Particle size dependent sinterability and magnetic properties of recycled HDDR Nd–Fe–B powders consolidated with spark plasma sintering

Abstract

The dependence of the magnetic properties on the particle size of recycled HDDR Nd–Fe–B powders was investigated, with the aim to assess the reprocessing potential of the end-of-life scrap magnets via spark plasma sintering (SPS). The as received recycled HDDR powder has coercivity (Hci) = 830 kA/m and particles in the range from 30 to 700 μm (average 220 μm). After burr milling, the average particle size is reduced to 120 μm and subsequently the Hci of fine (milled) powder was 595 kA/m. Spark plasma sintering was exploited to consolidate the nanograined HDDR powders and limit the abnormal grain coarsening. The optimal SPS-ing of coarse HDDR powder at 750 °C for 1 min produces fully dense magnets with Hci = 950 ± 100 kA/m which further increases to 1200 kA/m via thermal treatment at 750 °C for 15 min. The burr milled fine HDDR powder under similar SPS conditions and after thermal treatment results in Hci = 940 kA/m. The fine powder is further sieved down from 630 to less than 50 μm mesh size, to evaluate the possible reduction in Hci in relation to the particle size. The gain in oxygen content doubles for <50 μm sized particles as compared with coarser fractions (>200 μm). The XRD analysis for fractionated powder indicates an increase in Nd2O3 phase peaks in the finer (<100 μm) fractions. Similarly, the Hci reduces from 820 kA/m in the coarse particles (>200 μm) to 460 kA/m in the fine sized particles (<100 μm). SPS was done on each HDDR powder fraction under the optimal conditions to measure the variation in HCi and density. The Hci of SPS-ed coarse fraction (>200 μm) is higher than 930 kA/m and it falls abruptly to just 70 kA/m for the fine sized particles (<100 μm). The thermal treatment further improves the Hci to >1000 kA/m only up to 100 μm sized fractions with >90% sintered density. The full densification (>99%) is observed only in the coarse fractions. The loss of coercivity and lack of sinterability in the fine sized particles (<100 μm) are attributed to a very high oxygen content. This implies that during recycling, if good magnetic properties are to be maintained or even increase the HDDR powder particles can be sized down only up to ≥100 μm.