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The principle of the manufacturing process of sintered NdFeB permanent magnet materials
Raw material preparation → smelting → ingot casting → crushing and milling → magnetic field orientation and pressing → sintering → tempering → machining and surface treatment → inspection. The following describes the process principle according to the sequence of the process flow.
The magnetic properties of sintered NdFeB magnets are mainly determined by the Nd2Fe14B matrix phase. Because of its magnetic polarization Js (Js = μ0Ms, Ms is the saturation magnetization) and the anisotropy field HA depends mainly on the chemical composition of the Nd2Fe14B phase. Although the remanence Br, the coercive force Hci, and the energy product (BH)max are tissue-sensitive quantities, the limit of Br is Js, the limit of Hci is HA, and the limit of (BH)max is (Js2)/4μ0. Therefore, alloy composition design and raw material selection are crucial.
The purpose of the smelting is to melt the pure metal material (Fe, Nd, B-Fe, Dy, A1, Nb, Co, Cu, etc.) and ensure that (1) all metal materials are melted. The melting points of pure Fe and metallic Nd, etc. are higher and they should be managed to completely melt; (2) The design composition of the alloy is accurate. The reason for the inaccurate composition is the loss of volatility and oxidation of the metal (collectively referred to as burn-in). For this purpose, the vacuum induction furnace is generally used for smelting, and the vacuum degree should reach 10-2~10-3Pa or more; (3) ensure that the alloy composition is uniform; (4) ensure that the alloy is clean and prevent the inclusions and gas pollution.
The ingot structure has an important influence not only on the milling, orientation, and sintering processes but also on the powder properties and the final sintered magnetic properties. Without excellent ingot structure, it is impossible to produce a high-performance sintered permanent magnet. Ingot casting is one of the key technologies that restrict the performance of magnets. A good ingot microstructure should be: columnar grains grow well, their size is small, and the Nd-rich phase is uniformly distributed along the grain boundaries, but there must be no large Nd-rich phases, and there is no α-Fe crystal. Solidification of ingots is a nucleation process. In the crystallization process, the greater the nucleation rate, the more crystal nuclei grow at the same time. In this way, the size of the lamellae obtained will be smaller. In order to produce a high-performance Nd-Fe-B permanent magnet, it is preferable to control the size of the ingot crystallite to be 5 μm or less.
The purpose of milling is to break large alloy ingots into a certain size of powder. Including rough broken and milling two processes. There are two methods of coarse crushing: one is hydrogen crushing (HD) and the other is mechanical crushing. The coarsely broken medium powder of 246 μm to 175 μm (60 to 80 mesh) was ground to a fine powder of 3 to 4 μm. Most of the magnetic powder was single crystal. Ball milling or air milling powders are generally used. Ball mill milling rolling ball mill, vibration mill, high-energy ball mill. Air-milling powders use air flow to accelerate powder particles to supersonic speeds, colliding with each other and breaking them up. Current manufacturers with smaller production scales use rolling ball mills, and most Nd-Fe-B production plants use jet mills to produce magnetic powders.
Powder magnetic field orientation is another key process technology for manufacturing high-performance sintered Nd-Fe-B permanent magnets. The magnetic properties of sintered Nd-Fe-B permanent magnets mainly originate from the Nd2Fe14B matrix phase with a tetragonal structure. It is a uniaxially anisotropic crystal. The c axis is the easy axis and the a axis is the hard axis. For a single crystal, there is a maximum remanence Br=μ0Ms when magnetized along its easy axis of magnetization. If the c-axis of each powder particle of the sintered permanent magnet is chaotically oriented, an isotropic magnet is obtained, Br = μ0Ms/2 = Js/2, which is the lowest. If the direction of easy magnetization (c-axis) of each powder particle is oriented in the same direction to make an anisotropic magnet, there is maximum remanence in the direction of c-axis orientation of the powder particles. The 3 to 5 μm powder particles obtained during the milling stage are generally single crystals, but not monodomains, so the orientation of the powder particles in the magnetic field is accomplished in two stages. The first stage is that each powder particle becomes a single domain body. The second stage is the magnetic moment rotation process in the magnetic domain.
Powder compaction has two purposes: (1) pressing a powder into a compact of a certain shape and size; and (2) maintaining the degree of crystal orientation obtained in the orientation of a magnetic field. At present, there are three commonly used pressure molding methods, namely, compression molding, molding and cooling static pressure, rubber molding pressure (cold isostatic pressure). It can also be divided into dry pressure and wet pressure.
The sintering process is to heat the Nd-Fe-B powder compact to a temperature of about (0.70-0.85) T melting below the melting point of the powder matrix phase, and perform the heat preservation treatment for a period of time. The purpose is to increase the compact density, improve the contact properties between the powder particles and increase the strength. Makes the magnets have high permanent magnet performance microstructure characteristics. Sintering can be broadly divided into solid phase sintering and liquid phase sintering.
Nd-Fe-B permanent magnets sintered and cooled rapidly (sintered) have lower magnetic properties. The tempering treatment can significantly improve the magnetic properties of Nd-Fe-B alloys, especially the coercivity. The tempering treatment includes one tempering and two tempering treatments. Two-stage tempering gives better magnetic properties.
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