Rare earth neodymium iron boron permanent magnet material
Grade and performance of NdFeB permanent magnet materials
Neodymium iron boron is an alloy magnet composed of metal neodymium, iron, boron and other trace metal elements. It is currently the strongest rare earth permanent magnet with high magnetic energy product and good coercivity. Sintered NdFeB permanent magnet materials are divided into low coercive force N, medium coercive force M, high coercive force H, ultra-high coercive force SH, ultra-high coercive force EH, and extreme. There are 7 types of high coercivity UH and AH products, each of which is divided into several brands according to the maximum magnetic energy product. Table.1 refers to the product standards of domestic NdFeB manufacturers and summarizes the permanent magnet materials listed Brand and performance and its application.
Table.1 The grades, properties and applications of sintered NdFeB permanent magnet materials
|Grade||N series||M series||H series||SH series||UH series||EH series||AH series|
|Characteristic||Coercivity（kOe）||Low coercivity||Medium coercivity||High coercivity||Very high coercivity||Ultra high coercivity||Very high coercivity|
|Nominal value of magnetic energy product (MGsOe)||25||25AH|
|Main application areas||MRI equipment, magnetic separator, VCM (hard disk drive voice coil motor)||High-performance magnetic separator, consumer electronics||Elevator traction machine, linear motor, magnetic levitation||1. Megawatt pole wind turbine||EV/HEV (Electric/Hybrid) cars, energy-saving petroleum pumping units|
|2. Elevator traction machine|
|3. Servo Motor|
|4. Energy saving and environmental protection air conditioner compression motor|
|5. EPS (Automotive Electric Power Steering System)|
|Note: In the high-performance NdFeB permanent magnet material industry, it generally refers to series products with (magnetic energy product + coercivity) above 60.|
Composition design and performance of high-performance sintered NdFeB magnets
NdFeB permanent magnet material is a permanent magnet material based on the intermetallic compound RE2Fe14B. The main components are rare earth (RE), iron (Fe), and boron (B). Among them, rare earth Nd can be replaced by some other rare earth metals such as dysprosium (Dy), praseodymium (Pr), gadolinium (Gd), holmium (Ho), terbium (Tb), etc. in order to obtain different properties. Iron can also be replaced by cobalt (Co), aluminum ( Al), copper (Cu), niobium (Nb), gallium (Ga), zirconium (Zr) and other metal partial replacements, see Table 2. The content of boron is small, but it plays a role in the formation of tetragonal crystal structure intermetallic compounds The important role makes the compound have high saturation magnetization, high uniaxial anisotropy and high Curie temperature.
In the past, due to the relatively large performance difference between domestic NdFeB enterprises and foreign countries, they paid more attention to technological progress and regarded performance improvement as the key to enterprise development. However, today, NdFeB backbone enterprises can basically achieve N45 in performance. , 45M, 42H, 40SH, 35UH, 32EH, etc. (some companies can even do N50, 48M, 45H, 38UH, 35EH, etc.), so the key now is “stable performance, good product intrinsic quality, timely delivery, and lowest cost To achieve the above points, the alloy composition must be optimized during product formulation design, and certain elements must be reasonably added (to minimize the addition of high-priced elements <Dy, Nb, Co, Tb, Ga>, and use low-priced elements <Cu , A1, etc.>Replace the alloying element ratio of some high-priced elements), reduce the content of rare earth neodymium, and produce high-coercivity, high-temperature heat-resistant sintered NbFeB permanent magnets with low-cost ratio raw materials. R&D and component design have become an important part of the production and development of enterprises.
|1||Substitution elements of Nd||Dy、Pr、Ho、Gd、Ce、La||Improve the coercivity and ensure that the magnet has a higher magnetic energy product|
|2||Substitution elements of Fe||Co、Ni、Cr、Al|
|3||Alloy elements that increase coercivity||Dy、Tb、Nb、Ga 、Al 、Sn 、Mo、Zr、Ti||Improve the coercivity and ensure that the magnet has a higher magnetic energy product|
|4||Alloying elements that increase working temperature||Co、 Ga 、 Sn 、Si、 Ga 、 Nb、W|
Table.3 List of alloying elements added
|Add element Add amount||N series||M series||H series||SH series||UH series||EH series||AH series|
|Dy content||Effectively improve the coercivity and temperature stability of the alloy||0~0.5%||0~1%||0~2%||1~4.5%||3~6%||4~10%|
|Co||<10%, significantly increase the Curie temperature of the alloy.|
|Nb||The main reason for increasing the coercivity is to inhibit the growth of grains, refine grains, and isolate grain coupling.|
|0.02~2%, (aluminum, niobium, gallium, titanium, tungsten, molybdenum) effectively improve the coercivity and temperature stability of the alloy.|
Table.4 Ingredient list of different products
|RE||B||Fe||TM（Co、Al 、Cu、 Ni、 Ga 、Mo）|
Preparation process of neodymium iron boron permanent magnet material
The sintered NdFeB permanent magnet material is manufactured by powder metallurgy. The process flow is as follows: raw material ratio → vacuum melting → ingot casting → crushing powder → magnetic field orientation forming → isostatic pressing → sintering and tempering heat treatment → machining and surface treatment, According to the equipment selection, there are two processes: ordinary and (SC+HD): ordinary process (ordinary ingot, coarse, medium and fine crushing, jet milling, vertical magnetic field forming (TDP), cold isostatic pressing (ClP), three-stage sintering ) The production of sintered NdFeB magnets has poor consistency of magnetic properties; the rapid cooling thick strip (SC)/hydrogen crushing (HD)/primary magnetic field forming process produces NdFeB sintered magnets with high density, high remanence and magnetic energy product, and coercivity Ligao. The above two processes are used to prepare alloys with the same composition, and the properties of the magnets are quite different. The properties of the magnets of the latter are significantly better than those prepared from ordinary ingots. Due to the renewal and development of domestic rare earth permanent magnet production equipment, stable production of high-performance NdFeB can be realized.
