Microstructure And Performance Optimization Of 2:17 Type SmCo Permanent Magnet Based On Commercial Magnet

Because of the high Curie temperature,superior magnetic properties and good environmental stability,Sm（CoFeCuZr）_z permanent magnet materials have been extensively used in high technology fields,such as electric vehicles,high speed rail,etc.In this paper,based on the study of Sm（CoFeCuZr）_z magnets（32H and 32L brands）,clarified the causes for differences in magnetic properties of commercial magnets.The secondary aging treated the commercial magnet with low coercivity（32L brand）,and the influence of the second aging process on the magnetic properties of the magnet was analyzed.The sintered Sm（CoFeCuZr）_z magnets with different Fe content are prepared by the traditional powder metallurgy through the composition control and the optimization of heat treatment process.The effects between the microstructure,element and magnetic properties of the Fe-rich magnets are studied,discussed the magnetic hardening mechanism of the magnets.The researchs of the microstructure and magnetic properties of 32H and 32L magnets show that the difference of the orientation of the magnets affects the remanence.Electron probe micro-analysis results show that the phenomenon of“Cu-lean grain boundary”is obvious in 32H magnet with high coercivity,which leads to the“step”of demagnetization curve.The first derivative of initial magnetization curve shows the pinning field of 32H magnet with 30.5 k Oe is much larger than that of 32L magnet with 7.7 k Oe,which explains the difference of coercivity.Furthermore,the difference gradient of Cu content between the cell boundary phase and the cell phase of 32H magnet is higher than that of 32L magnet,and the continuity of cell boundary phases are good,which is beneficial to increase the pinning effect of cell boundary phases to domain wall.In addition,the effect of secondary aging heat treatment on 32L magnet is studied.During the secondary aging,the coercivity of the magnet increased from 11.2k Oe to 30.6 k Oe.After the secondary aging,the Cu element in the original magnet redistributes and the phenomenon of“Cu-lean grain boundary”appears.It is worth to do that the gradient of Cu element between the cell boundary phase and the cell phase increases significantly,and uneven distribution of Cu element in the cell boundary phase.These reasons increase the difference of domain wall energy at the cellular interface and improve the pinning force.The continuity of cell boundary phases is also improved and the cellular structure becomes much uniform,which is conducive to the improvement of coercivity.After the secondary aging treatment,the uneven distribution of domain and the phenomenon of wide domain appear in the magnet,which results in the squareness of demagnetization curve being destroyed.Based on the results of commercial magnets,the ingot with composition of Sm(Co_0.69Fe_0.22Cu_0.07Zr_0.02)_7.6 was adjusted and designed by simulating the composition of 32H magnet.Through the optimization of heat treatment process,the magnet with the best magnetic properties of B_r=11.0 k G,H_cj=26.7 k Oe,（BH）_max=28.2MGOe,S_r=92.8%was obtained by holding for 5 h at solution temperature of 1175℃.However,there is an obvious“Kink”phenomenon in the magnet,so in order to prepare a Sm（CoFeCuZr）_z magnet with higher performance.It is worth to continue to optimize the process,especially the solid solution process.Continued to add Fe element on the basis of optimizing heat treatment process,Sm(Co_0.91-xFe_xCu_0.07Zr_0.02)_7.6（x=0.22,0.30,0.35）sintered magnet was designed and prepared.The relationship between the composition,microstructure and magnetic properties of Fe-rich magnet is studied.In Sm(Co_0.56Fe_0.35Cu_0.07Zr_0.02)_7.6 magnet,due to the excessive addition of Fe element and the elements of Sm and Cu remain unchanged.The solubility of Fe in the main phase reaches the limit,which results in the segregation of elements and induces the appearance of precipitates with Sm-rich and Cu-rich.It consumes a large amount of Cu element,and results in the incomplete growth of cell boundary phase in the aging process.The incomplete growth of cell boundary phase will reduce the coercivity of the magnet.It is worth noting that the superposition state diffraction pattern of Th₂Zn₁₇ structure of 2:17R phase and Th₂Ni₁₇structure of 2:17H phase is found in the selected area electron diffraction results of Sm(Co_0.56Fe_0.35Cu_0.07Zr_0.02)_7.6 solid solution stage parallel to the orientation direction,which is not conducive to the amplitude modulation decomposition in the subsequent aging process.