Preparation And Optimization Of Magnetic Properties Of Anisotropic HDDR NdFeB Magnets

As the third generation of rare earth permanent magnet materials,Nd Fe B is widely used in information,energy,medicine,transportation,and national defense due to its excellent magnetic properties.In recent years,under the goal of"carbon neutrality and emission peak",the rapid development of new energy vehicles and energy-saving appliances and other emerging fields has led to a continuous increase in demand for high-performance Nd-Fe-B magnets.Through the Hydrogenation-Disproportionation-Desorption-Recombination（HDDR）process,nanocrystalline Nd-Fe-B magnetic powder with good anisotropy can be prepared,which is an excellent raw material for preparing high-performance neodymium Nd-Fe-B magnets.However,the industrial application mainly uses bonded magnets to prepare anisotropic magnets,which have lower magnetic density and poorer magnetic properties and thermal stability due to the introduction of adhesives,making them unable to meet the requirements for high-end applications.Therefore,how to use HDDR magnetic powder to prepare high-performance dense magnets is an urgent problem to be solved.In this article,using commercial HDDR magnetic powder as raw material,high-density HDDR Nd-Fe-B magnets were prepared by orientation sintering,selective laser melting（SLM）,spark plasma sintering（SPS）,and hot deformation（HD）technology and their magnetic properties were optimized.The research results are as follows:（1）Firstly,sintered magnets were prepared by traditional orientation sintering.Through multiple magnetic field orientations,the magnetic powders are uniformly arranged along the orientation direction to obtain anisotropic magnets.In order to increase the density of sintered magnet,the magnetic powder particle size was refined using an airflow mill,and low-melting Pr₇₀Al₂₀Cu₁₀alloy powder was doped to promote sintering,resulting in a density of 6.57g/cm³and coercivity of 1.07 T.（2）Using SLM technology to prepare 3D printed magnets.It is found that the microstructure of the printed magnet is distributed in layers,and theα-Fe soft magnetic phase is precipitated due to the unbalanced recrystallization of the molten metal,resulting in the disappearance of the coercivity.By doping Pr₇₀Al₂₀Cu₁₀powder into the magnetic powder for printing,the precipitation ofα-Fe phase was successfully avoided.In addition,a large amount of rich rare earth intergranular phase was distributed around the columnar Nd₂Fe₁₄B grains,which played a good magnetic isolation role,and a 3D printed magnet with coercivity of 0.1T was obtained.（3）A dense anisotropic sintered magnet was prepared by spark SPS,and the best magnetic properties were obtained by holding at 675°C for 5 minutes under 60 MPa pressure,with residual magnetism,coercivity,and maximum magnetic energy product of 11.5 kGs,1.23 T,and 29.8 MGOe,respectively.The magnetic properties of the sintered magnet were optimized by grain boundary diffusion technology.By selecting Pr₄₀Tb₃₀Al₂₀Cu₁₀alloy containing heavy rare earth elements as the diffusion source,the coercivity of the sintered magnet was increased to 1.62 T.The uniform intergranular phase distribution and the formation of a core-shell structure in the up and down surfaces of the easy axis direction of the diffusion magnet increased the nucleation energy of the two areas with the largest macroscopic demagnetization field,resulting in a significant improvement in the coercivity.（4）Using SPS-sintered magnets as precursors for HD process,magnets with a height reduction by 35%at a temperature of 750°C exhibit optimal magnetic properties,with remanence,coercivity,and maximum magnetic energy product of 12.8 kGs,0.91 T,and 36.9MGOe,respectively.The initial orientation of Nd₂Fe₁₄B grains within the SPS magnets reduces the grain shape anisotropy necessary for grain rotation and rearrangement during the hot deformation process,resulting in a reduced degree of hot deformation required to obtain the optimal orientation,and a decrease in mechanical anisotropy of the hot-deformed magnets.Finally,the coercivity of the hot-deformed magnet is increased to 1.4 T by intergranular diffusion in a Pr₄₀Tb₃₀Al₂₀Cu₁₀alloy.