Study On The Grain Boundary Diffusion Process For Hot-deformed And Sintered NdFeB Magnets

With the continuous expansion of application fields,the higher magnetic properties of the NdFeB magnets are required,especially the improved coercivity and stability at high temperatures.In addition,a large amount of rare earths are used in the NdFeB magnets,which makes the cost of the magnets a problem that must be considered.Grain boundary diffusion process is proposed under these circumstances,which can effectively improve the coercivity while relatively saving cost.In recent years,the research on grain boundary diffusion process has atratcted great attention.In this work,hot-deformation magnets,sintered magnet wastes,and commercial sintered magnets were used as diffusion substrates.The different diffusion sources and diffusion processes were employed to improve the coercivity.Based on the characterizations,the relationship between the performance and the microstructure was discussed.The main research contents and conclusions are as follows:Firstly,the fabrication process and grain boundary diffusion process of the hot-deformed NdFeB magnets were studied.It was found that the hot-deformed NdFeB magnets prepared by Pr-containing and Zr,Nb-free powders?MQP-02?have higher coercivity than the magnets prepared by Pr-free and Zr,Nb-containing powders?MQP-01?.The results suggest that the RE-rich phase in MQP-01 powders has better mobility,which contributes to the formation of c-axis orientation,but also causes the aggregation of the RE-rich phase and thickness reduction of the grain boundary phase.As a result,the magnets prepared from MQP-01powder have large local demagnetizing fields and a weak pinning field,leading to relatively low coercivity.The effects of in-situ diffusion of Tb₇₀Cu₃₀ alloy and diffusion of Pr₄₀Tb₃₀Cu₃₀alloy by different methods on the properties and the microstructures of hot-deformed NdFeB magnets were studied.It was found that the heavy rare earth Tb can effectively improve the coercivity of the magnets,mainly because the Tb element enters into the main phase and forms the?Tb,Nd?₂Fe₁₄B phase with a strong anisotropic field.In addition,the orientation degree,microstructure,and element distribution of the magnets after diffusion are also analyzed in detail,and the relationship among them and underlying mechanism are discussed.Secondly,the magnetic properties of sintered NdFeB wastes were improved by grain boundary diffusion process.After the diffusion of Pr₄₀Tb₃₀Cu₃₀ alloy,the coercivity of the magnet was increased from 763 k A/m to 1287 k A/m without a significant reduction in remanence.It was found that during the diffusion process,the diffusion depth of Pr element is much larger than that of Tb element,but Tb tends to enter into the main phase to form?Tb,Nd?₂Fe₁₄B phase.At the same time,the Nd,Ce,and Gd elements which are contained in the original magnet diffused to the surface of the magnets,and showed a different tendency to be replaced by Tb elements.In addition,the corrosion resistance of the magnets after grain boundary diffusion decreased.The reasin is attributed to the introduction of new elements in the diffusion process,which might increase the electrochemical activity of the magnet surface.Finally,the effects of grain boundary diffusion of non-rare earth metals and alloys on the structure and properties of NdFeB were studied.N50 commercial sintered magnets were used as the substrates.Zn,Al,Cu and their alloys were employed as diffusion sources.It is found that the diffusion of Al can increase the coercivity of the magnets to some extent,which may be related to the formation of the Nd₂?Fe,Al?₁₄B phase.When Zn is diffused at high temperatures?above 800 ^oC?,it will enter into the main phase and seriously damage the magnetic properties.At lower the diffusion temperature,the Zn element can achieve diffusion along the grain boundaries,but the heat treatment process at low temperature has a significantly negative effect on the coercivity.The diffusion of Cu element in this case was not significant,and no positive effect on the coercivity was observed.The present research in this thesis involves different NdFeB substrates and different diffusion sources.The behavior of diffusion sources and the relationship between microstructure and magnetic properties were understood.This work provides a valuable reference for enriching the choice of diffusion source and further developing new grain boundary diffusion process.