Research On Electron Beam Welding Technology,Microstructure And Properties Of The 1.3 GHz Single-Cell Niobium Superconducting RF Cavities In Particle Accelerators

The superconducting radio frequency(SRF)cavity is the core component of particle accelerator devices,applied in accelerator driven subcritical clean nuclear energy system,synchrotron radiation source devices,free electron laser devices and other accelerator facilities.Electron beam welding is one of the most important stage in its manufacturing process.The surface smoothness of the superconducting cavity is significantly impacted by the surface welding defects and the humps on the inner side of the weld seam,which may lead to quenching of the cavity and do harm to its acceleration performance.At the same time,the extremely low operating temperatures of the cavity place stricter demands on the low-temperature mechanical properties of the welded joints.In addition,it is necessary to take suitable measurements to reduce the welding distortion,which has the potential to alter the resonance frequency of the cavities.In order to perform a thorough investigation of the technical challenges in the welding of 1.3 GHz single-cell niobium SRF cavities,this study includes process experiments,microstructure observation,mechanical property testing,and numerical simulation.Based on the calculation of energy deposition distribution in the electron beam butt welded joint,the ideal welding parameters of the half cells in the SRF cavity were initially determined by orthogonal experiments.Different oscillating patterns were then employed for comparison studies based on the optimal parameters from orthogonal experiments.The dynamic behavior of the molten pool was numerically simulated to study the weld formation of niobium sheets with different oscillating patterns.The optimal oscillating pattern was ultimately determined after a comparative analysis on the microstructure and 77 K tensile properties of these joints.Eventually,the optimized welding parameters of half cells were obtained.In addition,in the subsequent optimization of weld humps,it was found that undercuts of a reasonable size at the bottom of the joint can more effectively minimise the humps on the rear side of the weld.For the electron beam welding of niobium plates with various thicknesses used in the welding of tubes and half cells,the welding parameters were optimized by adjusting the inner and outer welding beam currents as well as analyzing the weld formation,microstructure,and tensile properties at 77 K.At the same time,the welding experiment was also carried out for the butt welding of the tubes after rolling with increased thickness based on the butt welding experiments of half cells.The outcomes of the tube butt welding experiment were contrasted with those of other niobium joints used in the superconducting cavities,and differences in the optimum welding power,grain structure,and low-temperature mechanical properties of niobium sheets with various thicknesses were outlined.On the basis of electron beam welding experiments of niobium sheets,the welding quality of Nb/Nb55Ti dissimilar materials used in the welding of tubes and flanges is improved by optimizing the oscillating frequency and patterns.The tensile properties at 77K and the impact toughness at 4.2 K of the joints with different oscillating patterns were tested.By analyzing the grain structure of the joint and the element distribution in the weld zone,the optimal beam oscillation parameters for the joint were obtained.A new area heat source was created for the oscillating electron beam welding of thin-walled niobium workpieces based on the distribution characteristics of electron beam energy deposition in oscillating welding.Numerical simulations of the welding of the superconducting cavity at different stages were performed using the designed heat source and the Gauss body heat source.The simulation was used to design and optimize the clamping scheme,welding direction,starting point position,etc.,effectively reducing the welding deformation.In order to assess the validity of the welding experiment results of the sheets,the actual superconducting cavity was welded.The results of this study have certain engineering application value and offer a theoretical foundation and technical support for the manufacturing of 1.3 GHz singlecell niobium SRF cavities as well as other types of superconducting cavities.