Fabrication of Flux-Free REBCO CC Joints by Hybridizing Ultrasonic Welding and Soldering

Joining rare-earth barium copper oxide (REBCO) coated conductor (CC) tapes using hybrid welding (HW), a combination of ultrasonic welding (UW) and soldering methods, has shown superior joint characteristics and electromechanical properties compared to soldered CC joints. HW can be advantageous in terms of its quick and simple joining process that is suitable for on-site fabrication. In the conventional preparation of soldered CC joints, solder flux is typically employed to mitigate the oxide layers and impurities on the surface of CC tapes, thereby improving the wettability of the solder. However, the corrosive nature of the solder flux can be detrimental to the longevity and performance of the joints during the superconducting device operation. The authors' group has recently reported the feasibility of a flux-free HW method in fabricating lap-joints of differently stabilized GdBCO CC tapes. The flux-free HW CC lap-joints have impressively exhibited marginally higher joint strengths while maintaining comparable electromechanical properties to the soldered CC joints and HW with flux cases. This study applied the flux-free HW method to join Cu-stabilized CC tapes with butt-joint structures typically used in coil fabrication. The joint resistance and retained critical current of the flux-free HW CC joints were measured at 77 K and self-field, while electromechanical properties were evaluated using lap-shear and tension-bending tests.