Effect of welding time on joint formation, microstructural evolution, and mechanical properties of resistance-upset-welded 316L stainless steel fuel-rod joints

<title>Abstract</title> <p>Reliable tube-to-end-plug joints are essential for maintaining the sealing and structural integrity of nuclear fuel rods. This study examined the effect of welding time on the formation, microstructure, and tensile behavior of resistance-upset-welded 316L stainless steel tube–plug joints. Welding time was varied from 9 to 24 ms at a constant current of 12 kA and an axial upset force of 2500 N. Macroscopic observations, electron backscatter diffraction (EBSD), tensile testing, and fractography were used to characterize joint evolution. Increasing welding time enlarged the softened plastic layer and effective bonding region, promoted interfacial plastic flow, and improved joint continuity. EBSD revealed substantial grain refinement in the weld zone, consistent with severe plastic deformation and dynamic recrystallization; the grain-orientation distribution became more dispersed as welding time increased from 9 to 21 ms. The average maximum tensile load increased from 9.84 kN at 9 ms to 11.53 kN at 21 ms. At 21 ms, fracture shifted from the weld zone to the base metal, indicating that the weld zone was no longer the weakest region. At 24 ms, the average maximum load decreased to 10.39 kN, indicating performance deterioration under an excessively long welding cycle. The results identify a transition from incomplete bonding to stable joining and subsequent degradation and show that 21 ms provides the best overall performance under the investigated current and force conditions.</p>