Applicability of ultrasonic welding to Ag-stabilized REBCO CC joints and evaluation of their electromechanical properties

Abstract

Manufacturing a single long-length rare-earth barium copper oxide (REBCO) coated conductor (CC) tape with uniform critical current (Ic) is still a challenge; therefore, joining between multiple-length CC tapes is needed in the production of longer CC tapes for superconducting cables, coils, and magnets. Various joining techniques have been developed to achieve acceptably low joint resistance (Rj) with no Ic degradation and good electromechanical properties. The authors established ultrasonic welding (UW) and hybrid welding (HW) methods for joining Cu-stabilized CC tapes with different configurations. However, these methods have yet to be applied to Ag-stabilized CC tapes to produce compact joints and longer tapes during in-line production. This study used the UW and HW methods to fabricate Ag-stabilized CC joints to have low Rj without Ic degradation through a direct connection at the overlapped Ag layers between both CC tapes. Two CC tape samples with different thicknesses of ~2 µm and ~6 µm Ag stabilizers were supplied to produce UW and HW Ag-stabilized CC joints. These were compared with soldered CC joints fabricated by a mechanically controlled soldering method. In the case of UW, the design of experiment (DOE) using the Taguchi method was used to systematically determine the optimized weld parameter combination that yields a lower Rj without any Ic degradation. This study is a systematic attempt to evaluate the applicability of UW for solid-state joining between thin Ag layers, unlike the conventional joining of thick Cu stabilizers. Moreover, the electromechanical properties of differently processed Ag-stabilized REBCO CC joints were evaluated using both lap-shear and double-bending tests. As a result, the optimum UW parameter combinations were obtained to achieve Ag-stabilized CC joints. However, some variations were probably due to the differences in the production batch and thickness of the Ag layers. The UW Ag-stabilized CC joints showed superior joint characteristics and electromechanical properties compared to soldered CC joints. Good solid-state bonding of the Ag layers was observed through microscopic observation of the cross-section at the joint region of the UW CC joint. The joint characteristics and electromechanical properties of the UW Ag-stabilized CC joints can be further improved using the HW method.

Performance characteristics of REBCO coated conductor joints fabricated by flux-free hybrid welding

“…Following the successful implementation of flux-free HW to GdBCO Cu-stabilized CC tapes, we further investigated the feasibility of using the same method for GdBCO Ag-stabilized CC tapes. While a recent report has shown that UW and HW methods can join Ag-stabilized CC tapes [37], the specific application of the flux-free HW method on these tapes has not yet been studied.…”

“…To prevent slipping and misalignment during the joining process, a pair of toggle clamps was used to fix the overlapped samples. The horn and anvil have a contact area ratio of 20.9% for Custabilized CC tapes from the truncated pyramidal tip patterns (Horn C/Anvil C) [38], while for Ag-stabilized CC tapes, a different anvil with a finer tip pattern and contact area ratio of 11.1% (Horn C and Anvil B) was used [37]. For UW CC joints, we used the optimized parameter combination 50% vibration amplitude (∼31 µm displacement), 5 bar (∼3.9 kN) welding pressure, and 1 s welding time for Cu-stabilized CC tapes, as in our previous studies [35,36,38,39].…”

“…By applying ultrasonic vibration along the width of the CC tapes, solid-state bonding among the stabilizing layers can be achieved through frictional heat. Studies have reported successful UW joining of CC tapes with different stabilizers and joint structures [34][35][36][37][38]. To ensure high-quality UW joining, carefully selecting horn and anvil tip tools and patterns is crucial to avoid damage to superconducting layers.…”

“…The joint characteristics of UW CC joints vary depending on the stabilizers, tape manufacturing process, and production batch [35,36]. Recently, the authors reported successful UW joining of Ag-stabilized CC tapes, which have been challenging to join due to their thin and delicate Ag layer [37]. By optimizing the vibration amplitude, welding pressure, and welding time, sufficient heat can be generated within the joint interface, reducing the overall resistance and avoiding damage to the superconducting layer.…”

“…This is due to the geometry of suitable horn and anvil tip patterns and the brittle nature of the ceramic superconductor layer, making it difficult to apply repeated or additional UW processes to the same samples. The hybrid welding (HW) method was developed to overcome these challenges, combining UW and soldering methods [36,37,39]. HW can be done using either pre-solder insertion or post-solder infiltration.…”