The Preparation of sintered NdFeB magnets

In this article, the development process, performance requirements and main types of rare earth permanent magnet materials are introduced, with emphasis on the production process of sintered NdFeB magnets. Finally, the application of sintered NdFeB magnets in production, scientific research, life and other fields is summarized, and its development direction is considered. It is pointed out that the production process of sintered NdFeB magnets should be further studied, Only by improving the product quality of NdFeB magnets in China can the competitiveness of enterprises be increased.

This series of articles will be divided into four parts to explain the following five main production links:

1. Raw material preparation (alloy melting and casting)
2. Milling
3. Orientation shaping
4. Sintering, heat treatment and machining

Raw material preparation (alloy melting and casting)

We start with the preparation of raw materials and alloy preparation. Sintered magnets usually use pure metal or master alloy as raw materials. Based on the electromagnetic induction heating principle of eddy current generated in the raw materials by alternating magnetic field, medium and low frequency induction melting of raw materials is carried out in vacuum or inert gas environment to heat and melt the raw materials. At the same time, the melt is stirred to make it uniform. The melting points of rare earth metals are between 800 ℃ and 1500 ℃, Fe and Co are 1536 ℃ and 1495 ℃ respectively, and pure B is 2077 ℃. The melting points of some high melting point metals as additives, such as Ti, Cr, Mo or Nb, are between 1600 ℃ and 3400 ℃. Considering the inhibition of the volatilization of rare earth elements, the melting temperature is usually controlled at 1000 ~ 1600 ℃, and the high melting point elements are melted by the alloying action of rare earth metal melt, or the alloys of high melting point elements (usually ferroalloys) are directly used as raw materials, such as B-Fe (melting point ~ 1500 ℃) and Nb Fe (melting point ~ 1600 ℃). In order to ensure the low oxygen environment of smelting and casting, it is necessary to vacuumize the smelting and casting furnace body, and fully vent all parts and raw materials in the furnace. The vacuum level usually reaches 10-2 ~ 10-3, and the pressure rise rate (internal gas release and external gas leakage) of the furnace body before heating also needs to be controlled at a low level. For example, the pressure rise rate of 1 t smelting furnace should be less than 5% × 10-4~1 × 10-3 L/s。 Vacuum melting can make the molten liquid fully deflate, remove low boiling point impurities and harmful gas elements, and improve the purity of the alloy. However, because the vapor pressure of rare earth metals is very low (less than 1pA), the volatilization loss is very considerable, so the furnace body is usually filled with inert gas in the melting process, and the ambient pressure is increased to inhibit the volatilization of rare earth. It is more convenient to use high-purity argon, Generally, it is charged to the level of 50kPa. After the homogenization, exhaust and slagging of the alloy melt are fully completed, the casting can be carried out. Alloy casting is a very critical process, because the phase composition, crystalline state and spatial distribution are very important to the performance of sintered magnets. Alloy ingots have experienced heavy “shells”, 20 mm thick “books”, 5 mm thick “pancakes”. At present, they have developed to 0.3 mm thick quick setting sheet. The industry is trying to avoid composition segregation and formation of impurity phase Many efforts have been made to distribute nd rich phases reasonably.

Melting

Rare earth materials are usually pure metals, and rare earth alloys, such as praseodymium neodymium, lanthanum cerium, mixed rare earth and dysprosium ferroalloy, are often used for cost reasons; High melting point elements (such as B, Mo, Nb, etc.) are mostly added in the form of ferroalloy. Nd-Fe-B magnets have the characteristics of multi metallic phase. The rich nd phase is the necessary condition for high coercivity, and the rich B phase must coexist. Therefore, it is usually required that the content of rare earth and B in the original formula is higher than that of r2fe14b, but sometimes the content of B is slightly lower than that of r2fe14b in order to adjust the composition of grain boundary phase (especially when adding Cu, Al and GA). Because of the reaction between rare earth metal and crucible material, melting and sintering volatilization, the loss of rare earth metal should be considered in the formulation. In order to reduce the impurity content in the alloy, the purity of raw materials should be strictly controlled, and the oxide layer and adhesion on the surface should be fully removed. The heat source of medium and low frequency induction melting is the induced eddy current formed in the raw material by alternating magnetic field. The skin effect of 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 can not penetrate into the center of the block, so the core can only be melted by heat conduction. This is very unrealistic in actual production, so it is necessary to adjust the size of the raw material according to the selection of frequency, Control it at 3 ~ 6 times of the skin depth. The figure below shows the relationship between power frequency skin depth raw material size. It can be seen that the higher the frequency is, the more significant the skin effect is, and the smaller the raw material size is required.

