How to judge the pros and cons of sintered NdFeB magnets?

- Nov 22, 2017 -

Sintered Nd-Fe-B magnets are widely used in the fields of computer hard disks, magnetic resonance imaging, electric vehicles, wind power generation, industrial permanent magnet motors, consumer electronics (CDs), and other electronic devices as one of the important materials for promoting contemporary technology and social progress. DVD, mobile phones, stereos, copiers, scanners, cameras, cameras, refrigerators, TVs, 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 the industrialization of the past 30 years, the global permanent magnet materials industry to flourish, the magnetic properties continue to set new record, material varieties and grades continue to increase. With the expansion of the market, its manufacturers are also increasing, and many customers will inevitably fall into such a puzzled, how to judge the pros and cons of the product?

The most comprehensive way to judge: First, the magnet performance; Second, the magnet size; Third, the magnet coating.


First, the protection of the performance of the magnet comes from the control of the raw material production process


1, According to the requirements of manufacturing high-grade or medium-grade or low-grade sintered NdFeB, the raw materials are purchased according to the raw material components stipulated by the national standards.


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 Small capacity vacuum induction smelting furnace (10kg, 25kg, 50kg) has been replaced by large capacity (100kg, 200kg, 600kg, 800kg) vacuum induction furnace.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


3, the magnetic field orientation, China is the only country in the world using two-step compression molding, orientation with a small vertical pressure molding, and finally the use of quasi-static pressure molding, which is China's sintered NdFeB industry one of the most important features .


4, and the monitoring of the quality of the production process is very important, and can be controlled by measuring the thickness of SC sheet and the distribution of JM powder particle size. High-quality products are controlled by the production process, but the customer will be very puzzled, how to determine the performance of my purchase? 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, primarily for high coercivity permanent magnets that are required in the field of electric vehicles and large permanent magnet motors.


The customer can choose NdFeB grades according to the parameters of magnet (remanence), Hcb (coercive force), Hcj (intrinsic coercive force) and (BH) max (maximum energy product) Parameter is to determine whether the product is produced in accordance with customer standards.


Secondly, the protection of magnet size depends on the processing power of the factory


The practical application of NdFeB permanent magnet shape is varied, such as wafer, cylindrical, cylindrical (with holes); square, square, square columnar; tile, fan, trapezoid, polygon and various Irregular shape and so on. Each shape of the permanent magnet has different sizes, the production process is difficult to achieve a molding. The general production process is as follows: The company produces large (large size) billets, which are sintered and tempered and then machined (including cutting, punching) and grinding, surface coating (coating), and then Magnet performance, surface quality and dimensional accuracy of the test, then magnetizing, packaging and shipping.


1, machining is divided into three categories:

(1) cutting: the cylinder, square columnar magnets cut into disc-shaped, square sheet;

(2) Shape processing: the round, square magnets processed into fan, tile or groove or other complex shape of the magnet;

(3) drilling process: the round bar, square bar magnets processed into cylindrical or square tube magnet. Its processing methods are: grinding slice processing, EDM cutting and laser processing.


2, sintered NdFeB permanent magnetic components generally require a smooth surface and reach a certain accuracy, the delivery of the surface of the need for surface grinding of the mill. Square NdFeB permanent magnet alloy grinding methods commonly used are planar grinding, double-sided grinding, cylindrical grinding, cylindrical grinding and so on. Cylindrical commonly used coreless grinding, double-end grinding and so on. Multi-station grinders are used for tile, sector and VCM magnets.


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

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


Experiments show that 1cm3 sintered NdFeB permanent magnet placed in the air at 150 ℃ to stay for 51 days, will all be oxidized corrosion. It is more susceptible to corrosion in weak acid solutions. In order to make the NdFeB permanent magnet durable, requiring it to have a service life of 20-30 years, it must withstand the surface anti-corrosion treatment, in order to resist corrosion of the magnet against corrosion of the magnet. At present, sintered NdFeB permanent magnet manufacturing industry commonly used electroplating metal, electroplating + electroless metal, electrophoretic coating and phosphating and other methods, the magnet surface with an additional layer of insulation, the magnet surface and the corrosive medium isolated , To prevent the media against the magnet.


1, generally to galvanized, nickel + copper + nickel, nickel + copper + electroless nickel plating the main three kinds of other metal plating requirements, usually after the nickel plating and then impose other metal plating.


2, in some special cases will also use the phosphate: (1) in the NdFeB magnet products because of turnover, storage time is too long and not clear the follow-up surface treatment method, the use of phosphating simple and easy; ( 2) When the magnet needs epoxy adhesive, paint, etc., glue, paint and other epoxy organic matter adhesion needs of the substrate has a good invasion performance. Phosphating process can improve the invasion of magnet surface ability.


3, electrophoretic coating has become widely used anti-corrosion surface treatment technology. Because of its not only with the porous surface of the magnet very good, but also has salt spray, acid, alkali and other corrosion, corrosion resistance is excellent. However, it has poor wet and heat resistance properties compared to sprayed coatings.


Customers can choose the coating for their product work requirements. With the expansion of motor applications, customers have higher requirements on the corrosion resistance of NdFeB. The HAST experiment (also known as PCT experiment) is designed to test the sintered NdFeB permanent magnet in wet and high temperature corrosion resistance.

How does the customer judge whether the coating meets the requirement? The purpose of the salt spray test is to make a fast anti-corrosion test on the sintered NdFeB magnet with anti-corrosion coating on the surface. At the end of the experiment, the sample is taken out of the test chamber, dried, and observed with or without a spot on the sample surface with an eye or a magnifying glass , The size of the spot area box color change.

In summary, the customer only understand the production process, understand the requirements of the product factors, in order to correctly judge the product's eligibility. To summarize briefly, performance control, dimensional tolerance control, coating inspection and evaluation of appearance.


In performance, Br (remanence), Hcb (coercive force), Hcj (intrinsic coercive force), (BH) max (maximum energy product), and demagnetization curve can be detected by performance; dimensional tolerances can be performed by a vernier caliper Measuring the accuracy of the coating, the coating can be visually observed coating color brightness and through the binding force, salt spray test and other detection methods; overall appearance, mainly with the naked eye or magnifying glass, or optical microscope (for less than 0.2mm line products) Magnets surface smooth, no visible particles and foreign bodies, no spots, no off the corner, it is the appearance of qualified.


Article from NdFeB Industry Network

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