Microstructure And Magnetic Properties Of SmCo₅ Spark Plasma Sintered Magnets

SmCo₅ permanent magnetic alloy has huge magnetocrystal anisotropy field,high Curie temperature and excellent comprehensive magnetic properties,which are widely used in national defense,aerospace and other fields of science and technology.In this paper,SmCo₅ hot-pressed magnets（SPSed magnets）and hot-deformed magnets were prepared by smelting,rapid quenching and spark plasma sintering technologies（SPS）.The effects and evolution of sintering,hot deformation,heat treatment processes and magnetic powder state on the phase composition,microstructure,magnetic properties and high temperature stability of SmCo₅-based permanent magnet alloys were studied.Firstly,the magnetic properties and microstructure of SmCo₅ commercial quick setting tablets were tested and characterized.SmCo₅ and Sm2Co7 phases were mainly present in the quick setting tablets.The magnetic properties of SmCo₅ and Sm2Co7 phases were Jr = 0.32 T,Hcj = 116 k A/m,（BH）max = 8 k J/m3.The full compact SmCo₅-based SPSed was prepared by spark plasma sintering.The effects of sintering process on magnetic properties,phase transformation and microstructure of magnets were mainly explored.The optimum hot-pressing sintering process is 1000 °C/50 MPa/5 min by control variable method.When the sintering temperature is between 700 °C and 900 °C,the density of the magnets are much lower than the theoretical density,which is also the main reason for the low magnetic properties of the magnet.When the sintering temperature is 1000 °C,the density of the magnet is 8.40 g/cm3,close to the theoretical density.The sintering temperature increases gradually from 700 °C to 1000 °C,the remanence increases from 0.24 T to 0.45 T,the coercivity increases from 106 k A/m to 985 k A/m,and the maximum magnetic energy product increases from 5 k J/m3 to 34 k J/m3.When the sintering temperature reaches 1050 °C,there are not only SmCo₅ main phase and Sm2Co7 second phase,but also Sm2Co17 phase in the magnet,which is an important reason for the deterioration of magnetic properties of the magnet.The SmCo₅ SPSed magnet prepared at 1000 °C has good stability at high temperature,with coercivity of 335 k A/m and remanence of 0.39 T at 200 °C.The coercivity temperature coefficients β and remanence temperature coefficients α are-0.25%/ °C and-0.15%/ °C respectively over the temperature range of 27-400 °C.By optimizing the heat treatment process,the microstructure distribution of SmCo₅ SPSed magnet is more uniform,and the coercivity is increased to 1073 k A/m.Then the SmCo₅ SPSed magnets prepared by the optimal hot-pressing process was used as the precursor of hot-deformed magnets,and the hot deformation was carried out by spark plasma sintering.The optimal hot deformation process is 900 °C/10 k N/3 min,and the deformation is close to 70%.The ratio of characteristic peak I（002）/I（111）is 1.180,the formation of c-axis texture is the best,and the anisotropy is strong.BSE and TEM show that the deformed magnets exhibited excellent c-axis texture.Magnetic properties: Jr = 0.63 T,Hcj = 2088 k A/m,（BH）max = 68 k J/m3.By testing the demagnetization curves of SmCo₅ hot-deformed magnets at high temperatures,the coercivity temperature coefficient and remanence temperature coefficient α and β are-0.25 %/ °C and-0.11 %/ °C,respectively,in the range of 27-400 °C.At 200 °C,the remanence of the deformed magnet still has 0.58 T and the coercivity is 793 k A/m,showing a certain magnetic stability at high temperature.The hot-deformed magnets show good anisotropy and the coercivity mechanism is nucleation.At the same time,by testing the mini loops and recoil loops,it is concluded that the pinning effect also exists in the coarse crystal magnet,but the pinning effect is weak because of the large grain size and the fewer grain boundary.Finally,the SmCo₅ SPSed magnets and hot-deformed magnets were prepared by SPS using magnetic particles in different states as raw materials by using the optimized hot-pressing process and hot deformation process.The volume fraction of Sm2Co7 in the second phase of the SPSed magnets and the hot-deformed magnets obtained from strip and ingot strip powders decrease inordinately by comparing the SPSed magnets and the hot-deformed magnets obtained from strip and ingot strip powders.The results show that the volume fraction of the main phase increases,the remanence and the maximum magnetic energy product decrease slightly,the coercivity increases by 388 k A/m,and the square degrees is improved.However,due to the decrease of the volume fraction of the secondary phase of Sm2Co7,the deformation is not enough,and the deformed magnet shows weak anisotropy.In conclusion,this paper systematically explored the influence of hot-pressing process,heat treatment,hot deformation process and the magnetic powder state on the magnetic properties,phase composition and microstructure of SmCo₅ SPSed magnets and hot-deformed magnets,and also explored the high-temperature stability and coercivity mechanism of hotdeformed magnet,providing technological exploration and theoretical guidance for the preparation of new anisotropic magnets.