[P1] RE-Fe-B Magnets

[P1-31] On the Impact of Particle Size and Filling Density on the Magnetic Texture of Recycled Nd-Fe-B Magnets obtained via Pressless Processing

*Wagner Costa Macedo¹, Leonardo Antunes¹, Caio Ferreira Sampietro¹, Marcelo Augusto Rosa¹, Luis Torres Quispe¹, Paulo Wendhausen¹ (1. Federal University of Santa Catarina (Brazil))

Keywords:

Nd-Fe-B magnets,Pressless Processing (PLP),Powder metallurgy,Recycling

As the demand for NdFeB magnets grows significantly, so does the supply of scrap magnets, making recycling a focal point concerning sustainable magnet production. In that sense, the magnet to magnet approach is the most explored recycling method so far, where the scrap is disassembled and reintroduced in the powder metallurgy route, following the same steps usually used for commercial alloys.However, direct reprocessing results in additional oxygen uptake. This increase in oxygen content contributes to the degradation of magnet properties, meaning that a recycling strategy that minimizes oxidation is preferred. Sagawa et al introduced a potential strategy to overcome this issue, which is the Pressless Processing PLP.PLP does not involve a pressing step, which facilitates the handling of the powder with less oxygen exposure throughout processing, allowing for the use of finer powders and enhancing final coercivity. Nevertheless, the larger surface areas of these finer particles affect their rotation during the magnetic alignment step, hindering the development of a proper magnetic texture. It is possible to circumvent that by properly tailoring PLP parameters such as particle size, filling density, and presence of a lubricant.Although such features were investigated by Popov et al, they were not explored in the context of recycling magnets, which is the purpose of this work. Hence, the effect of PLP processing parameters on the final properties of the recycled magnets, especially the degree of alignment, is explored.N40M end of life magnets were subjected to hydrogen decrepitation and jet milling to achieve various mean particle sizes. Subsequently, graphite molds were filled with the NdFeB powders at different filling densities from zero point five to three point five grams per cubic centimeter. The filled molds underwent a four point five tesla magnetic pulse followed by vacuum sintering. All processing steps were conducted in a glovebox under anaerobic conditions with less than three parts per million of oxygen. Demagnetization curves of the recycled PLP magnets were obtained using a Brockhaus hystograph. The texture of the recycled magnets was assessed by determining the degree of alignment as described by Quispe et al.In addition to evaluating the degree of alignment after sintering, some additional molds were also prepared to study particle mobility under different conditions. These molds were subjected to a four point five tesla pulse, encapsulated under an argon atmosphere, and then removed from the glovebox for measurement in the hystograph. The demagnetization of powdered samples occurs through particle rotation, and the measured coercive force CF is related to the freedom of particle rotation. Thus, assessing CF provides insight into particle behavior during magnetic alignment.CF values increased with higher filling densities. A greater filling density indicates closer packing of particles, which limits their rotation during magnetic alignment. This effect was more pronounced for finer powders, which exhibited higher CF compared to coarser powders.After sintering, the recycled magnets in all conditions exhibited coercivities above eight hundred kiloamperes per meter. Differences in CF were reflected in the texture of the recycled PLP magnets, influenced by both particle size and filling density. The impact of filling density was less pronounced for coarser powders, likely due to their smaller surface area relative to finer powders. This phenomenon presents a tradeoff between remanence and coercivity, with the latter benefiting from smaller grain sizes. Fine tuning the filling density and particle size are critical factors to ensure a high degree of alignment in recycled magnets while maintaining a balance between high remanence and high coercivity, preventing degradation during recycling.
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