 MPa due to the smallest grain size and the largest number of low angle grain boundary, whereas the tensile strength of the base metal was the lowest in the whole butt joint. The best plastic deformation capacity in the base metal in which the strain approached 13 % was attributed to the largest number of high angle grain boundary in the Widmannstätten structure. Moreover, there were many deformation twins and faults in fusion zone to enhance the resistance to deformation. The tip of the nail-shaped weld was the brittlest location in the entire joint. The experiment also explored the fracture mechanism by in situ observation of tension and fractography, which involved the following three processes: the nucleation and growth of microvoids, the formation of microcracks produced by microvoids, and the occurrence of fracture caused by the growth and propagation of microcracks.

本研究旨在系统地研究 Ti-6Al-4V 合金电子束焊接接头钉状焊缝的显微组织演变。显微组织演化表明，熔合区的显微组织为针状马氏体α'相，晶粒尺寸沿深度方向减小，Widmannstätten 组织层状α相覆盖母材。熔合区由于晶粒尺寸最小，小角度晶界数量最多，抗拉强度最强，达到980 MPa，而母材的抗拉强度在整个对接接头中最低。在应变接近 13% 的基体金属中，最好的塑性变形能力归因于 Widmannstätten 结构中最大数量的高角度晶界。而且，熔合带中存在较多的变形孪晶和断层，增强了抗变形能力。钉状焊缝的尖端是整个接头中最脆的位置。实验还通过以下方式探讨了断裂机制张力和断口的原位观察，它涉及以下三个过程：微孔的形核和生长，微孔产生的微裂纹的形成，以及微裂纹的生长和扩展引起的断裂的发生。