Laser powder bed fusion of NbTi low-temperature superconductors

This study investigates the superconducting properties of additive manufactured in-situ NbTi alloy using laser powder bed fusion. Laser parameters were optimised for a maximum microstructural density and lowest Nb segregation fraction. These characters have been achieved at the sample built with an energy density of 3.33 J/mm², where it shows a density value of 6.12 g/cm³, undissolved Nb fraction of 0.05 %, and randomly distributed equiaxed grains with an average size of 10–20 μm. Post-processing of the as-fabricated (AF) sample was explored as a route to collapse residual porosity and dissolve residual segregated Nb particles using hot isostatic pressing (HIP) and heat treatment, respectively. HIPing resulted in a decrease of the undissolved Nb particle fraction to 0.035 %. Heat treatment at 1250 °C for 24 h, followed by aging at 400 °C for 2 h, was found to result in the complete dissolution of Nb particles and precipitation of the ɑ-Ti phase that works as pinning centres to increase the superconducting properties. The grain size increased in both post-processes to 400–500 μm. Electrical and magnetic properties were also measured for the AF, HIP and heat-treated samples. The thermal variation of electrical resistivity showed a critical temperature (T_c) range of 9.8–9.0 K, 9.6–9.1 K and 9.9–8.8 K for the AF, HIP, and heat-treated samples, respectively, which were close to the values obtained from magnetic measurements. The critical current density (J_c) was calculated using Bean's model employing the magnetic hysteresis loops. The AF condition showed a negligible value of J_c (5 × 10⁻³ kA/mm²), which improved to 0.28 kA/mm² and 2.65 kA/mm² for the HIPed, heat treated conditions, respectively (at 4.2 K, 5 T). Mechanical tensile testing was performed for the heat-treated sample to ensure both mechanical and superconducting properties would achieve the required performance, where achieves ultimate tensile strength value of 930 MPa, with 8 % elongation.