Spark Plasma Sintering as an Effective Texturing Tool for Reprocessing Recycled HDDR Nd-Fe-B Magnets with Lossless Coercivity

Abstract

The low-pressure hot-deformation methodology was applied to reprocess the nanocrystalline hydrogenation–disproportionation–desorption–recombination (HDDR) Nd-Fe-B powders from end-of-life (EOL) permanent magnets’ waste to determine the mechanism of texture development and the resultant improvement in remanence (and BHmax) in the recycled material. Both the hot-pressed and hot-deformed magnets produced via spark plasma sintering (SPS) were compared in terms of their magnetic properties with respect to forging pressures. Also, a comparison was established with the microstructure to cite the effectiveness of texture development at low deformation rates and pressures which is pivotal for retaining high coercivity. The hot-pressed magnets maintain the high coercivity (better than 100%) of the original recycled powder due to the control of SPS conditions. The hot deformation pressure was varied from 100–150 MPa at 750 °C processing temperature to identify the optimal texture development in the sintered HDDR Nd-Fe-B magnets. The effect of post-hot-deformation thermal treatment was also investigated, which helped in boosting the overall magnetic properties and better than the recycled feedstock. This low-pressure hot deformation process improved the remanence of the hot-pressed magnet by 11% over the starting recycled powder. The Mr/MS ratio which was 0.5 for the hot-pressed magnets increased to 0.64 for the magnets hot-deformed at 150 MPa. Also, a 55% reduction in height of the sample was achieved with the c-axis texture, indicating approximately 23% higher remanence over the isotropic hot-pressed magnets. After hot deformation, the intrinsic coercivity (HCi) of 960 kA/m and the remanence (Br) value of 1.01 T at 150 MPa is indicative that the controlled SPS reprocessing technique can prevent microstructure related losses in the magnetic properties of the recycled materials. This route also suggests that the scrap Nd-Fe-B magnets can be treated with recoverable magnetic properties subsequently via HDDR technique and controlled hot deformation with a follow-up annealing.

Limitations in the Grain Boundary Processing of the Recycled HDDR Nd-Fe-B System

“…The overall surge in production volume and application demand has necessitated the utilization of recycled REPMs into the industrial feedstocks [ 4 , 5 ]. The effectiveness of hydrogen gas towards decrepitating (HD) and disproportionation (HDDR) of the end-of-life rare-earth (RE) scrap has convincing been proven in previous studies [ 6 , 7 , 8 , 9 , 10 , 11 , 12 ]. Contrary to the theoretical models, the HDDR Nd-Fe-B system lacks high coercivity as a translation of high magnetocrystalline anisotropy ( µ 0 H a ) ~ 7.2 T because of crystal structure inhomogeneities, grain morphology, surface defects, oxidation, nonferromagnetic grain boundaries and localized exchange interactions at the grain interfaces, which restrict their usability [ 13 , 14 , 15 , 16 , 17 ].…”

Feasibility Study on Modified Edge-Free Spark Plasma Sintering (SPS) on Metal Powder – Al7075 Case Study

“…Subsequently, the pre-sintered sample is transferred into a larger-diameter die for the second stage, facilitating edge-free deformation during the subsequent synthesis to the required temperature and pressure. The utilization of a larger die in the second stage of SPS serves multiple purposes, including the elimination of die-wall interaction [12], removal of residual pores, and the promotion of grain refinement and deformation [1,13,14].…”

Pre-orientation sintering: an alternative process to prepare high-performance anisotropic Nd–Fe–B HDDR magnets

“…Figure11. Summarized remanences Br and coercivities H cj of the our prepared and reported Nd-Fe-B HDDR magnets, the data were extracted from[15,16,21,25,[44][45][46][47][48].…”