Lubricant-assisted powder alignment using a damped AC magnetic field for high-energy-density Nd–Fe–B magnets prepared via the pressless process

Fine Nd–Fe–B powders are essential for fabricating sintered magnets with high intrinsic coercivity; however, their magnetic alignment is hindered by the low magnetic torque and high resistance to particle rotation and rearrangement encountered during pressless processing. In this study, a damped AC magnetic-field waveform and 2-nonanone lubrication were applied to improve the alignment of fine Nd–Fe–B powders in the pressless process. Repeated impulse-field application induced post-alignment particle rearrangement and green-body cracking, whereas the damped AC waveform promoted progressive particle rotation and rearrangement while preserving green-body integrity. The addition of 2-nonanone further reduced interparticle friction and increased μ0Mr from 1.315 T without lubricant to 1.408 T under 0.5 wt.% lubricant addition. However, increasing the lubricant content also increased the residual carbon concentration and eventually reversed the μ0Hc-enhancing response to annealing. In a high-carbon magnet, the NdOHCO₃ triple-junction phase transformed during annealing into spatially separated c-Nd₂O₃ and tetragonal Nd-carbide phases. This transformation was accompanied by localized strain at the Nd-carbide/Nd₂Fe₁₄B interface, which is considered a major reason for the negative μ0Hc response. Hydrogen-assisted processing reduced the residual carbon content from 2,290 to 740 ppm and changed the annealing-induced μ0Hc response from -0.159 to +0.217 T. These results demonstrate that simultaneous control of particle mobility during magnetic alignment and lubricant-derived carbon during thermal processing is essential for achieving high energy density in the pressless process.