Revealing tensile behaviors and fracture mechanism of Ti–6Al–4V titanium alloy electron-beam-welded joints using microstructure evolution and in situ tension observation

Abstract

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.