Preparation Of High-performance,Low-cost NdFeB Magnets By Dy Thermal Diffusion Technology And Its Coercive Force Growth Mechanism

NdFeB sintered rare-earth permanent magnet materials are key magnetic functional materials because of it’s extremely high magnetic properties.The NdFeB magnets have been widely used in energy-saving motors,new energy vehicles,electronics,information,aerospace and many other fields.In recent years,a large amount of heavy rare earth elements(such as Dy,etc.)need to be added into the magnet to increase the coercivity of the magnet in order to improve the temperature characteristics of the magnet.However,the heavy rare earth is expensive,which makes the cost of NdFeB materials increase year by year.How to prepare high-performance,low-cost NdFeB magnets has become a research difficulty and hot spot in this field.Based on above practical problems,this work performed a systematic study on the high-performance and low-cost NdFeB magnets prepared by using Dy thermal diffusion technology.The 30M and N35 commercial magnets were chosen as experimental materials.The rare earth permanent magnet nondestructive testing system,metallographic microscope,scanning electron microscope and electron probe were used to analyze and characterize the magnetic properties,microstructure and element distribution of the samples,respectively.Some valuable research results have been obtained on the basis of experiments.The coercivity of the N35 and 30M samples have been greatly improved after thermal diffusion through different processes,but the remanence hardly decreased.The corcivity of the magnet increases and then decreases with the increase of the thermal diffusion time and temperature.The optimal process of thermal diffusion for N35 sample is at 940℃ for 10 h.The coercivity enhanced from 12.5 kOe to 18.11 kOe(increase of 44.88%).The best thermal difusion process for a 30M magnet is at 900℃ for 8 h.Under this process,the coercivity of the magnet increased from 14.16 kOe to 20.43 kOe(increase of 44.28%).The grain size of the samples increased with the increase of the thermal diffusion time and temperature.Under the same conditions,the thermal diffusion effect on the 30M magnet is better than that of the N35 magnet.This is due to that 30M sample has a smaller grain size than that of the N35 sample,and more neodymium-rich phases are evenly distributed on more grain boundaries,which is more conducive to the diffusion of Dy.Based on Fick’s second formula,the diffusion coefficients of Dy element in grain boundary and in the main phase grain were calculated.The thermal diffusion process of Dy was analyzed with the electron probe.The mechanism of coercivity increasing was deduced.Dy has a diffusion coefficient D1=2.3328×10-3μm/s in grains,and a diffusion coefficient D2=5.1640μm/s in grain boundaries.D2 is much larger than D1,indicating that the diffusion rate of Dy element in the grain boundary is much faster than that in the grain,so Dy element preferentially penetrates into the grain boundary.At the beginning stage of thermal diffusion,Dy element penetrates into the magnet deeper along the grain boundary,the Dy element concentration gradually decreases from the surface area to the core area.With the extension of thermal diffusion time,Dy element in the grain boundary starts to diffuse into the grain,and Dy element infiltrating into the outer layer of the main phase grain will replace Nd element and form(Nd,Dy)2Fe14B modified zone around the main phase grain,which is a hard magnetic phase with high magnetic crystal anisotropy.In addition,the Dy and neodymium-rich phases form a non-magnetic thin layer in the grain boundary to separate the main phase grains from each other,which has a decoupling effect and can also improve the coercivity of the magnet.The increase of the thermal diffusion time and thermal diffusion temperature make the thermal diffusion process more sufficient.When the distribution of Dy elements in the magnet is even and the concentration is high,the magnet has the highest magnetic performance.However,when the thermal diffusion time is too long,the Dy element will diffuse into the main phase grains and even fill the entire grains,which will deteriorate the performance of the magnet.In addition,an excessively long thermal diffusion time and an excessively high thermal diffusion temperature will also make the grain size coarse,and further reduce the coercive force of the magnet.After Dy thermal diffusion treatment,the performance of the magnet is obviously improved from N to H,and from M to SH.In the traditional process,about 3%of Dy needs to be added to prepare high-grade SH magnets,and the thermal diffusion technology only consumes about 0.8%of Dy to obtain SH magnets.Therefore,thermal diffusion technology is an important development direction for the preparation of high performance and low cost magnets.