Preparation,Microstructure,and Properties Of Anisotropic Hot Worked Nd-Fe-B Permanent Magnets

Nd-Fe-B magnets are the important rare earth permanent magnets,which can be prepared various processes including sintering,bonding and hot working.Hot worked magnets exhibit magnetic anisotropy produced by hot deformation and have the advantages of high magnetic properties,strong corrosion resistance,high temperature stability,low rare earth content and simple process.However,due to the immature preparation process and the imperfect establishment of microstructure-magnetic properties relationships,the production and application of hot worked Nd Fe B magnets are still limited.In this work,isotropic and anisotropic Nd Fe B magnets were prepared by hot pressing,hot deformation,and back extrusion.The effects of original magnetic particle state,processing parameters and alloy composition on the microstructure,magnetic properties and other application performance of the magnets have been studied.The magnetic properties have been further improved by in-situ grain boundary diffusion process.Based on the theoretical and experimental investigations,the relationships between the process,microstructure,magnetic properties and other properties have been deeply analyzed.The main research contents and conclusions are as follows:Firstly,the hot-pressed and hot-deformed magnets were prepared using both amorphous and nanocrystalline magnetic powders with the composition of Nd_28.47Pr_0.56Fe_balB_0.97Co_3.63Al_0.13Nb_0.56Zr_0.44.The effects of the process on the microstructure and magnetic properties have been analyzed.It is found that the remanence（J_r）,coercivity(H_cj)and maximum magnetic energy product(（BH）_max)of the hot-pressed magnet increase with the increase of hot-pressing temperature,pressure or holding time.The hot-pressed magnets prepared by amorphous Nd Fe B magnetic powders exhibit the grain pre-orientation,and their magnetic properties are better than those prepared by nanocrystalline magnetic powders.The magnetic properties of J_r=0.79 T and H_cj=1519 k A/m were obtained by the optimum hot-pressing process of 700 ^oC/400 MPa/90 min.The magnetic properties of subsequent hot-deformed magnets are mainly determined by the deformation temperature.Based on the optimum hot-deformation temperature of 750 ^oC,high（BH）_max of 302 k J/m³ was obtained.Under the same process conditions,the hot-deformed magnets prepared by amorphous magnetic powders show higher magnetic properties than those prepared by nanocrystalline magnetic powders,due to the smaller grain size and more uniform grain boundary phase in the former.Then,the coercivity of hot-pressed and hot-deformed magnets is further improved by in-situ grain boundary diffusion technology.The effects of different contents of low melting point diffusion sourcers including Pr₇₀Al₂₀Cu₁₀,Pr₇₀Cu₃₀ and Pr₇₀Al₃₀ alloys on the magnetic properties,temperature stability,mechanical properties,and corrosion resistance of hot pressed and hot deformed magnets were studied.The results show that the improvement of coercivity of hot-deformed magnet is dependent on the composition and amount of diffusion source and the deformation ratio.The coercivity of magnet increases with the increase of diffusion source content.Diffused by Pr₇₀Al₂₀Cu₁₀ alloy,the coercivity of the hot-pressed magnet increases from1380 k A/m to 2019 k A/m,and the coercivity of the hot-deformed magnet increases from 930k A/m to 1723 k A/m.It is also found,for the first time,that in the hot-deformed magnet prepared by amorphous magnetic powders added with 10 wt.%Pr₇₀Al₂₀Cu₁₀ alloy,the main phase grains are fine nanosized equiaxed grains with c-axis grain orientation.The evolution of this anisotropic equiaxed grain structure and its influence on the properties have been studied in detail.It is found that this unique microstructure cannot only enhance the coercivity and thermal stability,but also improve the corrosion resistance and mechanical properties of the magnets.In order to further simplify the process,the anisotropic magnets are prepared by one-step hot-deformation technology based on Nd_28.47Pr_0.56Fe_balB_0.97Co_3.63Al_0.13Nb_0.56Zr_0.44 powder.The effects of hot deformation temperature and deformation ratio on the microstructure,hard magnetic properties and corrosion resistance were studied.The optimum magnetic properties of J_r=1.2 T,H_cj=960 k A/m,and（BH）_max=262 k J/m³ can be obtained by the optimum process condition of 750 ^oC and deformation ratio of 80%.The magnetic properties,thermal stability,and corrosion resistance of the magnet first increase and then decrease with the increase of deformation temperature and deformation ratio.The microstructure of one-step deformed magnet is uniform and no coarse grain region is observed.The results show that one-step hot deformation can be used as an effective short process for hot working magnets.In addition,it is found that there is segregation of Co in the hot deformed magnet,and its segregation behavior varies with the deformation ratio,which has effects on the Curie temperature and thermal stability of the magnets.Based on above process optimization,the hot pressed and hot deformed magnets containing high abundance rare earth elements of La and Ce are prepared using commercial MQP-9-6.5HD powder with H_cj=520 k A/m,J_r=0.63 T,and（BH）_max=65 k J/m³.The preparation process,microstructure,magnetic properties and other properties were studied.The results show that the hot-pressing process does not change the microstructure and composition of the2:14:1 grain,but Ce Fe₂ phase will precipitate in the hot deformed magnet at increased temperature.One step hot-deformation process can inhibit the formation of Ce Fe₂ phase in the magnet.Compared with hot-deformed Nd Fe B,the hot deformed La and Ce-containing magnets show lower magnetic properties and inferior orientation.The magnetic properties of La and Ce-containing magnet prepared by ordinary hot pressing and hot deformation at 750 ^oC are H_cj=390k A/m,J_r=0.72 T,and（BH）_max=81 k J/m³,while the magnet prepared by one-step hot deformation are J_r=0.72 T,H_cj=430 k A/m,and（BH）_max=81 k J/m³.Finally,the anisotropic radial oriented Nd-Fe-B ring magnets are prepared by back extrusion process with self-designed extrusion die.It is found that the density,magnetic properties,and mechanical properties of the magnets are mainly affected by the wall thicknessof the raing,deformation temperature and deformation rate.The magnetic properties and orientation of the magnet increase first and then decrease from bottom to top of the ring.In the ring magnet with thick wall,the magnetic properties and orientation also increase first and then decrease from inner wall to outer wall.The best magnetic properties of the magnetic ring prepared by Nd_28.47Pr_0.56Fe_balB_0.97Co_3.63Al_0.13Nb_0.56Zr_0.44 amorphous magnetic powder are J_r=1.28 T,H_cj=930 k A/m,and（BH）_max=270 k J/m³.Overall,based on the studies of the hot working process and performance optimization of Nd-Fe-B magnets with different compositions,this work can provide both theoretical and practical basis for the further development of hot deformed RE-Fe-B magnets.