The Role of Dehydrogenation Conditions on Sinterability and Magnetic Properties of Fine NdFeB Powders Consolidated With Hydrogen-Containing Sintering

The dependence of the dehydrogenation conditions on the sinterability and magnetic properties of fine neodymium-iron–boron (NdFeB) powders consolidated with hydrogen-containing sintering was investigated. The new process proposed in this study adopts jet milling (JM) powder refinement to enhance the coercivity. After JM, the average particle size was reduced to
$1.6~\mu $
m and subsequently added lubricant to the powder in an increment of 0.3 wt% to avoid the deterioration of the magnetic field alignment; with the incremental addition of lubricant, the magnet’s impurity content increased, resulting in a decrease in magnetic properties. Unlike the traditional process, the dehydrogenation step is postponed until the sintering process in this study to address the problem of residual impurities in the magnets. Then, dehydrogenation treatment under special conditions, such as heating rate and atmosphere, was exploited to improve the sinterability of fine powders and simultaneously further reduce impurities in the magnet. Decreasing the heating rate of the dehydrogenation step in the sintering stage resulted in a lower rate of hydrogen release from the magnet. Under this condition, the green body remains intact after sintering without cracking and achieves better sinterability. While heating with argon (Ar) partial pressure in the dehydrogenation step, the rate of hydrogen removal can be controlled more slowly. As a result, the hydrogen gas will continue to exist in the dehydrogenation atmosphere, which would make the maximum amount of hydrogen gas react with the carbon and nitride compounds in the green body, eventually further reducing the impurity content in the magnet. Herein, we report a new commercial-scale process (100 kg batches) integrating all the above-modified procedures that reached the best results of Br =13.6 kG, iHc =20.8 kOe, (BH)max =44.5 MGOe, and BHH =65.3, far beyond the specifications of N45H grade commercial magnets with the same composition. The coercivity enhancement by our modified process shows promise for developing NdFeB sintered magnets with improved thermal stability. Controlling dehydrogenation conditions in the sintering stage is crucial for balancing magnetic properties and sinterability.