The sintering mechanism of fully dense and highly coercive Nd-Fe-B magnets from the recycled HDDR powders reprocessed by spark plasma sintering

The goal of this study was to justify the reprocessing of recycled HDDR Nd-Fe-B powders with spark plasma sintering (SPS) and to investigate the dependence of the final magnetic properties on SPS and thermal treatment. The initial recycled HDDR powder of the composition Nd _13.4Dy _0.6Fe _78.6B _6.1Nb _0.4Al _0.7 with 4760 ppm O ₂ content, coercivity (H _Ci) = 830 kA/m, and large particle size distribution <50–600 μm, was treated using the SPS parameters as follows: T = 650–850 °C for 1–5 min and 50 MPa pressure. The post SPS thermal treatment was performed at 750 °C for 15 min in vacuum. The optimal SPS conditions at 750 °C for 1 min, yielded fully dense magnets with the coercivity H _Ci = 1060 kA/m, which was boosted to 1160 kA/m after the post-SPS thermal treatment. The grain growth was obvious in samples SPS-ed above 800 °C and subsequently, the H _Ci was reduced. The SPS reprocessing beyond 850 °C was found to be detrimental to the overall magnetic properties due to the formation of bimodal grain size distribution origination from the abnormal grain growth (700–2600 nm). The redistribution of Nd-rich grain boundary phase between the Nd ₂Fe ₁₄B matrix grains in thermally treated magnets and the relaxation of the internal stresses induced via SPS are the possible reasons for the H _Ci surpassing the H _Ci of the starting recycled HDDR powder. It was shown that the SPS consolidation technique is suitable for producing fully dense nanograined bulk Nd-Fe-B magnets, with coercivities even exceeding the initial HDDR-powder and is one of the most suitable routes for revitalizing the Nd-Fe-B scrap magnets.