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2024

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Magnet company to judge the quality of sintered NdFeB magnets

Sintered NdFeB permanent magnets, as one of the important substances to promote contemporary technology and social progress, are widely used in the following fields: computer hard disks, nuclear magnetic resonance imaging, electric vehicles, wind power generation, industrial permanent magnet motors, consumer electronics (CD, DVD, mobile phones, stereos, copiers, scanners, cameras, refrigerators, televisions, air conditioners, etc.) and magnetic machinery, magnetic levitation technology, magnetic transmission and other industries. Since 1985 in Japan, China and Europe and the United States at the same time began industrialization, the past 30 years, the global permanent magnet material industry to flourish, magnetic properties continue to break new records, material varieties and grades continue to increase.


Sintered NdFeB permanent magnets, as one of the important substances to promote contemporary technology and social progress, are widely used in the following fields: computer hard disks, nuclear magnetic resonance imaging, electric vehicles, wind power generation, industrial permanent magnet motors, consumer electronics (CD, DVD, mobile phones, stereos, copiers, scanners, cameras, refrigerators, televisions, air conditioners, etc.) and magnetic machinery, magnetic levitation technology, magnetic transmission and other industries. Since 1985 in Japan, China and Europe and the United States at the same time began industrialization, the past 30 years, the global permanent magnet material industry to flourish, magnetic properties continue to refresh the record, material varieties and grades continue to increase. With the expansion of the market, its manufacturers are also increasing, and many customers are inevitably caught in such confusion, how to judge the merits of the product? The most comprehensive judgment method: first, magnet performance; second, magnet size; third, magnet coating. First of all, the guarantee of magnet performance comes from the control of the raw material production process.

1. According to the requirements of the enterprise to manufacture high-grade or mid-grade or low-grade sintered NdFeB, the raw materials are selected according to the raw material composition specified by the national standard.

The advanced production process directly determines the performance quality of the magnet. At present, the most advanced technologies are scale ingot (SC) technology, hydrogen crushing (HD) technology and jet mill (JM) technology. Small capacity vacuum induction smelting furnaces (10kg, 25kg, 50kg) have been replaced by large capacity vacuum induction furnaces (100kg, 200kg, 600kg, 800kg). SC(StripCasting) rapid casting technology has gradually replaced large ingots (ingots with a thickness of more than 20-40mm in the cooling direction), hydrogen crushing (HD) technology and jet mill (JM) have replaced jaw crusher, disc mill and ball mill (wet milling), ensuring the uniformity of powder and is conducive to liquid phase sintering and grain refinement.

3. In terms of magnetic field orientation, China is the only country in the world that adopts two-step pressing molding, which is one of the most important characteristics of China's sintered NdFeB industry.

4. In addition, it is very important to monitor the quality of the production process, which can be controlled by means of SC sheet thickness measurement and JM powder particle size distribution. High-quality products depend on the control of the production process, but customers will be very confused, how to judge the performance of the products I buy? China Institute of Metrology has developed a variety of models of permanent magnet material technology magnetic parameter measurement instruments. Pulsed magnetic field magnetometer (PFM) is a test instrument for testing ultra-high coercivity permanent magnets, mainly to adapt to the high coercivity permanent magnets required by the field of electric vehicles and large permanent magnet motors. Customers can select the NdFeB brand they need according to magnet parameters Br (remanence), Hcb (coercivity), Hcj (intrinsic coercivity) and (BH)max (maximum magnetic energy product). At the same time, these four parameters are the standards for judging whether the product is produced according to customer requirements.

Secondly, the protection of the magnet size depends on the processing strength of the factory. The actual application of NdFeB permanent magnets has a variety of shapes, such as discs, cylinders, cylinders (with inner holes); square, square, square and columnar; tile, fan, trapezoid, polygon and various irregular shapes. Each shape of the permanent magnet has a different size, the production process is difficult to achieve a single molding. The general production process is: Mr. output large (large size) blanks, after sintering and tempering treatment, and then through mechanical processing (including cutting, drilling) and grinding, surface coating (coating) treatment, and then the magnet performance, surface quality and dimensional accuracy of the detection, and then magnetizing, packaging and factory.

