Process technologies and magnetic property optimization of hot deformed NdFeB magnets

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In order to develop hot deformed anisotropic NdFeB magnets with high magnetic performance, commercial MQU-F powders were adopted to make isotropic NdFeB MQ2 magnets followed by hot deformed process to prepare anisotropic NdFeB MQ3 magnets. The main results are summarized as follows: (1) Effect of hot pressed and hot deformed processes on the magnetic properties of MQ3 magnets: Firstly, the effects of hot pressed processes on the magnetic properties of hot pressed NdFeB magnets were studied. Shorter heating rate and lower cold pressed pressure could increase the density and the uniformity of MQ2 magnets and also refine the grain size of the MQ2 magnets. Furthermore, the effects of reduction ratio, deformation temperature, and strain rate on the magnetic properties of hot deformed MQ3 magnets are also discussed. In this work, the optimized magnetic properties of Br =14.4 kG，iHc=15.8 kOe，(BH)max=51.5 MGOe，iHc+(BH)max=67.3 were obtained in the MQ3 magnets made with MQ2 magnet with the density of 7.58 g/cm3 followed by hot deformation for 70 % height reduction at 780 oC for 120 s. (2) Coercivity enhancement of hot deformed NdFeB magnets Coercivity enhancement of hot deformed NdFeB magnets consisted of commercial MQU-F powder by doping various powders and low-melting alloys are demonstrated. Firstly, doping DyF3 and TbF3 powders into MQ3 magnets could increase iHc from 15 kOe to 18.0-19.3 kOe. Besides, the magnets made from NdFeB MQU-F powders doped with 2 wt% low-melting R70Cu30 and R80Ga20 alloy powders also show improved coercivity. The coercivity enhancement results from not only the improved HA with Dy and Tb in 2:14:1 phase, also the strengthened pinning effect and decoupling effect due to the preference of Cu and Ga at grain boundary. Furthermore, (Nd0.5Dy0.5)70Cu30-doped magnets show an almost identical enhancement of coercivity of about 4.4 kOe to Dy70Cu30-doped ones, and it shows a beneficial effect of reducing the usage of Dy from 1.6 wt.% to 0.8 wt.%. Finally, coercivity enhancement of hot deformed NdFeB magnets by doping HRE-free Mm70Cu30 alloy powders (Mm = Mischmetal) is unexpectedly found. The increased coercivity up to 20.1 kOe can be obtained by doping (Pr72Nd11.5Ce11La5.5)70Cu30 alloy powders. The coercivity enhancement is mainly resulted from grain refinement, low aspect ratio of the grains, the improved HA with Pr into 2:14:1 phase, meanwhile, the preference of Ce at the grain surface and boundary phase might strengthen the magnetic isolation effect between grains. The above results provide easier and cheaper way to enhance the coercivity of hot deformed NdFeB magnets It can also be adopted for making thicker NdFeB MQ3 magnets for the applications at high temperature.