Coercivity enhancement of sintered NdFeB magnets by powder Refinement, lubricant increment and Hydrogen-Containing sintering manufacturing

Abstract

This study investigates powder refinement, lubricant increment, and hydrogen-containing sintering procedures to develop high coercivity in NdFeB magnets. To improve the intrinsic coercivity iHc of sintered NdFeB magnets, it is proposed to control the average particle size and distribution of the magnetic powder by increasing the classification wheel speed of the jet mill during the jet milling process. Additionally, the incremental addition of lubricant is carried out in the additive mixing stage after the magnetic powder is refined. Through the increase of lubricant, the amount of lubricant is enough to completely cover the surface of the powder; hence, the alignment of the powder in the magnetic field forming process is improved, thereby increasing the maximum magnetic energy product (BH)max of the magnet. However, the magnetic properties of the magnet are improved through the refinement of the powder and the increase of the lubricant, but it also leads to the residue of impurities inside the magnet, which actually deteriorates the iHc. Different from the traditional process, this study moved the dehydrogenation procedure to the sintering process, thus reducing the residue of impurities inside the magnet, which is beneficial to the improvement of the magnetic properties of the magnet. Herein, we report a new commercial-scale process (100 kg batches) integrating all the above-modified procedures that reached the best experimental results of iHc = 20.79 kOe, (BH)max = 44.46 MGOe and BHH = 65.25 (i.e., BHH= iHc + (BH)max). The iHc, (BH)max, and BHH of the NdFeB magnet obtained by the new process proposed in this study are 16.47 %, 0.33 %, and 5 % higher than the traditional process, respectively. As a result, iHc has been greatly improved, making NdFeB magnets more conducive to meeting a wide variety of end-user applications, including high-coercivity (>20 kOe) sintered magnets suitable for equipment operating in high-temperature environments.