Study On Sintered NdFeB Magnets With Dy Grain Boundary Diffusion

At present,the main way to prepare NdFeB permanent magnets with high coercivity is to add the heavy rare earth elements(Tb and Dy).However,heavy rare earth resources are important non-renewable resources.It is scarce and expensive.In order to promote the efficient use of rare earth resources and meet the requirement of high performance rare-earth permanent magnet material on the new energy vehicles,wind power generators and other emerging fields,it is an important goal in the research field of sintered Nd-FeB magnets to reduce the demand on heavy rare earth elements for rare earth permanent magnetic materials with high coercive force.In this paper,the influence on the properties of sintered NdFeB magnets by Dy vapor heat treatment was studied,the mechanism of coercivity enhancement of the grain boundary diffusion(GBD)processed magnet was elucidated;Dy vapor heat treatment technology suitable for bulk magnets was developed and optimized,the relationship between process parameters and magnet performance was clear;the key technology problems in the industrialization of the GBD processed sintered NdFeB magnets were investigated through rolling contact heat treatment process.The main results and achievement are summarized as follows:1.The influence of the key process parameters(magnet size,heat treatment temperature,heat treatment time,etc.)on the properties and structure of NdFeB magnets by Dy vapor heat treatment was obtained.Small size,high temperature and long time are favorable for the diffusion of Dy elements and the enhancement of coercivity.The coercivity of magnets prepared under optimized heat treatment conditions is increased by 38%.The mechanism of coercivity enhancement of the GBD processed magnet was illustrated by microstructure analysis: it’s the primary cause for the increase of coercivity that shell structure with high magnetocrystalline anisotropy field on the main phase grain surface was formed.At the same time,the continuous and uniform distribution of rich Nd grain boundary has reduced the exchange couple interaction between grains,which makes further enhance for coercivity of mangets.2.The process of Dy diffusion was combined with the sintering process of magnet.The GBD processed magnet was prepared by vapor heat treatment under the pressed compact,which the effective penetration depth of Dy in the magnet exceeded 15 mm.The ΔHcj/ΔBr is up to 23.08 for GBD processed magnet,which is much more than the magnets by nanoparticles doping.The research showed that the internal properties of the magnet is obviously inhomogeneous after diffusion.The coercivity of the external surface of magnet is significantly higher than the interior,and the coercivity of magnet at the corner is the highest.(ΔHcj = 6.05 kOe)3.The GBD processed magnet was preparated by rolling contact heat treatment process,which has the characteristics of diversification of process system,stabilization of product performance,universalization of application field and maximization of industrial benefit.That established foundation for the quantitative production of NdFeB by GBD technology.