Solder-Free Low Resistive Joining of Metal Stabilization Layers of REBCO Coated Conductors by Use of Sonic-Welding Technique

We have developed a solder-free sonic welding technique for joining coated conductor tapes using only sonic energy. This method eliminates the need for surface treatments such as oxide film removal and avoids the use of intermediate metals like solder or indium foil. In this study, a joint resistivity of 17.3 nΩcm² at 77 K, less than half that of typical solder joints, was achieved with solder-free sonic-welding process by using appropriate welding parameters and 15 kHz sonic vibration. Scanning Hall-probe microscopy (SHPM), which enables 2-D visualization of the critical current density ($J_{\mathrm{c}}$) distribution in the joint area, was also used to confirm the soundness of the joints without local degradation. In addition, we proposed an analytical model to describe the joint resistivity as a function of welding parameters such as load, vibration amplitude and sonic energy based on a consideration of a physical model of the sonic-welding process. It has been demonstrated that the analytical model reproduces the experimental results quantitatively, allowing us to control the joint resistivity and to understand the dominant factors in the sonic-welding process, which has been difficult to achieve in the past.