Studying On The Effect Of Grain Boundary Diffusion On The Properties Of Sintered Nd-Fe-B Permanent Magnets

As its excellent magnetic properties and easy machining features,s-NdFeB permanent magnets are widely used in all aspects of our lives.s-NdFeB permanent magnets gradually applied in the field of new and high technology to promote the development of high technology,such as computer,hard disk,magnetic resonance imaging,electric vehicle,wind power,consumer electronics and magnetic levitation technology.However,with the development of science and technology especially the development of the hybrid electric vehicles and pure electric vehicles,sintered NdFeB permanent magnets are required a high thermal stability and coercivity properties in high temperature environment.But the Curie temperature of s-NdFeB permanent magnets is low,then it’s anisotropy field rapidly decreases with the increase of temperature.The coercivity of s-NdFeB permanent magnet quickly decreases with the increase of temperature,thus it is difficult to meet the requirements of high temperature.Therefore,in order to further improve the coercivity and the magnetic properties of s-NdFeB permanent magnets,two methods of grain boundary diffusion are accepted in this paper,namely,Multi-arc ion coating technology and Hot pressing sintering technology.The main results are as follows:1.Under the heat treatment process of first annealing at 750℃ for 3h,second annealing at 550℃ for 2h,the coercivity of the Nd-Fe-B magnets containing cerium finished Nd80Cu20 alloy grain boundary diffusion has been effectively improved,then the coercivity of magnets with 5.0wt%of Nd80Cu20 alloy gets the maximum one,1.24T.And its coericivity temperature dependence is-0.572%/℃.According to the microstructure analysis,the grain boundaries of Nd-Fe-B magnets containing cerium coated Nd80Cu20 alloy finished the grain boundary are increased and thickened,and the ferromagnetic coupling is weakened.Thus the coercivity of magnets effectively enhance.2.With hot pressing sintering experiment of Nd13.5Fe79.5B7 alloy doped with RE70Cu30 alloy,benchmark NdFeB alloy gets the maximum coercivity of 1.54T under the process of hot press sintering at 575℃;annealing at 625℃,and the coericivity temperature dependence coefficient is-0.571%/℃.After the diffusion of RE70Cu30 alloy,the coercivity increases significantly,and the coercivity temperature coefficient is improved.The sample diffused of Dy70Cu30 alloy not only achieves the highest coercivity of 3T,but also achieves the best temperature coefficient of coercivity is-0.410%/℃.3.According to the coercivity enhancement mechanism of Nd13.5Fe79.5B7 alloy doped with RE70Cu30 alloy by hot pressing sintering experiment,the contribution to the enhancement of coercivity caused by ferromagnetic decoupling can be roughly estimated by La70Cu30 diffusion process;The contribution to coercivity enhancement due to the remedied grain surface not only ferromagnetic coupling by Nd70Cu30 diffusion process;Compared with the diffusion process of Nd70Cu30 alloy,the diffusion process of Dy70Cu30 is forming magnetic hardened Dy-rich shell on the grain surface.