Spark plasma sintering as an effective texturing tool for reprocessing recycled HDDR Nd-Fe-B magnets with lossless coercivity

Ikram, Awais (author), Awais, Muhammad (author), Sheridan, Richard Stuart (author), Walton, Allan (author), Kobe, Spomenka (author), Pušavec, Franci (author), Žužek Rožman, Kristina (author)

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 BH$_{max}$) 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 M$_r$/M$_S$ 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 (H$_{Ci}$) of 960 kA/m and the remanence (B$_r$) 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.

• Language: English
• Keywords: rare earth permanent magnets, anisotropic HDDR Nd$_2$Fe$_{14}$B, recycling, spark plasma sintering, texture development, hot deformation
• Work type: Article
• Typology: 1.01 - Original Scientific Article
• Organization: FS - Faculty of Mechanical Engineering
• Publication status: Published
• Publication version: Version of Record
• Year: 2020
• Number of pages: 17 str.
• Numbering: Vol. 10, iss. 3, art. 418
• PID: 20.500.12556/RUL-133245
• UDC: 620.1/.2
• ISSN on article: 2075-4701
• DOI: 10.3390/met10030418
• COBISS.SI-ID: 33278247
• Publication date in RUL: 18.11.2021

Record is a part of a journal

• Title: Metals
• Shortened title: Metals
• Publisher: MDPI AG
• ISSN: 2075-4701
• COBISS.SI-ID: 15976214

Licences

• License: CC BY 4.0, Creative Commons Attribution 4.0 International
• Link: http://creativecommons.org/licenses/by/4.0/
• Description: This is the standard Creative Commons license that gives others maximum freedom to do what they want with the work as long as they credit the author.
• Licensing start date: 24.03.2020

Projects

• Funder: EC - European Commission
• Funding programme: H2020
• Project number: 674973
• Name: Training Network for the Design and Recycling of Rare-Earth Permanent Magnet Motors and Generators in Hybrid and Full Electric Vehicles
• Acronym: DEMETER