Rolling process plays an important role in the manufacture of Bi-based high temperature superconductor tapes, and the plastic flow regularities of the superconducting wires during deformation will directly affect the ultimate quality of the tapes. In order to investigate the effect of cross-sectional shapes before flat rolling on the performance and homogeneity of the tapes, some numerical models of Bi-2223/Ag wires with different cross-sectional shapes including circular, square, elliptical and racetrack cross-sections are constructed during the rolling process. By comparing the relative density, logarithmic strain ratio and length-width ratio on the filaments, it is revealed that Bi-2223/Ag wire with special-shaped cross-section can achieve better conductivity than the round wire, in particular, the racetrack cross-sectional wire has the second best performance among four wires. Based on material processability and experimental condition, tri-pass racetrack drawing technique is employed to optimize the process and obtain racetrack cross-sectional wire. The rolling process of Bi-2223/Ag wire with racetrack cross-section causes more intensive deformation of filaments in the center of the tape and achieves the filaments with larger length-width ratio. Also, the deformation distribution of filaments verifies the numerical results. Consequently, the racetrack drawing technique can be utilized for a reference during the mechanical processing and to increase the current transmission capacities of Bi-2223/Ag tapes.
The traditional rolling process of Bi-2223/Ag wires with circular sections is an inhomogeneous process. For the purpose of achieving a uniform filaments distribution, a novel technique for round wire through racetrack-type dies during the last several passes of drawing is necessarily brought forward. In order to investigate the processing behaviors of superconducting filaments, a numerical model for the three passes of racetrack drawing was constructed, and subsequently, the optimization of three racetrack-type dies was designed based on the simulation results. Then the three passes of racetrack drawing was applied to fabricate Bi-2223/Ag tape and the density distribution of each filament of the wire was studied by Vickers microhardness. The experimental results showed that the racetrack drawing can obtain dense superconducting filaments, and improve the mechanical properties and critical current of the final tape.
LU YongJinZENG PanLEI LiPingQU TiMingFANG GangSUN JianFeng