Power frequency/Hz		50	150	1000	2500	4000	8000
Skin depth/mm		73	42	16	10	8	6
Optimum raw material size/mm		220-440	125-250	50-100	30-60	25-50	15-35

The choice of melting frequency is subject to another important role of induction melting electromagnetic stirring, that is, the interaction between molten metal and alternating magnetic field is used to promote the melting of unmelted solid and homogenization of molten metal. The magnitude of electromagnetic force is inversely proportional to the square root of current frequency. Too high frequency will weaken the electromagnetic stirring effect of alternating power supply. The frequency band used in actual production is about 1000 ~ 2500Hz, and the size of raw materials should be controlled below 100 mm.
When the raw materials are stacked in the crucible, the space distribution of the induced magnetic field and the temperature in the melting process should be considered. Generally, the induction coil is wound around the outer side of the crucible. The magnetic field on the inner side of the crucible is the strongest and gradually weakens to the center. However, the side, bottom and upper opening of the crucible are the main ways of heat leakage. Therefore, the temperature of the lower side of the crucible is in the middle, and the temperature of the upper and bottom middle is low, The middle part has the highest temperature. Therefore, small pieces with low melting point should be placed at the bottom of the pot; The high melting point material and bulk material are placed in the middle and lower parts; The bulk material with low melting point is placed on the top and is loose to prevent bridging. Nowadays, continuous melting casting technology has been widely used. Raw materials are added into crucible at high temperature through the charging chamber. In order to control the volatilization of rare earth materials, pure iron is usually added first to melt them, then high melting point metals or alloys are added in sequence, and finally rare earth is added.

Casting

The binary or ternary alloys of rare earth are inevitably formed under the condition of slow (approaching equilibrium) cooling α- Co or α- The soft magnetic properties of Fe phase at room temperature will seriously damage the permanent magnetic properties of the magnet, and the formation of Fe phase must be restrained by rapid cooling.
In order to achieve the desired quenching effect, the traditional ingot mold casting technology has been working hard to reduce the thickness of the alloy ingot. The advantages of ingot mold casting are low equipment cost and simple operation, which can meet the requirements of general magnet production. The disadvantages are that the grain size is not uniform and there are many defects α- Co or α- Fe phase precipitates. Long time heat treatment at a temperature lower than the melting point of the alloy is helpful to eliminate the corrosion 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 grain boundary phase.
In order to further reduce the thickness of alloy ingot, a “disk scraper” structure similar to pancake spreading was developed, which makes the alloy thickness about 1cm. However, the increase of alloy area brings a lot of trouble to the charging of large capacity smelting furnace. Another effective technology development path is the opposite. Starting from the extremely high cooling rate of preparing rapidly quenched Nd-Fe-B alloy, we try to reduce the cooling rate to prepare rapidly quenched crystalline alloy. The technology called strip casting or SC emerges as the times require, The alloy sheet with thickness of 0.2 ~ 0.6 mm and ideal phase composition and texture was obtained. In the strip cast alloy structure, the nd rich phases are uniformly distributed and have no effect on the microstructure α- The suppression of Fe reduces the total rare earth content, which is beneficial to obtain high performance magnets and reduce the cost of magnets; Due to the decrease of the volume fraction of Nd rich phase, the brittleness of the magnet increases and the difficulty of post-processing increases compared with the magnet produced by ingot casting.

