Correlation between hot pressed and hot deformation processes on the magnetic properties and microstructure of NdFeB permanent magnets

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In this study, commercial isotropic NdFeB powders, MQU-F, were adopted to make isotropic MQ2 magnets followed by hot deformation process to prepare anisotropic MQ3 magnets. Firstly, the effects of height reduction, deformation temperature and strain rate on the microstructure and magnetic properties of hot-deformed MQ3 magnets were discussed. The intensity ratio of I(006)/I(105) was evaluated from XRD analysis and the grain morphologies were observed by SEM to correlate with the magnetic performance of the MQ3 magnets. Among all samples, the optimal magnetic properties of Br = 13.4 kG、iHc = 15.5 kOe、(BH)max = 45.1MGOe、iHc+(BH)max = 60.6 was obtained in the magnet for 70 % height reduction and hot pressing at 780 oC for 240 s (strain rate 5*10-3 s-1). Secondly, the results show that with the decrease of strain rate, the uniformity of magnetic properties of the magnets was improved. Thirdly, with increasing the density of MQ2 magnets, the grain size could be decreased. The smallest grain size of ~70 nm was obtained for MQ2 magnet with the density of 7.58 g/cm3. In addition, the (00L) texture and magnetic properties of MQ3 magnet, which made from above high density MQ2, were enhanced effectively. The optimal magnetic properties of Br = 14.4 kG, iHc = 15.8 kOe, (BH)max = 51.5 MGOe, and iHc+(BH)max = 67.3 was obtained (70 % reduction, hot pressing at 780 oC for 120 s (strain rate 9.9*10-3 s-1)), if 12.5% thickness in both sides of the magnet was polished.