Mechanism of ductility improvement while maintaining strength in TC4 electron beam welded joints induced by thermo-ultrasonic stress relief

This study addresses the issues of residual stress concentration and insufficient deformation compatibility in TC4 titanium alloy electron beam welded joints. The effects of different post-weld treatments on the microstructure, residual stress distribution, deformation behavior, and mechanical properties of the welded joints were systematically investigated. The results indicate that post-weld heat treatment significantly reduces the overall residual stress level in the weld metal and markedly enhances the plastic deformation capability of the joint, increasing the elongation from 4.7% in the as-welded condition to 12.7%. Ultrasonic treatment mainly introduces compressive residual stress on the weld surface, which suppresses early-stage strain localization to a certain extent, resulting in an elongation of 7.0%. By contrast, the combined heat treatment-ultrasonic stress relief reconstructs the spatial distribution of the residual stress field, promoting the formation of more dispersed and homogeneous plastic deformation paths in the weld metal during tensile loading. Consequently, a noticeable improvement in ductility is achieved while the joint strength remains essentially unchanged. After the combined treatment, the elongation reaches 11.1%, the overall deformation angle is reduced to approximately 2.5°, and the fracture location stably shifts from the weld metal to the base metal. These results indicate that the spatial distribution of residual stress and its regulation of plastic deformation paths and damage evolution constitute the key physical mechanism by which the combined heat treatment–ultrasonic stress relief improves the plastic deformation capability of welded joints while maintaining their strength level.