Effects Of Grain Boundary Diffusion Process Using Various Alloys On The Microstructure And Properties Of Sintered NdFeB Magnets

NdFeB permanent magnets have been widely employed in various fields.With the rapid development of 5G communication technology,new energy power vehicles,wind power generators and so on,the requirements for NdFeB magnets are getting higher and higher.Improving the magnetic properties and at the same time reducing the cost has been the main target for the NdFeB industry.The grain boundary diffusion process(GBDP)is a novel technology for improving the coercivity of the NdFeB magnets by adding the rare earth(RE)elements or RE-containing compounds and eutectic alloys into the grain boundary(GB)of the magnets,which can form the hard magnetic shell of RE2Fe14 B phases and improve the GB phase distribution.By this approach,the total RE content,especially the heavy RE content,in the magnets can also be reduced.Currently most of the diffusion agents used in GBDP are the compounds containing the heavy RE,such as Dy and Tb.In this thesis,aiming at developing the GBDP with low cost and preparing NdFeB magnets with high magnetic properties,different alloys are employed as the diffusion agents.The coercivity has been enhanced based on optimized GBDP.The underlying physical mechanism of coercivity enhancement is clarified based on the investigations of the microstructure improvement after diffusion and diffusion kinetics for the diffusion elements.In addition,the effects of GBDP on the corrosion resistance and mechanical properties of the magnets have been also investigated.The main research contents and results are as follows:Firstly,the low melting point Pr-Al-Cu alloys were used for GBDP.After diffusion heat treatment of 800??2 h-500??3 h,the coercivity was increased from 1000 kA/m to 1714 kA/m.The microstructure of the Pr-Al-Cu alloy diffused magnet showed the formation of(Nd,Pr)-FeB phase around the main phase,and the improved grain boundary phase distribution,both of them contribute to the coercivity enhancement.Based on the diffusion coefficient analysis,Pr element diffused the fastest in the grain boundary among Pr,Al and Cu elements.Compared to the original magnet,the corrosion resistance of the Pr-Al-Cu diffused magnets is a little lower,but the micro hardness and compressive strength are both improved.Secondly,the low cost high abundance RE elements of La and Ce containing alloys(LaxCe1-x)-Al-Cu were used as the diffusion agents.The coercivity was enhanced from 990 kA/m to 1030 kA/m by the heat treatment of 600??1 h-500??1 h.The microstructure and phase investigation showed the different behavior of La and Ce in GB.The wettability of GB was improved when Ce was contained in the diffusion alloys.However,the REFe2 phase was formed in the diffused magnet,which could deteriorate the magnetic properties of magnets.When La was containing in the diffusion alloys,the formation of REFe2 phases can be reduced.However,it can easily get oxidized and the ?-Fe phases were formed.The coercivity of the(LaxCe1-x)-Al-Cu alloys diffused magnets are affected by the combining factors of both wettability of GB and the deteriorated REFe2 phase.The diffusion kinetics analysis showed that Ce diffused faster than La in the GB.After GBDP by(LaxCe1-x)-Al-Cu alloys,the corrosion resistance of the magnets was reduced,but the mechanical properties of magnets were enhanced.Lastly,the non-RE Al-Cu alloys were used as the diffusion agents.After Al-Cu alloy diffusion at 800??2 h-500??3 h,the coercivity was enhanced from 1000 kA/m to 1125 kA/m.The investigations on the microstructure and phase structure of the Al-Cu diffused magnets showed that,Al and Cu elements in the grain boundary can react with GB phase,leading to the formation of fcc-Nd2O3 phases,which can reduce the nucleation of reversal domain,and improve the grain boundary phase distribution.Both are contributed to the coercivity enhancement.It is interesting the corrosion resistance was improved after Al-Cu diffusion,and at the same time,the micro hardness and compressive strength were improved.Based on the above results,the coercivity of NdFeB has been successfully enhanced by different diffusion alloys GBDP.The effect of the diffusion process on coercivity has also been analyzed by experiments and theories.This work can provide good guidance for preparing the NdFeB magnets with low cost and high performance.