The electron beam welding of the tubes and the half-cells for our 1.3 GHz single-cell superconducting radiofrequency (SRF) cavities is complex due to the different thicknesses of the tubes and the half-cells in the iris region. However, the mechanical properties and microstructure of the iris welds in niobium SRF cavities have barely been explored in previous studies. For high-quality iris welds, welding experiments of niobium sheets of 2 mm and 2.8 mm were carried out under different oscillating conditions. The results show that welding with no oscillation or sinusoidal oscillation may not be applied in actual welding owing to the large misalignment of the bottom surface. The weld grains were not significantly refined through beam oscillation. The joints with infinity oscillation had a higher elongation than circular oscillation, which exhibited a brittle fracture in the tensile tests at 77 K. Nevertheless, the texture of the weld with infinity oscillation implies poor formability, so the feasibility of infinity oscillation in actual welding needs verification in future study.

The China Fusion Engineering Test Reactor vacuum vessel has strict tolerances requirements and a large number of fully penetrated joints in the manufacturing and assembly processes. As the most complicated component of vacuum vessel, port stubs have stricter tolerance requirements, to guarantee the high assembly accuracy with the vacuum vessel. Due to the small deformation of electron beam welding, this method is adopted for the welding of port stub. However, welding deformation cannot be avoided, thus it is necessary to carry out finite element analysis to analyze and control the welding deformation of the port stub. In this paper, based on thermo-elastic–plastic theory, the double ellipsoid and cone-shaped hybrid heat source can accurately outline the electron beam weld pool contour of the 316L stainless steel with 50 mm thickness, which is further verified by the welding process experiment. The above hybrid heat source is applied to analyze the welding deformation of the lower port stub, and according to the deformation simulation results, the welding fixture is designed to reduce the welding deformation. The welding deformation analysis can effectively provide theoretical data support and practical guidance for the actual electron beam welding process of port stub.

Oscillating electron beam welding (EBW) is necessary for the fabrication of pure niobium superconducting radiofrequency (SRF) cavities. Due to the negative effects that welding deformation and residual stress have on the performance of the SRF cavities, it is crucial to predict residual stress and welding distortion with high precision for the design of welding tooling and the optimization of welding parameters. However, the stress and distortion simulation of oscillating EBW received little attention in the previous research. In order to accurately predict the welding stress and distortion of niobium cavities with thin walls, a novel heat source with two reverse 2D Gaussian heat sources was used for the first time in the finite element method (FEM) simulation of EBW with beam oscillation. Additionally, a computational fluid dynamics (CFD) simulation of the molten pool was run as a guide for adjusting the parameters of the designed heat source. The FEM simulation with 2D Gaussian heat source was taken as a comparison. An EBW experiment of niobium sheets was performed to verify the simulation. The simulated molten pool of this model has a wider width, which is significantly closer to the actual measurement. Compared with the result estimated by 2D Gaussian heat source. The joint simulated with the designed heat source displays a smoother temperature gradient. The mechanical results suggest that the peak longitudinal stress in the weld center, the peak transverse stress in the weld center, the longitudinal contradiction, and the transverse contradiction estimated by the designed model deviate from the experimental results by only −4.88%, −4.03%, −9.32%, and −5.98%, respectively. The error of the simulation by the proposed method is dramatically smaller than the evaluation by the 2D Gaussian model. The designed heat source and the CFD validation provides a reliable simulation scheme for the oscillating EBW of thin sheets, and the oscillating EBW of thick plates will be simulated using the suggested method with the improvement of the heat source model in the future.

The high-temperature superconducting (HTS) current lead is one of the core components of the International Thermonuclear Experimental Reactor (ITER) magnet feeder system. Due to high-quality requirements, electron beam welding (EBW) was chosen to weld the room temperature terminal, the finned heat exchanger, and the HTS segment. The short mock-up was created to assess the performance of the welded joints. The analysis of macroscopic morphology and microstructure indicates that the welded joint of the short mock-up appeared to be free of visible defects. The tensile strength of the welded samples ranges from 185 to 208 MPa, which is higher than the design requirement of 150 MPa. The side bending test also shows no open discontinuity of the samples. Additionally, the thermal shock test was carried out. The dye penetrant test and the ultrasonic test after thermal shock shows that there are no obvious defects inside or on the surface of the weld seam. Furthermore, the leakage rate of the welded sample measured in the helium tightness leak test was 2.2 $\times$ 10 $^{{-10}}$ Pa $\cdot$ m $^{{3}}$ /s, indicating no cracks detected in the joints in the joints. The same parameters were also used for the welding of the long mock-up equipped with fins. The maximum temperature during the welding process does not go over the prescribed limit. The actual HTS current lead was welded, and its performance was evaluated, based on the welding experiments of mock-ups. The welding quality of the HTS current lead was confirmed by all test results.

Electron beam welding (EBW) is a crucial stage in the manufacturing of superconducting radiofrequency (SRF) cavities. The EBW joint between Nb half cells and the joint between Nb half cell and Nb beam tube have both been researched over the past few decades, but the EBW of Nb55Ti flange and Nb tube, especially the optimization of welding process, has not received enough attention. In this study, the weld formation, mechanical properties and microstructure of Nb55Ti-Nb weldments with non-oscillation, sinusoidal oscillation, circular oscillation, and infinity oscillation were investigated to improve the quality of the joints. The joints were all fully penetrated. The undercut was detected in the non-oscillation sample. Small depressions can be found at Nb side for the joints with beam oscillation. The EBW joints with beam oscillation have a slightly higher tensile strength than the non-oscillating one, which has a tensile strength of only 568.8 MPa. When using sinusoidal oscillation, the joint has weak ductility as evidenced by its extremely low elongation and cleavage fracture. The circular and infinite oscillated samples have comparable ductility and strength. The tensile strength of these joints are 597.1 and 583.9 MPa, respectively, and the elongation after fracture of these samples are 26.5% and 27.6%, respectively. The tensile properties are affected by the movement trajectory of beam oscillation. Compared to weld metal and Nb55Ti alloy, Nb base metal and Nb HAZ have superior impact toughness. The weld metal of infinity oscillated joint has the highest impact toughness of any of the weld metal samples, at 94 J/cm2. The macrosegregation structure of Nb can be used to explain why the impact toughness of the weld metal varies in different joints.