Modification Of Sintered Nd-Fe-B Magnets By Grain Boundary Diffusing Low Melting Point Alloys

Sintered Nd-Fe-B magnets are widely used in wind power generation,aerospace,medical instrument,energy saving appliances and many other fields due to their excellent magnetic properties.However its poor temperature stability and corrosion resistance limit its application in a specific environment.Therefore,people usually improve coercivity,thermal stability and corrosion resistance by adding heavy rare earth elements.The addition of heavy rare earth elements by traditional single alloy method can improve the coercivity and thermal stability,which consumes more heavy rare earth elements.For this reason,two kinds of low-melting-point alloys,Dy₇₀Al₁₀Ga₂₀ and(Pr₇₅Dy₂₅)₇₀Al₁₀Ga₂₀?at.%?,were designed and used as diffusion sources.Grain boundary diffusion technology,consumed less heavy rare earth elements,were applied in the sintered Nd-Fe-B magnets.The reasons for improving magnetic properties,temperature stability and corrosion resistance of diffused magnets were studied by exploring the best heat treatment process.Firstly,the grain boundary diffusion process was optimized by diffusing Dy₇₀Al₁₀Ga₂₀low melting point alloys on the surface of sintered Nd-Fe-B magnets in the thesis.The optimum diffusion process is 850°C/6 h+490°C/3 h,under which H_cj=1604.74 kA/m,J_r=1.44 T,?BH?_max=400.95 kJ/m³.The coercivity increased from 1014.10 kA/m to 1604.74kA/m,an increase of 58.2%with no reduction in remanence.The recoil loops of the magnet changed from the open state to the closed state,which is contributed by the improvement of grain boundaries.The absolute value of the remanence temperature coefficient increased from 0.1081%/K to 0.1088%/K,the absolute value of the coercivity temperature coefficient decreased from 0.6758%/K to 0.4613%/K,irreversible flux loss rate decreased from 75.5%to 48.6%.Electrochemical experiments results show that the corrosion resistance of Dy₇₀Al₁₀Ga₂₀ alloy diffused magnets were improved compared with original magnet.The XRD shows that the peak of the main phase shifts to a large angle after the diffusion,which originates from the Nd partly replace by Dy.Next,based on the diffusion of Dy₇₀Al₁₀Ga₂₀ low melting point alloys,the(Pr₇₅Dy₂₅)₇₀Al₁₀Ga₂₀ alloy diffusion source was prepared by using partial light rare earth element Pr to replace heavy rare earth element Dy.It shows that the comprehensive magnetic properties of the diffused magnets are optimal under 850°C/6 h+490°C/3 h.The specific magnetic properties are H_cj=1441.56 kA/m,J_r=1.41 T,?BH?_max=382.56 kJ/m³.Compared with the original magnet,the coercivity increased from 1014.10 kA/m to 1441.56kA/m,an increase of 42.2%with only a slight reduction in remanence.The temperature coefficient of remanence increased slightly,the coercivity temperature coefficient and the irreversible flux loss rate decreased,which indicate the temperature stability were improved.Electrochemical experiments results show that the diffused magnets have slower corrosion rate,which improved corrosion resistant.XRD shows that the main phase peak moves to a small angle,which is caused by the combined substitution effect of Pr and Dy,and the amount of Pr substitution Nd is more than that of Dy.