1. Machining is divided into three categories:

(1) Cutting processing: cutting cylindrical and square cylindrical magnets into round sheets and square sheets;

(2) Shape processing: processing round and square magnets into sectors, tiles or magnets with grooves or other complex shapes;

(3) punch processing: the round bar, square bar shaped magnet processing into a cylindrical or square tube shaped magnet. Its processing methods are: grinding slice processing, EDM processing and laser processing.

2. The surface of sintered NdFeB permanent magnet components is generally required to be smooth and reach a certain precision, and the surface of the magnet delivered by the blank needs to be ground. The commonly used grinding methods of square NdFeB permanent magnet alloy include plane grinding, double-end grinding, internal grinding, external grinding, etc. Cylindrical commonly used without core grinding, double end grinding, etc. Multi-station grinders are used for tile, sector and VCM magnets.

Whether a qualified magnet, not only need performance standards, dimensional tolerance control also directly affect its application. The size of the guarantee directly depends on the processing strength of the factory, processing equipment with the economic and market demand is also constantly updated, more efficient equipment and the trend of industrial automation, not only to meet the customer's growing demand for product accuracy, but also save manpower and cost, so that it is more competitive in the market.

Again, the quality of the magnet coating directly determines the application life of the product.

According to the experiment, 1cm3 sintered NdFeB permanent magnet will be completely oxidized and corroded when it is placed in the air at 150 ℃ for 51 days. It is more susceptible to corrosion in a weak acid solution. In order to make the neodymium iron boron permanent magnet durable, it is required to have 20-30 years of service life, it must be subjected to surface anti-corrosion treatment, so as to resist the corrosion of the corrosive medium to the magnet. At present, the manufacturing industry of sintered NdFeB permanent magnets generally adopts methods such as electroplating metal, electroplating electroless plating metal, electrophoretic coating and phosphating treatment, etc., and the surface of the magnet is plated with an additional separator to isolate the surface of the magnet from the corrosive medium to prevent the medium from infringing the magnet.

1, generally galvanized, nickel-plated copper nickel, nickel-plated copper electroless nickel plating three main process, other metal plating requirements, are generally in the nickel plating and then applied to other metal plating.

2. Phosphating will also be used under some special circumstances:(1) Phosphating is simple and easy to use when NdFeB magnet products are not clear about the subsequent surface treatment method due to long turnover and storage time;(2) When the magnet needs epoxy glue bonding, painting, etc., the adhesive force of epoxy organic compounds such as glue and paint requires the matrix to have good invasion and wetting properties. The phosphating process can improve the wettability of the magnet surface. 3. Electrophoretic coating has become one of the widely used anti-corrosion surface treatment technologies. Because it not only has a good bonding force with the surface of the porous magnet, but also has corrosion resistance such as salt spray, acid and alkali resistance, and has excellent corrosion resistance. However, compared with the spray coating, its wet heat resistance is poor.

Customers can choose the coating according to the requirements of their products. With the expansion of motor applications, customers have higher requirements for the corrosion resistance of NdFeB. The HAST experiment (also known as the PCT experiment) is specifically to test the corrosion resistance of sintered NdFeB permanent magnets in humid and high temperature environments.

The purpose of the salt spray test is to do a rapid anti-corrosion test on the surface of the sintered NdFeB magnet treated by the anti-corrosion coating, at the end of the experiment, the sample is taken out of the test box, dry, with eyes or a magnifying glass to observe the sample surface without spots, the size of the spot area box color changes.

In summary, customers only understand its production process, understand the requirements of the product factors, in order to correctly judge the conformity of the product. In a simple summary, it is the grasp of performance, the control of dimensional tolerances, the detection of coatings and the evaluation of appearance.

In terms of performance, Br (remanence), Hcb (coercivity), Hcj (intrinsic coercivity), (BH)max (maximum magnetic energy product) and demagnetization curve can be detected through performance. Dimensional tolerance can be measured by vernier caliper. On the coating, the color and brightness of the coating can be observed by naked eyes, as well as through binding force, salt spray test and other detection methods. The overall appearance is mainly with naked eye, or optical microscope (for products with linearity less than 0.2mm), the appearance of the magnet is qualified if the surface is smooth, there are no visible particles and foreign matters, no spots, no edges and corners.

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Source: magnet manufacturer Youlian Magnetics www.youliancy.com

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