High–coercivity HRE–free NdFeB magnets by laser powder bed fusion

Additively manufactured NdFeB permanent magnets with no Heavy Rare Earth (HRE) such as Dy and Tb were fabricated by Laser Powder Bed Fusion (LPBF). As-built samples exhibited relative densities up to 97 % and magnetic properties up to Br = 0.667 T, H_cJ = 1173 kA.m⁻¹ and (BH)_max = 59.4 kJ.m⁻³. After an adequate annealing, a substantial increase of the magnetic properties up to Br = 0.691  T, H_cJ = 1443 kA.m⁻¹ and (BH)_max = 77.9 kJ.m⁻³ was achieved. These magnets have been elaborated using a specific powder fabricated with an in-house experimental pilot line allowing the possibility to adapt both the size (in the case of this study a narrowly-distributed powder with a volumetric D50 of 40  μm) and the chemical composition of the material as desired. The microstructure of the printed magnets has been investigated and is shown to be mainly constituted of untextured submicronic magnetic Nd₂Fe₁₄B grains surrounded by an intergranular Nd–rich phase, the same phases as reported in standard sintered magnets but with an average grain size several times smaller. Due to the presence of the Nd–rich intergranular phase, the obtained intrinsic coercivity is significantly higher than the one reported in studies of the literature working on LPBF with a commercial MQP–S powder. Nanosized Nd-rich oxide inclusions are often observed within the Nd₂Fe₁₄B grains. It is suggested that these Nd-rich oxide inclusions could act as nucleants for the Nd₂Fe₁₄B grains during the solidification process.