The Study Of High Performance Die-upset Magnets Based On Misch Metal

Consumptions of medium and heavy rare earth（RE）elements such as Pr,Nd,Dy and Tb are increasing quickly with the development of permanent magnet（PM）industry,meanwhile large amounts of highly abundant and inexpensive light RE elements such as La and Ce have been left used.In order to balance the use of RE resources and reduce manufacturing cost of Nd-Fe-B PMs,which may contribute to improve market competitiveness and promote the sustainable development of RE PM industry,there has been an intense worldwide research effort to develop alternative,economically more attractive materials by substitute highly-abundant La/Ce for Nd in Nd-Fe-B based magnets.In this thesis,the magnetic properties,phase composition and microstructure of the hot-pressed/hot-deformed magnets prepared by replacing the Nd with misch metal（MM）on different substitution amounts were studied.In addition,the stability of the high abundance hot-deformed magnets was investigated,and the remanence（B_r）and maximum energy product(（BH）_max)of the magnets were improved by the low temperature heat treatment process,the coercivity(H_cj)of the magnets is greatly improved by the method of liquid phase addition process.High performance RE-Fe-B anisotropic nanocrystalline magnets based on La-Ce-Pr-Nd MM alloys were fabricated by hot pressing and hot-deforming process in this experiment,the detailed contents and results are shown as follows:MM-Fe-B melt spun powders with B_r⁶.85kGs,（BH）_max⁸.35MGOe were prepared by induction melting and melt spinning process.In this experiment,industrial MM alloys which contains La 23.17 wt.%,Ce 53.96 wt.%,Pr 5.16 wt.%,Nd 17.6 wt.%from Bayan Obo mine were chosen.These MM-Fe-B powders were mixed with commercial MQU-F powders at a mass fraction of x（x=0,0.2,0.3,0.4,0.5 and 1）.R_M magnets were fabricated by these as-mixed powders through hot-pressing and hot-deforming.The optimum deformation temperature rises with the MM content increased.An excellent comprehensive high-performance R_M30 hot-deformed magnet which with B_r=13.46 kGs,H_cj=10.69 kOe and（BH）_max=43.5 MGOe is obtained.The XRD peaks confirm that the tetragonal RE₂Fe₁₄B phase is the dominant phase in R_M magnets,and no RE-Fe binary phases such as CeFe2 phase or RE oxides are detected as MM mass fraction increases.The strong intensity（00l）characteristic peaks indicate that obvious c-axis crystallographic texture formed in all samples.The microstructures of R_M magnets are actually inhomogeneous,which contents Nd-rich main phase and MM-rich main phase.Microstructure analysis revealed that Ce element tended to diffuse into2:14:1 matrix phase uniformly,while the excessive La element tended to assemble in the RE-rich phase in coarse grains.The magnetostatic interactions and resultant exchange coupling across grains ensure the high performance of R_M hot-deformed magnets.Furthermore,the stability of the R_M hot-deformed magnets was investigated.The hot and humid corrosion resistance of the R_M magnets added with MM are comparable to those of the magnets without the addition of MM,while the MM-Fe-B magnets are less resistant to wet heat.The acid corrosion resistance of the R_M magnets added with MM is inferior the magnets without MM.Low temperature tempering can promote the flow of rare earth in the magnet and the diffusion of elements,thereby enhancing the magnet’s remanence and energy product.The annealed magnets show high energy product,when MM≤40 wt.%,B_r≥13.2 kGs,（BH）_max≥43.1 MGOe.With the addition of 2wt.%PrCu powders,the coercivity rises from 10.69 kOe to 14.1 kOe,which results from the modification of grain boundary and the optimization of microstructure.