A new method for simulating electron beam welding process of niobium sheets with beam oscillation

Abstract

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.