Magnetic Properties And Microstructure Regulation Of RE-Fe-B Permanent Magnet

Due to the excellent magnetic properties,Nd-Fe-B sintered magnet has become an indispensable permanent magnet material in industrial applications.Under the continuous development of science and technology,commercial demand for rare earth permanent magnet materials put forward higher requirements for performance.It is bound to rely excessively on rare earth elements such as Pr,Nd,Dy and Tb in order to develop high-performance rare earth permanent magnet materials.La and Ce account for a relatively high proportion in rare earth intergrowth ores.A large number of medium and heavy rare earths have led to a large backlog of high abundance rare earths La and Ce,which results in the extremely unbalanced utilization of rare earth resources.In recent years,rare earth metals（such as Pr,Nd,Pr-Nd alloy,etc.）have been rising sharply,which limits the application of traditional Nd-Fe-B magnets in price-sensitive areas.In order to reduce the cost of raw materials and improve the balanced utilization of rare earth resources,the research on high abundance rare earth has become a research hotspot in rare earth permanent magnets.With the increase of La and Ce content,magnetic properties of high-abundance magnets,especially coercivity,have obviously decreased,which can no longer meet the requirements of current commercial production.Therefore,it has become the key point of current research on how to make full use of high abundance rare earth elements while making efficient use of heavy rare earth elements.In this paper,one or more of high abundance rare earth elements La,Ce,Y,MM（Misch-Metal,MM）and Pr Nd alloy were used to prepare permanent magnet materials with high abundance rare earth by single alloy method.The magnetic properties,microstructure and phase composition of the prepared permanent magnet materials were characterized,and the effects of different high abundance rare earths on the properties and microstructure of the permanent magnet materials were analyzed.The main experimental results are as follows:The permanent magnet（MM magnet）with high abundance of rare earth MM was prepared by powder metallurgy process,in which the mass ratio of MM accounted for50%of the total rare earth.The optimum heat treatment process of MM magnet was found by choosing different sintering temperature:Sintering at 1050℃for 180 min,heat treatment at 855℃and 465℃for 180 min.The optimum magnetic properties of MM magnet were obtained as follows:remanence B_r=12.90 k Gs,intrinsic coercivity H_cj=9.25k Oe,maximum magnetic energy product（BH）_max=39.05 MGOe.Due to the low endogenous properties of high abundance rare earth,the magnetic dilution effect of MM on magnets was significant,and the magnetic properties,especially coercivity,of MM magnets were greatly reduced,which can no longer meet the needs of commercial production.As a result,the grain boundary diffusion technology was adopted,which used heavy rare earth element Tb as diffusion source and MM magnet as substrate.The high anisotropy of terbium,iron and boron was used to improve the anisotropic field in the edge layer of main phase grains,so as to improve the coercivity of MM magnet and meet the application requirements of high coercivity of MM magnet.It can make full use of high abundance rare earth elements while making efficient use of heavy rare earth elements.MM magnet with remanence B_r=12.90 k Gs,coercivity H_cj=8.29 k Oe and maximum magnetic energy product（BH）_max=38.12 MGOe was selected as substrate.After Tb diffusion treatment by magnetron sputtering,the weight gain ratio of the magnet reached 0.3%.The magnetic properties of MM magnet were remanence B_r=12.90 k Gs,coercivity H_cj=15.68 k Oe and maximum magnetic energy product（BH）_max=40.15 MGOe.Comparing the results of magnetic properties before and after Tb diffusion treatment,it can be found that the coercivity of the magnet after Tb diffusion increased obviously from 8.29 k Oe to 15.68 k Oe,and the coercivity increased by 89.1%,and the remanence did not deteriorate obviously,so the maximum magnetic energy product increased from38.12 MGOe to 40.15 MGOe.The maximum magnetic energy product increased by 5.3%.According to the current national standard of sintered Nd Fe B permanent magnet materials（GB/T 13560-2017）,the magnetic properties of diffused MM magnets reached the commercial N40M level,and the raw material cost was reduced by about 1.0%compared with Nd Fe B magnets of the same brand.The magnetic properties and microstructures of（Pr Nd,Ce）-Fe-B magnets and（Pr Nd,MM）-Fe-B magnets with approximately the same mass ratio of high abundance rare earths to rare earths were compared.After sintering at 990℃for 3h and annealing at 660℃for 2h,the remanence B_r=12.56 k Gs,coercivity H_cj=12.93 k Oe and maximum magnetic energy product（BH）_max=36.65 MGOe were obtained for（Pr Nd,Ce）-Fe-B magnets.Compared with the（Pr Nd,MM）-Fe-B magnet,the magnetic property B_r=12.90 k Gs,the coercivity H_cj=9.25 k Oe,the maximum magnetic energy product（BH）_max=39.05 MGOe,the coercivity H_cjof（Pr Nd,Ce）-Fe-B magnet was 40%higher.The remanence B_rand the maximum magnetic energy product（BH）_maxwere lower than those of（Pr Nd,MM）-Fe-B magnets,with a decrease of 2.6%and 6.1%,respectively.It provides a theoretical basis for the commercial feasibility study of mixed rare earths associated with Baiyun obo.Compared with（Pr Nd,Ce）-Fe-B magnet,the mixed rare earth magnet not only reduces the separation cost,but also avoids the industrial wastewater produced by separation and protects the environment.The influence of the substitution of high abundance rare earth element Y on the magnetic properties and microstructure of MM magnets was discussed.It was found that Y tended to enter the main phase.With the increase of Y content,Nd elements in the main phase migrated to the grain boundary,which affected the remanence of magnets.