Study On Electrodeposition Process Of High Performance Sintered Nd-Fe-B

So far,sintered Nd-Fe-B magnets with the best magnetic properties among rare earth permanent magnet materials are widely used in wind power generation,hybrid vehicles,information technology and so on.However,the poor temperature stability limits its application in high temperature.The traditional method is to add Dy or Tb into the alloy to improve coercivity,but it leads to a sharp decrease in remanence and increase in cost.The grain boundary diffusion technology can effectively enhance coercivity,and remanence is hardly reduced.Electrodeposition is the most promising heavy rare earth diffusion process to realize industrial production.Therefore,we studied the high-performance sintered Nd-Fe-B magnets by electrodeposition grain boundary diffusion,which can provide basic data for industrial production.Firstly,Dy H₃ nanoparticles were used as diffusion sources to study the diffusion process.Five binders were selected to improve the coating adhesion.It was found that polyvinyl butyral(PVB)enhanced the adhesion without damaging magnetic properties.The effects of deposition time,voltage and anode plate spacing on the coating quality were studied.It shows that the weight gain ratio increases with the increasing of time and voltage,and decreases with the increasing of anode plate spacing.The optimum deposition voltage and anode plate spacing are 150 V and 4 cm,respectively.By adjusting the weight increase ratio and heat treatment process,the optimum weight increase ratio and time are 2.0 wt.%and 40 s,respectively;and the optimum heat treatment process is 925?-8 h-500?-3 h.The coercivity can reach21.16 k Oe,and the homogeneity of magnetic properties is good.Results show that Dy diffuses into the magnet along grain boundaries,partially replaces Nd in Nd₂Fe₁₄B grains,preferentially distributes in the epitaxial layer of main phase grains,forms(Nd,Dy)₂Fe₁₄B phases with a higher magnetic anisotropy field,inhibits the nucleation of reverse domains,and greatly improves coercivity.The coercivity increment decreases with the increasing of magnet thickness.EPMA shows that the diffusion depth of Dy increases with the increasing of magnet thickness,and Dy content decreases from the surface to inside,while Nd content changes conversely.The maximum coercivity of 1mm,2 mm,4 mm and 6 mm magnets was obtained at 925?,which were increased by 7.74,7.52,7.38 and 6.91 k Oe compared with that of original magnets,respectively.The coercivity of 14 mm magnets is still increased by 4.50 k Oe,indicating that the electrodeposition process combining with Dy H₃ nanoparticles can be used for thicker magnets.For 40UH,45SH,48H and 50M magnets,the coercivity was increased by4.93,5.41,6.79 and 8.16 k Oe,respectively.It shows that the larger the original coercivity is,the smaller the coercivity increment is.Tb H₃ nanoparticles were used as diffusion sources to further improve the coercivity and verify above conclusions.By adjusting the weight increase ratio and heat treatment process,the optimum weight increase ratio and time are 1.0 wt.%and 20 s,respectively.The optimum heat treatment process is 940?-8 h-500?-3 h,and the coercivity can reach 23.70 k Oe.EPMA shows that Tb diffuses into the magnet along grain boundaries and partially replaces Nd in Nd₂Fe₁₄B grains,enriches in the epitaxial region of main phase grains and forms(Nd,Tb)₂Fe₁₄B phases,thus enhancing the coercivity.Results show that the coercivity increment decreases with the increasing of magnet thickness.The diffusion depth of Tb increases with the increasing of magnet thickness,and Tb content decreases from the surface to inside,while Nd content changes conversely.The maximum coercivity of 1 mm,2 mm,4mm and 6 mm magnets was obtained at 940?,which were increased by 9.97,9.96,9.92 and 9.42 k Oe compared with that of original magnets,respectively.The coercivity of 14 mm magnets is still increased by 5.55 k Oe.The coercivity of 40UH,45SH,48H and 50M magnets were increased by 8.04,8.40,9.52 and 10.97 k Oe,respectively.It shows that the larger the original coercivity is,the smaller the coercivity increment is.Above conclusions are consistent with those of diffusing Dy H₃.Besides,the influence of Ga and Tb H₃ diffusion on sintered Nd-Fe-B magnets was studied.It is found that the corrosion resistance and coercivity of magnets can be improved by Ga diffusion.When Ga and Tb H₃ are combined,the influence on coercivity is different whether they are in different order.However,the magnetic properties are not as good as that of diffusing Tb H₃.That is to say,when they act alone,it can improve magnetic properties,but the combination of the two is not conducive to magnetic properties of magnets.