Study On The Coercivity Mechanism And Performance Of Nd-Fe-B Magnetic Powder Prepared By D-HDDR Process

Nowadays,Nd-Fe-B magnetic materials are widely used in computer,communication,aerospace,equipment manufacturing and other industrial fields,has already become one of the indispensable materials in the manufacturing industry,the world has invested a lot of resources for the research and development of magnetic materials,especially Nd-Fe-B magnetic materials.At present,the focus of global research has shifted from simply improving the magnetic properties of magnets to how to improve the magnetic properties of magnets without adding Dy,Tb and other heavy rare earth elements.After many relevant professional research,the most direct and effective method is grain refinement,which can ensure that Nd-Fe-B magnetic powder is not oxidized to reduce the grain size,One of the main method is to composite hydrogenated-disproportionation-dehydrogenation-recombination（hereinafter referred to as HDDR）process.However,in order to obtain high performance Nd-Fe-B magnetic particles,it is very important to improve the HDDR technology,the mechanism of magnetic particles and the magnetic properties.This paper uses independently designed HDDR magnetic powder special equipment-rotary hydrogen crushing furnace,combined with the advantages of the standard d-HDDR（dynamic-HDDR）process for hydrogen storage at low temperature and d-HDDR process for T_DR>T_HD,the improved d-HDDR process adopted in this experiment was designed,that is,under the premise hydrogen storage at 150℃before HD treatment and low hydrogen pressure dehydrogenation and recombination,the DR temperature is slightly higher than the HD temperature.The raw material is an industrial ingot.The coercivity mechanism of Nd-Fe-B magnetic powders prepared by the improved d-HDDR process and the improvement of magnetic properties by tempering heat treatment and ball milling process were investigated.The specific results of the study are as follows:（1）The coercivity mechanism of anisotropic Nd₂Fe₁₄B magnetic powder prepared by the improved d-HDDR process was studied by studying the magnetization characteristics,observing and analyzing the microstructure and theoretical calculation of coercivity.The results show that the coercive force comes from the interstice of Nd₂Fe₁₄B grains or grain clusters and the pinning of neodymium-rich relative magnetic domains at the grain boundaries,which is the decisive factor of coercive force.In addition,confirmatory experiments have been carried out and it is concluded that the magnetic properties of magnetic particles are better when DR temperature is higher than 40℃for HD temperature.The magnetic energy DOA=0.69,B_r=6.13 k Gs,H_cj=13.90 k Oe,（BH）_max=6.60 MGOe;the improved d-HDDR process has higher anisotropy orientation than other HDDR processes.（2）By studying the magnetic properties and structure of the magnetic powder prepared by tempering heat treatment,it is found that tempering heat treatment can promote the diffusion of Nd-rich phase,increase the content of Nd-rich phase at the grain boundaries,strengthen the Nd-rich phase of the magnetic domain movement,and also facilitate the rotation of the grain orientation,so that the degree of anisotropic orientation becomes larger.The coercivity,remanence and maximum magnetic energy product of magnetic particles are improved.When the tempering process conditions are as follows:the tempering temperature is 650℃,the tempering time is 20 min,the ideal magnetic properties of magnetic powder are obtained,the magnetic energy is DOA=0.73,B_r=6.55 k Gs,H_cj=12.26 k Oe,（BH）_max=7.12 MGOe.When DR temperature is 860℃,tempering temperature is 650℃,tempering time is 20 min,the magnetic energy of the prepared magnetic powder is DOA=0.69,B_r=6.24k Gs,H_cj=14.67 k Oe,（BH）_max=7.06 MGOe.（3）By studying the magnetic properties,microstructure and grain size of the magnetic powders prepared by ball milling,it is found that the grains can be refined by ball milling,and there are no obvious large Nd-rich particles,so that they can be evenly dispersed among the main grains of Nd₂Fe₁₄B,and the anisotropic orientation of the magnetic powders can be increased to form an ideal non-magnetic grain boundary phase,and the magnetic domain movement can be strengthened.Thus,the coercive force of magnetic powder is increased.Doped Cu has the effect of repairing grain boundary,achieving demagnetization coupling,and thus improving the coercivity of magnetic powder.When the milling process conditions are as follows:the grinding speed is 250 rpm and the grinding time is 2 h,the magnetic properties of magnetic powder obtained by ball milling are the best,and the magnetic energy is DOA=0.55,B_r=4.93 k Gs,H_cj=7.23 k Oe,（BH）_max=3.23 MGOe.On the basis of these conditions,the magnetic properties of magnetic powder obtained by adding 10wt.%Cu powder are the best,and the magnetic properties are DOA=0.62,B_r=4.96 k Gs,H_cj=9.86k Oe,（BH）_max=4.39 MGOe.