Formation mechanism and microstructure evolution during resistance upsetting welding of zirconium alloy

Abstract

The end plugs (Zr-Sn alloy) and cladding tubes (Zr-Nb alloy) were joined by resistance upsetting welding (RUW). RUW is a solid-state welding method involving thermal-electric-force-magnetic multi-field coupling process and has characteristics of both friction welding and resistance welding. The heat sources include joule heating, frictional heating, and plastic deformation heating of the metals. The forces include top forging force, frictional force, and lorentz force. Through the combined effects of temperature and force, the material undergoes the process of elemental diffusion, phase transformation, dynamic recrystallization (DRX) and plastic flow, thus forming the weld. According to different microstructure, the joint could be divided into: fine crystal zone (FCZ), thermal mechanical affected zone (TMAZ), heat-affected zone (HAZ), and base metal (BM). During the welding process, the grain size and morphology of the weld zone change due to the influence of high temperatures and pressures, leading to significant differences in the microstructure across different regions of the weld zone. The deformation process is subject to the synergistic effect of slip and twinning. The joint exhibits evident DRX phenomenon. The recrystallization forms of joint mainly consist of continuous dynamic recrystallization (CDRX), discontinuous dynamic recrystallization (DDRX) and geometric dynamic recrystallization (GDRX). Deformation texture and recrystallization texture occur simultaneously in the FCZ and TMAZ. The shape and distribution of the secondary phase vary across different regions of the joint.