Influence of Beam Oscillation Patterns on the Weld Formation, Mechanical Properties, and Microstructure of Nb55Ti-Nb Weldments

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 nonoscillation, 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 nonoscillation 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 nonoscillating 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/cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup>. The macrosegregation structure of Nb can be used to explain why the impact toughness of the weld metal varies in different joints.