Milling

The cast alloy needs to be made into powder by powder making process before it can enter the subsequent processing process. The basic purpose of powder preparation is to obtain appropriate powder shape, average particle size and particle size distribution. The above-mentioned characteristic differences of powders are shown as the changes of parameters such as bulk density, tap density, angle of repose, fluidity, compression ratio, internal friction coefficient and external friction coefficient, which are directly related to the powder filling, magnetic field orientation, blank pressing and demoulding, as well as the microstructure of magnets produced in sintering and heat treatment processes, Thus, the permanent magnetic properties, mechanical properties, thermoelectric properties and chemical stability of the magnet are sensitively affected.
The ideal microstructure of sintered magnet is that the fine and uniform main phase grains are surrounded by smooth and thin additional terms, and the easy magnetization direction of main phase grains is aligned along the orientation direction as much as possible. Voids, large grains and large size of soft magnetic phase will seriously reduce the intrinsic coercivity of the magnet, while grains with easy magnetization direction deviating from the orientation direction will reduce the remanence and demagnetization curve squareness of the magnet at the same time. Therefore, it is necessary to make the alloy ingot or rapid cooling sheet with an average grain size of 3 ~ 5 μ m. The maximum particle size is less than 20 μ m. In order to avoid the tendency of serious oxidation of the powder, it is necessary to control the proportion of super fine grains. If necessary, the surface treatment of the powder can enhance the anti-oxidation ability of the powder and improve the filling and compactability.

Conventional mechanical crushing method

The hardness and brittleness of rare earth intermetallic compounds are high, so the alloy ingot can be easily broken into small pieces by jaw crusher or similar machinery, and then mechanically crushed step by step to an average particle size of 3 ~ 5 μ However, the impurities brought in by equipment wear will inevitably affect the quality of powder. Due to the serious oxidation tendency of rare earth metals and their intermetallic compounds, coarse crushing (~ 10 mm level) and medium crushing (~ 100 mm level) occur μ M level) is usually carried out in a protective atmosphere such as nitrogen or argon, while fine grinding (average particle size 3 ~ 5 μ m) Then choose liquid ball mill or nitrogen or inert gas jet mill.

The double alloy method or multi alloy method of sintering Nd-Fe-B is also widely used, which is usually to grind the alloy close to the positive component of Nd2Fe14B and the rapid cooling alloy rich in Nd, and make the nd rich powder with smaller volume evenly distributed in the main body of the near positive component alloy powder.

Hydrogen decomposition (HD)

The research on the hydrogen absorption behavior of rare earth metals, alloys and intermetallic compounds and the physical and chemical properties of hydrides has always been an important topic in the application of rare earth. The most direct example is hydrogen battery. The alloy ingot of rare earth permanent magnet materials also has a strong tendency to absorb hydrogen. Hydrogen atoms enter into the interstitial sites in the main phase of intermetallic compounds and the grain boundary phase rich in rare earth, forming interstitial atomic compounds, which increase the atomic spacing and expand the lattice volume. The resulting internal stress causes grain boundary cracking (intergranular fracture) in the brittle alloy Grain fracture (transgranular fracture) or ductile fracture is called “hydrogen fracture” or “hydrogen burst” because of cracking or accompanied by crackling.

Ammonia flow grinding method

In the laboratory or large-scale production process, the fluidized bed jet mill with high pressure (0.6MPa) and high purity (99.995%) nitrogen as the power source is usually used. The median particle size D50 measured by laser particle size analyzer is 5 μ M. Considering that the gas pressure is proportional to the average kinetic energy of the gas molecules, under the same pressure, the small molecular weight gas has a greater flight speed, and the increase of gas velocity is conducive to improving the natural collision frequency of powder particles. Hydrogen and helium are the best candidates. However, due to the deflagration of hydrogen, helium is the best choice. The flow rate of helium is 2.9 times that of nitrogen, which can crush Nd-Fe-B coarse powder to D50 = 2 in a short time μ It is less than 5m.

