Preparation process of sintered NdFeB magnets (1) – Raw material
Today we start with the link of raw material preparation and alloy preparation. Sintered magnets usually use pure metals or intermediate alloys as raw materials, use the electromagnetic induction heating principle of eddy current generated by alternating magnetic fields in the raw materials, and conduct medium and low frequency induction melting on the raw materials in a vacuum or inert gas environment to heat and melt the raw materials. The melt is stirred to homogenize it. The melting point of rare earth metals is between 800 and 1500°C, Fe and Co are 1536°C and 1495°C respectively, and pure B is as high as 2077°C. Some high melting point metals used as additives such as Ti, Cr, Mo or Nb have melting points at 1600~3400℃. Considering the suppression of volatilization of rare earth elements, the smelting temperature is usually controlled at 1000~1600°C, and the high melting point elements are melted by the alloying of rare earth metal melts, or directly use alloys of high melting point elements (usually iron alloys) as raw materials, such as B-Fe (melting point ~1500°C), Nb-Fe (melting point ~1600°C) alloys, etc. In order to ensure a low-oxygen environment for smelting-casting, it is necessary to vacuumize the smelting and casting furnace body, and fully deflate the parts and raw materials in the furnace. The vacuum level usually reaches 10-2~10-3, and the furnace body is heated. The previous pressure increase rate (internal deflation and external air leakage) also needs to be controlled at a low level, such as a melting furnace with a capacity of 1t, the pressure increase rate should be lower than 5×10-4~1×10-3 L/s . Vacuum smelting can fully deflate the molten liquid, remove low-boiling impurities and harmful gas elements, and improve the purity of the alloy. However, due to the low vapor pressure of rare earth metals (less than 1Pa), the volatilization loss is very considerable, so usually in the smelting process. The furnace body is filled with inert gas to increase the ambient pressure to suppress the volatilization of rare earths. It is more convenient to use high-purity argon gas, which is generally charged to the level of 50kPa. After the homogenization of the alloy melt, degassing and slagging are fully completed, casting can be carried out. Alloy casting is a very critical process, because the composition, crystallization state and spatial distribution of phases are crucial to the performance of sintered magnets, alloy ingots have experienced heavy “cannonballs”, 20mm thick “books”, 5mm “pancakes” At present, it has developed to a quick-setting sheet with a thickness of only 0.3mm. People in the industry have made many efforts in avoiding composition segregation and impurity phase formation, and rationally distributing the distribution of neodymium-rich phases. NdFeB magnets Raw material
Rare earth raw materials are usually in the form of pure metals, and rare earth alloys are often used for cost reasons, such as praseodymium neodymium metal, lanthanum cerium metal, mixed rare earth and dysprosium iron alloy, etc.; high melting point elements (such as: B, Mo, Nb, etc.) It is mostly added in the form of iron alloy. Nd-Fe-B magnets have the characteristics of multi-metal phases. The Nd-rich phase is a necessary condition for high coercive force, and the B-rich phase must also co-exist. Therefore, it is usually required that the rare earth and B in the original formula are higher than the normal composition of R2Fe14B, but Sometimes in order to adjust the composition of the grain boundary phase (especially when Cu, Al, Ga is added), the B content will be slightly lower than the normal composition. Due to the reaction of rare earth metals and crucible materials and volatilization during smelting and sintering, a certain loss of rare earth metals needs to be considered when formulating. In order to reduce the impurity content in the alloy, the purity of raw materials must be strictly controlled, and the oxide layer and attachments on the surface must be fully removed. The heat source of medium and low frequency induction melting is the induced eddy current formed by the alternating magnetic field in the raw material. The skin effect of the eddy current makes the current concentrate on the surface of the raw material. If the size of the raw material block is too large, the eddy current cannot penetrate to the center of the block, and only The core can be melted by heat conduction, which is not realistic in actual production, so the size of the raw material should be adjusted according to the frequency selection, and it should be controlled at 3 to 6 times the skin depth. The figure below shows the relationship between power frequency-skin depth-material size. It can be seen that the higher the frequency, the more significant the skin effect, and the smaller the material size is required.
The selection of smelting frequency is subject to another important role of induction smelting—electromagnetic stirring, which uses the interaction between the molten metal liquid and the alternating magnetic field to promote the melting of unmelted solids and the homogenization of molten metal liquid. The size is inversely proportional to the square root of the current frequency, too high a frequency will weaken the electromagnetic stirring effect of the alternating power supply. The frequency band used in actual production is around 1000~2500Hz, and the size of raw materials needs to be controlled below 100mm.
The stacking of raw materials in the crucible should take into account the induced magnetic field and the spatial distribution of temperature during the melting process. Usually, the induction coil is wound on the outer surface of the crucible. The magnetic field is the strongest on the inner side of the crucible and gradually weakens towards the center. The opening is the main way of heat leakage, so the temperature of the lower side of the crucible is in the middle, the temperature of the upper layer and the middle of the bottom surface is lower, and the temperature of the middle part is the highest. Therefore, when charging, it is advisable to place the small pieces with low melting point on the bottom of the crucible more densely; the materials with high melting point and large pieces should be placed in the middle and lower parts; Continuous smelting-casting technology is widely used nowadays. The raw materials are added to the crucible which is still at high temperature through the feeding chamber. In order to control the volatilization of rare earth materials, pure iron is usually added first to melt it, then high melting point metals or alloys are added sequentially, and finally rare earths are added. NdFeB magnets Raw material
Rare earth binary or ternary alloys will inevitably generate α-Co or α-Fe phases under slow (approaching equilibrium) cooling conditions, and their soft magnetic properties at room temperature will seriously damage the permanent magnetic properties of the magnets. cooling to suppress its formation.
In order to achieve the desired quenching effect, the traditional ingot casting technology has been working towards reducing the thickness of the alloy ingot. The advantages of ingot mold casting are low equipment cost, simple operation, and can meet the requirements of general magnet production. The disadvantage is that the grain size is not uniform and α-Co or α-Fe phase is often precipitated. Long-term heat treatment of the alloy ingot at a temperature lower than the melting point of the alloy helps to eliminate the α-Co or α-Fe phase, but it will cause the accumulation of Nd-rich phase, which is not conducive to the optimal distribution of the grain boundary phase of the sintered magnet. NdFeB magnets Raw material
In order to further reduce the thickness of the alloy ingot, a “disc-scraper” structure similar to spreading pancakes was developed, so that the thickness of the alloy reached about 1cm, but the increase in the area of the alloy brought a lot of trouble to the collection of large-capacity melting furnaces . Another effective technology development path is the opposite. Starting from the extremely high cooling rate of the fast-quenching Nd-Fe-B alloy, try to reduce the cooling rate to prepare the fast-cooling crystalline alloy, which is called strip sheet The technology of casting or quick-setting thin slice (strip casting or SC) came into being. It pours the molten alloy through the diversion groove onto the fast-rotating water-cooled metal wheel to obtain a thickness of 0.2~0.6mm, ideal phase composition and texture. alloy flakes. In the strip-cast alloy structure, the uniform distribution of Nd-rich phase and the suppression of α-Fe reduce the total rare earth content, which is beneficial to obtain high-performance magnets and reduce the cost of magnets; the disadvantage is that due to the reduction of the volume fraction of Nd-rich phase , Compared with the magnets produced by ingot mold casting, the brittleness of the magnets increases and the difficulty of post-processing increases.