Effect of Fixture Release Timing in Butt Welding on Deformation and Residual Stress of High-Speed Train Sidewalls

A partially butt-welded 6005A aluminum alloy sidewall structure of a high-speed train was selected as the research object, and a thermo-mechanical sequential coupled finite element model considering the clamp constraint and release process was established. The post-weld deformation and residual stress distributions under four working conditions—free condition (FC), continuously clamped condition (CC), immediate release condition (IR), and release after cooling to 100℃ (TR)—were compared and analyzed. The results show that, under the condition of consistent welding temperature field distribution, the timing of clamp release mainly affects the post-weld mechanical response. Under all four working conditions, the out-of-plane deformation of the component is mainly negative displacement. The continuous constraint working condition has the most obvious inhibitory effect on the out-of-plane deformation after welding. In contrast, the release working conditions all show a certain degree of spring back effect after the clamp is released. The longitudinal residual stresses under all four conditions show the typical distribution characteristics of tensile stress concentration in the weld center area and gradually transforming into compressive stress away from the weld area. The peak value order is CC > IR > TR > FC. For transverse residual stress, the CC condition maintains a higher tensile stress level along the weld direction, which is significantly higher than the other working conditions. Studies have shown that the timing of clamp release essentially alters the distribution relationship between deformation release and stress retention during welding shrinkage. Appropriately extending the clamp holding time helps to achieve a better balance between post-weld deformation control and residual stress reduction.