Preparation And Grain Boundary Modification Of Hot-Pressed And Hot-Deformed Nd-Fe-B Magnets

Nd-Fe-B rare earth permanent magnet material for the first time in the world is in the 1980s,after continuous improvement and research,successfully used in industrial production.And because it has excellent magnetic properties and other advantages,so it is widely known as"Magneto".With the continuous development of society,the technology of rare earth permanent magnet materials is constantly updated.It also shows a broad market space in some emerging sunrise industries,such as wind power generation,new energy vehicles,magnetic levitation and other fields,and plays an increasingly important role.Nd-Fe-B rare earth permanent magnet materials can be divided into sintering,bonding and hot-deformed Nd-Fe-B according to the different production process.Compared with the traditional sintering Nd-Fe-B preparation process,the hot-deformed Nd-Fe-B magnet preparation method has the unique advantages of low process temperature,short process time and small grain size.Thermal deformation is the only practical technique for inducing textural orientation in nanocrystalline hard magnetic materials in the past 30years.In this paper,by exploring the hot pressing process parameters to prepare dense isotropic hot pressing magnets with superior comprehensive performance,and then according to different thermal deformation rates to prepare anisotropic magnets with excellent magnetic properties and good orientation.Moreover,the magnetic properties and thermal stability of the hot-deformed magnets were further improved by adding RE-Fe(RE is Pr,Nd,Dy)alloys at grain boundary and re-diffusion by heat treatment.The coercoerity enhancement mechanism and microstructure of the magnets were studied and analyzed.Firstly,the isotropic hot pressing Nd-Fe-B magnets were prepared by using commercial pure MQU-M magnetic particles at different hot pressing temperatures.Thermopressing magnets are mainly composed of coarse-grained equiaxed grains with random orientation,and their density gradually increases with the increase of hot pressing temperature.Finally,it was found that the composite magnetic properties of the hot-pressed magnet prepared at the hot-pressed temperature of 650 ^? were the best.Secondly,when the hot-deformed temperature of 820 ^?,different hot-deformed rates are selected to prepare of the hot-deformed magnets.It is found that as the deformation rate decreases,the remanence of the hot-deformed magnets also become lower.This is because the deformation rate is slower,the longer time of heat preservation in the high temperature to deform for a long time,and it is easy to cause the crystal grain size of the magnets,have greater resistance to deform,which results in the decrease of remanence.Therefore,we choose the hot-deformed magnet with the best comprehensive performance for the next experiment.When the deformation rate is 1 mm/s,the hot-pressed temperature and hot-deformed temperature are 650 ^? and 820 ^?,respectively.it has the best comprehensive magnetic properties,J_r=1.41 k Gs,(BH)_max=46.5 MGOe,H_cj=12.55k Oe.Finally,the hot-deformed magnet was prepared with deformation rate of 1 mm/s,and added Pr₈₀Fe₂₀,Nd₈₀Fe₂₀ and Dy₈₀Fe₂₀ alloys with different contents.It is found that the effect of adding Dy₈₀Fe₂₀ alloy is more excellent.When 2 wt.%Dy₈₀Fe₂₀alloy is added,the coercivity of the hot-deformed magnet increases from 12.55 k Oe to 18.23 k Oe,while J_r and(BH)_max decrease from 14.1k Gs and 46.5 MGOe to 12.9 k Gs and 39.3 MGOe,respectively.The decrease of remanence J_r is due to the decrease in the degree of orientation and the accumulation of excessive RE-rich phases at the interface of the powder flakes,which leads to the formation of coarse grain regions without orientation.By adding 2 wt.%Dy₈₀Fe₂₀,the remanence temperature coefficient?and coercivity temperature coefficient?increase from-0.1268%/^? and-0.6319%/^? to-0.1063%/^? and-0.5290%/^?,respectively.And the thermal stability was improved.The T_c of the hot-deformed magnet with 2 wt.%Dy₈₀Fe₂₀ is higher than that of the original hot-deformed magnet.Through the intergranular addition process,the Pr or Nd atoms in the main phase particles are partially replaced by Dy,forming(Pr/Nd,Dy)₂Fe₁₄B core-shell structure,which is also the reason for the increase of coercivity.In addition,the coercivity of the magnet can be increased again by re-diffusion at 600^?.At the same time,the heterogeneous aggregation of the RE-rich phase makes the demagnetization region non-uniform,which leads to the non-uniform magnetic anisotropy and the opening of the recoil loops.In this paper,the preparation method of anisotropic hot-deformed Nd-Fe-B magnet was studied and the coercivity was improved by the method of intergranular addition under the condition that the remanence was not reduced.This technique has great potential for the development of Nd-Fe-B magnets with high coercivity and high remanence.