Orientation shaping

Magnetic field orientation forming is to arrange the magnetization direction of powder particles by using the interaction of magnetic powder and external magnetic field, so that it is consistent with the final magnetization direction of magnet. This is the most common method to obtain anisotropic magnet. The powder preparation process breaks Nd Fe B alloy into single crystal particles, which are uniaxial anisotropy, each particle has only one easy magnetic axis, the c axis of the main phase cell. The powder is packed into the mold loosely, and the filling density is about 25% – 30% of the real density. Under the action of the external magnetic field above 0.8a/m, the powder particles are changed from multi domain to single domain, and the magnetization direction is adjusted to the direction of external magnetic field by rotation or movement.
In industrial production, the pressing forming methods are divided into two categories: one forming and two forming.
One way press (pressure is 50-100mpa, blank density is 55% – 60% of the real density) or cold and other static press (pressure is generally 200MPa and blank density is 60%).
One way press (pressure is generally 20-30Mpa, blank density is 45%) and cold isostatic press (pressure is generally 200MPa and blank density is 60%) can be used for two-time forming.
During the orientation forming, the alloy powder basically retains the c-axis orientation arrangement. After pressing, demagnetization is done to the blank (eliminating the damage of magnetic dipole interaction between the magnetic particles on the orientation of the adjacent particles), and then demoulding. The rough with good orientation in magnetization direction can be obtained.
The pressure up to 100 MPa will force the magnetic particle to obey the balance condition of mechanical force and magnetic force, which will inevitably cause the movement or rotation of magnetic particle, and may cause the c axis to deviate from the direction of external magnetic field and reduce the orientation of the blank. Therefore, the magnetic field formation process is to balance the relationship between the magnetic field strength and forming pressure reasonably and obtain the orientation as high as possible under the premise of reaching the blank density.
The orientation of powder is also affected by the friction force in the powder, especially when the loose density is large. In actual production, organic lubricant is used to reduce the internal friction. However, the lubricant must be completely removed before sintering reaction (usually around 200 ℃) to prevent the oxidation or carbonization of lubricating agent and reduce the performance of magnet.
There are three kinds of forming processes in actual production:

• Vertical direction pressing, TDP
• Axial direction pressing, ADP
• Isostatic pressing, IP

The most common is vertical pressure, which means that the direction of magnetic field H is vertical to the pressing direction P; Parallel pressure means that the direction of magnetic field is parallel to the forming pressure; The isostatic pressure is to apply pressure to the magnetic particle evenly in all directions through the medium such as liquid or rubber mold. When the parameters of magnetic particle filling, magnetic field strength and forming pressure are the same, the performance of the magnet obtained by isostatic pressure is the highest, the vertical pressure is the second, and the parallel pressure is the lowest. If the orientation degree is measured by the ratio of remanence to saturation magnetization, Rip is 94% – 96%, TDP is 90% – 93%, and ADP is only 86% – 88%, and the difference between BHmax can be 16-40kj / m3 (2-5mgoe). This difference typically reflects the competition relationship among mechanical pressure, magnetic dipole interaction and internal and external friction.

Cold isostatic pressure is also often used for the secondary pressure of one-way blank pressing. Under the condition of limited orientation magnetic field, the proper orientation degree is obtained by using lower pressure, and then isostatic pressure is used to further improve the density of blank without destroying the existing orientation level.

Sintering, heat treatment and machining

Sintering and tempering

The relative density of sintered NdFeB powder is relatively high, and the contact between particles is mechanical contact. The bonding strength is low. In order to further improve the density, improve the contact property between powder particles and enhance the strength, and make the magnet have the microstructure characteristics of high permanent magnet, it is necessary to heat the blank to the temperature below the melting point of the basic phase of the powder for a period of time, This process is called sintering.
After the sintered magnet is quenched at high temperature, the phase distribution of the grain boundary is uneven and the grain boundary is not clear. Therefore, tempering treatment is needed to optimize the structure and obtain the best magnetic energy. Tempering refers to the sintering of the powder billet to a certain temperature after heating up again, tempering temperature needs to pass the test or through the thermal difference analysis.

(vacuum sintering furnace)

Machining and surface treatment

The practical application of sintered NdFeB magnets has many shapes, such as round plates, cylinders, rings, squares, tiles, fan-shaped and irregular shapes. Because of the different shape and size of permanent magnet elements, it is difficult to make one-time forming for other magnets except large-size regular permanent magnet elements. Therefore, in the process of powder metallurgy, Mr. produced large pieces of blank, after sintering and tempering, and then produced magnetic materials of the shape and size that meet the needs of customers through mechanical processing (including cutting, drilling, etc.) and grinding and surface coating treatment. There are three types of machining, including

• 1. Cut the cylindrical and square column magnets into round and square piece shaped parts, which is called cutting and processing
• 2. The round and square magnets are processed into magnets with fan-shaped, tile type, or groove or other complex shapes, which is called shape processing
• 3. The round rod and square rod magnet are processed into cylinder or square cylinder shaped element, which is called hole drilling.

The machining methods include grinding slice processing, EDM and laser processing.

(multi wire cutter)

Quality inspection

The quality monitoring and final product quality inspection during the production of sintered NdFeB permanent magnet shall include the items listed in the table below, but not every item shall be tested, which shall be determined by the requirements of the product order contract.

Source: China Permanent Magnet Manufacturer – www.rizinia.com