Evaluations of Joint Resistance and Bending Performance of Various Ultrasonically Welded CC Joints

In superconducting magnet and coil applications, joints are used to connect lengths of superconducting tapes and to complete the windings. Certain coated conductor (CC) joint properties, such as joint resistance (Rj), must be considered during the design phase. Ultimately, the joints must meet all electrical and mechanical requirements. In this study, the parameters for ultrasonic welding (UW) of differently stabilized rare-earth barium copper oxide CC tapes were optimized using the Taguchi method. Along with critical current (I
_c
), R
_j
was measured at 77 K in joints formed using various types of CC tapes. Bending performance of the joints was measured as a practical evaluation for coil winding. Retained I
_c
showed no degradation after the UW process. Resistance at the joint was in the range of 100 to 200 nΩ (joint resistivity, about 120-240 nΩ·cm
²
). Bending tests showed that a bending diameter of 50 mm was tolerated without I
_c
degradation. Surface morphologies of the cross-sections and joint interfaces were observed and revealed an explanation for the behavior of R
_j
with bending.