Nb/Nb55Ti welding process test at the joints of tubes and flanges in superconducting cavities

The superconducting cavity is the core component of the particle accelerator. At present, the welding research of superconducting cavity at home and abroad is mainly focused on the pure niobium butt joints between half cells. There are few studies on the welding between the tube and the flange of the superconducting cavity. In order to explore the welding process of dissimilar materials at this joint, the L-shaped joint assembled vertically by the Nb plate and the Nb55Ti plate was used to simulate the joint between the tube and the flange during the electron beam welding process of superconductingcavity. The tests were carried out under beam currents of 8, 12 and 20 mA respectively. The macroscopic morphology, microstructure, chemical composition and microhardness of the joints under different beam current parameters were compared and analyzed after welding. The results showed that the weld was formed better when the beam current was 12 mA, and the grain growth in the HAZ on the Nb sidewas more obvious when the beam current was high. What’s more, the epitaxial growth of grains at the fusion boundary appeared in 3 groups of joints. As the welding current increased, coarse columnar crystals appeared near the fusion line. When the heat input was large, the liquid metal in the welding pool was fully mixed, which could be improved macrosegregation in the weld. The microhardness of the heat-affected zone on the Nb side decreased significantly after welding.