| NdFeB-type permanent materials is a kind of practical permanent materials with the best magnetic properties up to now. It is named "the king of permanent magnet" and paid wildly attention for many years. Many of research works focusing on composition designing, elements addition, microstructure improvement and manufacture crafts are carrying out. In this experiment, sintered Nd15Fe78-xNbxB7 (x is the atom contents of Nb) permanent magnets are prepared by conventional powder metallurgical process. Nb addition and manufacture crafts have been carefully researched.Systematic research about technology parameter's influences on the magnetic properties and microstructure of theses magnets have been done. The better parameters of production have been achieved. They are showed as follows: as-cast alloys with the nominal composition Nd15Fe78-xNbxB7 are prepared by induction-melting under argon atmosphere. As-cast alloys are broken under argon atmosphere and filtered through 40# screen. The coarse grain is ball-milled under 120# avgas for 80 min. The ratio of milling ball and grain is 15:1. Then fine powder is aligned and pressed in a magnet field of 1.5T. The pressure is 80MPa. The green compacts then are shaped by cold isostatic pressing under 280MPa pressure. The last compacts are sintered in vacuum at 1110℃ for 1h and annealed at 900℃ for 1h, then at 600℃ for 2h.Adding Nb to ternary Nd-Fe-B magnet can enhance the coercivity of magnet without decreasing remanence and maximum energy product obviously. At the same time, Nb addition can improve the stability of temperature. In this experiment, the roles of Nb element in sintered Nd-Fe-B magnets have been studied. The results show that: the magnetic properties at x=0.75 are better. The improvement of the magnetic properties of sintered Nd-Fe-B magnets with Nb addition is due to the improvement of the microstructure of magnets and as-cast alloys. In the magnets with Nb addition the size of Nd2Fe14B phase is homologous and the figure of Nd2Fe14B phase is regularer. The Nd-rich phase is distributed dispersively. Nb addition make the microstructure of as-cast alloys tochange. The figure of Nd2Fe14B phase change from coarse patch to fine particle. Nb addition also can partly prevent the appearance of a -Fe in as-cast alloys. By comparing the magnetic properties in room and high temperature, we find Nb addition can improve the stability of temperature of sintered Nd-Fe-B magnets.The magnetic properties of sintered Nd-Fe-B magnet strongly depend on its microstructure. Understanding the quantitative and qualitative relationships between the magnetic properties and the microstructure of magnets is useful to grasping the coercivity mechanism and all kinds of magnetic theories. In the paper, we discuss a formula derived by Chen Wenhao and test the reasonability of this formula using a practical magnet. |
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