How to judge the quality of sintered NdFeB magnets?

- Nov 02, 2018 -

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 disk, magnetic resonance imaging, electric vehicles, wind power, industrial permanent magnet motors, consumer electronics (CD, DVD, mobile phones, stereos, copiers, scanners, camcorders, cameras, refrigerators, televisions, air conditioners, etc.) and magnetic machinery, magnetic levitation technology, magnetic transmission and other industries.


With the expansion of the market, its manufacturers are also increasing, and many customers are inevitably caught in such confusion, how to judge the pros and cons of the product?

The most comprehensive judgment method: one, magnet performance; two, magnet size; three, magnet plating.

First of all, the guarantee of the performance of the magnet comes from the control of the raw material production process.

1. According to the requirements of the company to manufacture high-grade or medium-grade or low-grade sintered NdFeB, the raw materials are selected according to the national standard.

2, the advanced production process directly determines the performance quality of the magnet. The most advanced technologies available today are scale ingot (SC) technology, hydrogen fragmentation (HD) technology and jet mill (JM) technology. The small-capacity vacuum induction smelting furnace (10 kg, 25 kg, 50 kg) has been replaced by a large capacity (100 kg, 200 kg, 600 kg, 800 kg) vacuum induction furnace. SC (StripCasting) rapid solidification casting technology has gradually Instead of large ingots (ingots with a thickness greater than 20-40mm in the cooling direction), hydrogen crushing (HD) technology and jet mill (JM) replace the jaw crusher, disc mill, ball mill (wet milling), guarantee The uniformity of the powder is favorable for 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 press molding. It is vertically molded with small pressure during orientation, and finally adopts quasi-isostatic pressing. This is one of the most important characteristics of China's sintered NdFeB industry. .

4, and the monitoring of the quality of the production process is very important, can be controlled by SC sheet thickness measurement and JM powder particle size distribution. High-quality products are all dependent on the control of the production process, but customers will be very confused, how to judge the performance of the products I purchased? The China Institute of Metrology has developed a variety of magnetic parameter measuring instruments for permanent magnet materials. Pulsed Magnetic Field Magnetometer (PFM) is a test instrument for testing ultra-high-coercive permanent magnets. It is mainly used to adapt to high-coercive permanent magnets required in electric vehicles and large permanent magnet motors.


Customers can choose the NdFeB that they need according to the magnet parameters Br (remanence), hexachlorobenzene (coercive force), coercive force Hcj (inner coercivity), (BH)MAX (maximum energy product). The grade, and these four parameters are the criteria for determining whether the product is produced according to customer requirements.

Second, the guarantee of the size of the magnet depends on the processing strength of the factory.

The practical application of NdFeB permanent magnets is various in shape, such as wafer, cylinder, cylindrical (with inner hole); square, square, square column; tile, fan, trapezoid, polygon and various Permanent magnets of various shapes, such as irregular shapes, have different sizes, and the production process is difficult to achieve one-shot molding. The general production process is: Mr. produces large (large size) billets that are sintered and tempered, then machined (including cutting, perforating) and ground, surface coated (coated), then subjected to magnet properties, surface quality And dimensional accuracy is detected, then magnetized, packaged and shipped.

1, machining is divided into three categories:

(1) Cutting processing: cutting a cylindrical, square columnar magnet into a disk shape, square piece shape;

(2) Shape processing: processing circular, square magnets into sectors, tiles or grooves or other complex shapes of magnets;

(3) Punching processing: The round bar and the square rod magnet are processed into a cylindrical or square cylindrical magnet, and the processing methods are: grinding slicing, electric spark cutting processing and laser processing.

2. The surface of sintered NdFeB permanent magnet components is generally required to be smooth and to a certain degree of precision. The surface of the magnet delivered by the blank needs to be surface-ground. The commonly used grinding methods for the block NdFeB permanent magnet alloys include surface grinding, double end grinding, internal round grinding, and external grinding. The cylinder is usually made of coreless grinding, double-face grinding and other tiles. Type, sector and VCM magnets use multi-station grinders.

A qualified magnet not only requires performance compliance, but also the control of dimensional tolerance directly affects its application. The size guarantee is directly dependent on the processing strength of the factory. The processing equipment is constantly updated with the economic and market demand. The trend of more efficient equipment and industrial automation is not only to meet the increasing demand of customers for product precision, but also It saves manpower and costs, making it more competitive in the market.

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

It has been experimentally found that 1 cubic centimeter of sintered NdFeB permanent magnets are left in the air at 150 ° C for 51 days and will be completely oxidized and etched away. It is more susceptible to corrosion in weak acid solutions. In order to make NdFeB permanent magnets durable, it is required to have a service life of 20-30 years. It must be subjected to surface anti-corrosion treatment to resist corrosion of corrosive medium to magnets. Currently, sintered NdFeB permanent magnet manufacturing industry is common. Electroplating metal, electroplating + electroless metal plating, electrophoretic coating and phosphating treatment are applied to the surface of the magnet with an additional spacer to isolate the surface of the magnet from the corrosive medium to prevent the medium from invading the magnet.

1, generally galvanized, nickel-plated + copper + nickel, nickel-plated copper + + electroless nickel plating three processes, other metal plating requirements, generally after nickel plating and then applied other metal plating.

2, in some special cases will also use phosphating: (1) in the NdFeB magnet products due to turnover, preservation time is too long and the subsequent surface treatment method is not clear, the use of phosphating is simple and easy; 2) When the magnet needs epoxy bonding, painting, etc., the bonding force of epoxy, lacquer and other epoxy organic substances requires good infiltration performance of the substrate. The phosphating process can improve the infiltration ability of the magnet surface.

3, electrophoretic coating has become one of the widely used anti-corrosion surface treatment technologies. Because it has good adhesion not only to the surface of the porous magnet, but also has corrosion resistance to salt spray, acid and alkali, and excellent corrosion resistance. However, it has poor heat and humidity resistance compared to spray coating.

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

And how does the customer judge whether the coating meets the requirements? The purpose of the salt spray experiment is to conduct a rapid anti-corrosion test on the sintered NdFeB magnet treated with anti-corrosion coating on the surface. At the end of the experiment, the sample is taken out from the test chamber, dried, and the surface of the sample is observed with an eye or a magnifying glass. The size of the box size of the spot area changes.

In summary, customers can only correctly judge the product's eligibility if they understand their production processes and understand the requirements of the products. In a nutshell, it is the performance of the control, the control of dimensional tolerances, the detection of the coating and the evaluation of the appearance.

In terms of performance, it can be tested for its energy Br (remanence), hexachlorobenzene (coercivity), coercive force Hcj (inner coercivity), (BH)MAX (maximum energy product) and demagnetization curve; Tolerance, the accuracy can be measured by vernier caliper; on the coating, the brightness of the coating can be observed by the naked eye and the detection method by bonding force, salt spray experiment; the overall appearance, mainly with the naked eye or magnifying glass, or optical microscope (for line degree less than 0.2) Products below the millimeter), the surface of the magnet is smooth, there are no visible particles and foreign matter, no spots, no falling corners, it is qualified.


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