Research On Pulse Laser Spot Welding Characteristics Of Zirconium Alloys Intersection Point And Growth Mechanism Of Oxide Film Formed On Weld Bead

Zirconium alloys spacer grid is the essential assembly in reactor core. The manufacturing technology and corrosion performance in service of zirconium alloys spacer grid are still the limiting factors for high burn-up reactors. The welding process of zirconium alloys should be conducted in the chamber filled with pure argon, since zirconium alloys have a high absorption of oxygen, nitrogen, hydrogen in the atmosphere due to the chemistry activity. Thus, The elimination of heat and smoke produced during the laser processing is of vital importance. Besides, the weld beads size must be small enough to immune to wear the nuclear fuel rods and lead to a small pressure drop of the coolant which flows along the nuclear fuel rods. More importantly, the corrosion resistance of weld bead is crucial to the space grid, since the microstructure of fusion zone changed greatly compared to the base metal.In the present work, the research on multi-pulse laser spot welding and corrosion characteristics of zirconium alloys intersected weld bead was conducted. The optimization of the gas flow behavior in the chamber was conducted utilizing simulation method. In order to control the weld bead size, multi-pulse laser spot welding was proposed and the optimized parameters were obtained. The microstructures of fusion zone and base metal of laser welded zirconium alloys joints were comparatively investigated. The corrosion performances of fusion zone and base metal were comparatively studied and the measure to improve the corrosion performance of fusion zone was presented.The gas flow behavior in the chamber was adjusted using coanda air knives. The critical value of air flow for coanda effect and the reasonable distribution of air knives were obtained. Two air knives were necessary to optimize the gas flow behavior in the chamber. One of them was fixed to increase the gas flow rate in the chamber, the angle of the other one was flexible to make the gas flow over the workpiece and prevent the backflow of external gas. To control the setup time before laser welding, the auxiliary outlet was designed to reduce the moisture and oxygen contents in the chamber quickly. The function of the main outlet was increase the exhaust and cooling rates. The optimal gas flow behavior could be obtained while the angle of the adjustable air knife and gas flow rate were 30-45° and 60-120 L/min, respectively.In order to control the weld bead size, multi-pulse laser spot welding was proposed and the optimized parameters were obtained. To obtain the weld beads with qualified weld width and protrusion, the pulse peak power and number of pulse should be increased, the pulse width should be decreased properly. Compared with base metal of zirconium alloys, the grain size of fusion zone was bigger and second phase particles could not be observed. After the annealing treatment, the grain orientation was more consistent and the β-Nb and Zr(Nb, Fe)2 second phase particles were precipitated inter and intragranular of columnar grains. The transformation of residual β-Zr phase to α-Zr and β-Nb phases occurred.The corrosion performances of fusion zone and base metal were comparatively evaluated by investigating the different characteristics of oxide films formed on fusion zone and base metal. Phase transformation of oxide microstructure and second phase particles were the main factors to affect the corrosion resistance. The diffusion rate of oxygen into the metal was reduced, since the transformation from tetragonal to monoclinic Zr O2 phase could be suppressed due to the high macroscopic compressive stress. Higher level of stress was produced at the oxide/metal interface of base metal, which was ascribed to more pronounced undulation of the oxide/metal interface of base metal than fusion zone. Besides, the second phase particles was contribute to the improve the corrosion resistance of base metal due to the better corrosion resistance than α-Zr matrix phase. The corrosion resistance of fusion zone could be improved by the annealing treatment at 650℃ for 150 min due to the precipitation of second phase particles.