Development of UAM for REBCO CC Terminations in High Purity Copper

Low resistance terminations of Rare Earth Barium Copper Oxide (REBCO) Coated Conductors (CCs) into normal conductors are critical for remountable fusion magnet joints. However, making repeatable low resistance connections with CCs is difficult, especially when joining larger CC assemblies such as REBCO cables. For the first time, Ultrasonic Additive Manufacture (UAM) has been identified as a potential route to scalable manufacturing of REBCO cables and cable terminations. In the UAM process, solid components are manufactured by ultrasonic consolidation of successive layers of metal tapes and subsequent machining to net shape. This feasibility study investigated solder-free embedding of single CCs into copper by tailoring the UAM process parameters vibration amplitude, downforce and travel speed. The initial parameter space was narrowed down by conducting qualitative peel tests of exploratory welds. CC lap joint test pieces with intermediary copper were subsequently manufactured and critical current retention as well as joint resistance were measured at 77 K. Cross-sections of the samples were analysed to understand the microstructure of the bond. Based on these data, suitable UAM process parameters were identified. It has been demonstrated that UAM could be a promising manufacturing route for low resistance CC connections to copper with no significant critical current degradation.