Under the premise of certain purity of raw materials, the production process determines the performance of the magnet. Every process has a great influence on the final magnetic properties of the magnet. The uniform distribution of the Nd-rich phase during the preparation of the magnet can increase the magnet density; single crystal powder is beneficial to increase the degree of orientation, thereby increasing the remanence and magnetic energy product; the small and uniform magnet grains can increase the coercivity.
Smelting ingot production process
The microstructure of the alloy ingot has an important influence on the powder making process, the magnetic field orientation forming process, and the sintering process of the blank in the subsequent process, and further affects the performance of the sintered NdFeB magnet. The ideal microstructure of the alloy ingot should be that there is no α-Fe, and the main phase of Nd2Fe14B has fine grains and the Nd-rich phase is evenly distributed.
Table.5 Comparison of smelting and ingot casting processes
|Different processes||Ingot quality||Characteristic|
|Vacuum induction furnace||It is difficult to completely suppress the appearance of α-Fe dendrites||The process is very unstable and not suitable for industrialized mass production|
|Two-phase alloy method||Main phase and liquid phase are melted separately||The process is complex, not suitable for mass industrial production|
|SC scale technology||It inhibits the precipitation of α-Fe very well, and the pulverization of the scale is very good, and the scale technology improves the distribution of the neodymium-rich phase||Suitable for mass industrial production, producing NdFeB magnets with excellent magnetic properties|
Flour milling process
The fine and uniform NdFeB powder is one of the important conditions for obtaining ideal coercivity. High-performance sintered magnets require the average particle size of the powder prepared by jet milling to be 2.8-3.2μm, with narrow particle size distribution and low oxygen content. The particle shape has an important influence on the orientation of the magnet and the sintering process. Table 6 is the comparison of the results of ordinary ingot, coarse, medium and fine crushing, jet milling powder and hydrogen crushing + jet milling powder. Lubricants and antioxidants should be added when making powder, and inert gas protection can effectively reduce the oxygen content, improve the lubrication effect of the magnet, and increase the orientation of the magnet.
Table.6 Comparison of two milling processes
|Mechanical crushing + jet milling||The main phase grain structure of the alloy is severely destroyed. The powder has a wide particle size distribution, rough surface and high activity. The magnet grains are easy to grow during sintering, and high-performance magnets cannot be prepared.|
|HD process + jet milling powder||The particle size distribution of the powder is concentrated, the surface is smooth, and the activity is low. The magnet grains are not easy to grow during sintering, and are used to manufacture high-performance magnets; reduce equipment wear.|
The orientation of the sintered NdFeB magnet during forming affects the coercive force, remanence and magnetic energy product of the magnet. The magnetic field forming has two processes, TDP (vertical magnetic field forming) + CIP (cold isostatic pressing) and one forming. Among them, due to different production equipment, they are divided into the following types:
|1. Wet press molding technology||The slurry is not easy to oxidize, has a high degree of orientation, small crystal grain size, uniform particle size, and high magnet performance.|
|2. Pulse magnetic field orientation||Improve the orientation of the main phase particles and the bulk density of the powder.|
|3. Rubber molding technology||Rubber mold is compressed during isostatic pressing, magnet orientation and remanence are high.|
|4. Near-end forming magnetic field press||Special-shaped magnet, one-time molding, saving raw materials and subsequent processing procedures.|
When weighing the powder during the molding process, avoid oxidation of the powder, shorten the time of the process as much as possible and carry out it in an environment full of protective gas to ensure the performance of the magnet.
Sintering and tempering process
Sintering and tempering is the key to making sintered NdFeB magnets. Through this process, the magnets are densified and the grain size is uniform. Different grades of high-performance products correspond to different sintering temperature, time and tempering temperature and time. If the temperature and time are unreasonably selected, it is prone to abnormal growth of crystal grains and lower magnetic properties. Choosing a low-temperature, long-term sintering process can ensure the coercivity of the design grade. At the same time, the subsequent tempering process has a good stabilizing effect on the homogenization of the liquid phase.
Source: China NdFeB Magnets Manufacturer – www.rizinia.com