Comparison of shear strength and failure mechanisms of lap joint between REBCO tapes bonded by different joining techniques

Lap joints between Rare-Earth Barium Copper Oxide (REBCO) tapes are intended to be employed for segmented fabrication of high-temperature superconducting magnet as innovative design of fusion magnets. This study evaluates the shear strength and failure mechanism of lap joints between commercial REBCO tapes from two different manufacturers while using different mechanical joining procedures or soldering alloys. Additionally, the shear strength along the width direction of the different REBCO tapes was experimentally evaluated. Overall results using REBCO tapes with soldered Cu stabilizer revealed that they are unsuitable for field application due to low shear strength of the tapes. On the other hand, REBCO tapes with electro-plated Cu stabilizer are more versatile; soldered lap joints using Pb₃₇Sn₆₃ showed shear strength higher than required value. However, failure mechanism of the soldered joints always involve delamination of the REBCO tape which instantly increase joint resistance. Conversely, mechanical lap joints with indium fabricated using the improved method with heat treatment or indium pre-coating with heat treatment exhibited cohesive failure followed by plastic deformation of indium, which reduces the shear stress while maintaining acceptable joint resistance. This fact makes the mechanical lap joint a more reliable option so far.