Robust Joining Solution of REBCO-to-Copper for Current Lead Applications Using Ultrasonic Welding

This study investigates the utilization of ultrasonic welding (UW) to establish robust connections between REBCO coated conductor (CC) tapes (4 mm and 12 mm in width) and copper (Cu) sheets (0.2 mm and 1 mm thick) for current lead applications in superconducting magnet systems. Various joint configurations, including both single-layer and multi-layer CC-to-Cu joints, are examined, with welding parameters optimized using the Taguchi method. To enhance joint strength and joint resistance, a hybrid welding approach with solder and flux-free techniques is adopted. Electromechanical assessments conducted at 77 K include critical current (I-c) measurements, joint electrical resistance (R-j) evaluations, and lap-shear tests. The results reveal minimal I-c degradation, along with acceptable R-j values of around 600 n omega, and lap shear strength of over 700 N, particularly with the configuration of upper-positioned CC tapes on the horn side. Notably, the UW process, especially when implemented in a multi-layer configuration, successfully achieved a joint current capacity of over 1 kA, effectively meeting the rigorous demands of superconducting magnet systems.