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Judgment of the quality of sintered NdFeB magnets

The quality of sintered NdFeB magnets can be judged from three aspects: magnet performance, magnet size, and magnet coating.
 
Super strong NdFeB magnet, sintered NdFeB
 
Magnet performance
 
First of all, the guarantee of magnet performance comes from the control of the raw material production process.
 
1. According to the company's requirements for manufacturing high-end or mid-range or low-end sintered NdFeB, raw materials are selected according to the raw material composition specified by the national standard.
 
2. Whether the production process is advanced or not directly determines the performance and quality of the magnet. At present, the most advanced technologies are scale casting (SC) technology, hydrogen crushing (HD) technology and jet mill (JM) technology.
 
Small capacity vacuum induction smelting furnace (10kg, 25kg, 50kg) is replaced by large capacity (100kg, 200kg, 600kg, 800kg) vacuum induction furnace
 
SC (StripCasting) quick-setting casting technology has gradually replaced large ingots
 
Hydrogen crushing (HD) technology and jet mill (JM) replaced jaw crushers, disc mills, ball mills (wet milling)
 
3. In terms of magnetic field orientation, my country is the only country in the world that adopts two-step compression molding. It uses low pressure vertical compression molding during orientation, and finally adopts quasi-isostatic compression molding. This is one of the most important features of my country's sintered NdFeB industry. .
 
4. The monitoring of the quality of the production process is very important, and it can be controlled by testing methods such as SC sheet thickness measurement and JM powder particle size distribution.
 
Magnet size
 
Secondly, the guarantee of magnet size depends on the processing strength of the factory.
 
The practical application of neodymium iron boron permanent magnet shapes are various:
 
The permanent magnets of each shape have different sizes, and it is difficult to form them in one production process. The general production process is:
 
Produce large (large size) blanks→→sintering and tempering→→machining (including cutting and perforating)→→grinding→→surface coating (coating)→→testing (magnet performance, surface quality and size) Accuracy)→→magnetizing→→packing→→factory.
 
1. Mechanical processing is divided into three categories:
 
Cutting process: cutting cylindrical and square columnar magnets into discs and squares;
 
Shape processing: processing round and square magnets into fan-shaped, tile-shaped or grooved or other complex-shaped magnets;
 
(3) Punching processing: processing round bar and square bar magnets into cylindrical or square cylindrical magnets. Its processing methods include: grinding and slicing processing, electric discharge cutting processing and laser processing.
 
2. The surface of the sintered NdFeB permanent magnet element is generally required to be smooth and reach a certain accuracy. The surface of the magnet delivered as a blank needs to be surface ground.
 
Commonly used grinding methods for square NdFeB permanent magnet alloys are:
 
Square: surface grinding, double end grinding, internal grinding, external grinding, etc.
 
Cylindrical: commonly used coreless grinding, double end grinding, etc.
 
Tile-shaped, sector-shaped and VCM magnets: use a multi-station grinder
 
Whether a magnet is qualified or not, not only needs to meet the performance standards, but also the control of dimensional tolerances directly affects its application. The guarantee of size directly depends on the processing strength of the factory. Processing equipment is constantly updated with economic and market demand. More efficient equipment and the trend of industrial automation not only meet the customers' increasing demand for product accuracy, but also It saves manpower and costs and makes it more competitive in the market.
 
Magnet coating
 
Thirdly, the quality of the magnet coating directly determines the application life of the product.
 
Experiments have shown that 1cm3 of sintered NdFeB permanent magnets will be completely oxidized and corroded if they stay in the air at 150°C for 51 days. It is more susceptible to corrosion in weak acid solutions. In order to make the neodymium iron boron permanent magnet durable, it is required to have a service life of 20-30 years, and it must undergo surface anti-corrosion treatment.
 
At present, sintered NdFeB permanent magnets are commonly used in the manufacturing industry
 
Electroplated metal
 
Electroplating + electroless metal plating
 
Electrophoretic coating and phosphating treatment methods
 
1. Phosphating will also be used in some special cases: (1) Phosphating is simple and easy to use when the NdFeB magnet product is too long due to the turnover and storage time and the subsequent surface treatment method is not clear; ( 2) When the magnet needs epoxy adhesive bonding, painting, etc., the bonding force of the epoxy organic matter such as glue and paint requires the matrix to have good wettability. The phosphating process can improve the wettability of the magnet surface.
 
2. 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 porous magnet surface, but also has corrosion resistance such as salt spray, acid and alkali resistance, and excellent corrosion resistance. However, compared with spray coating, its resistance to humidity and heat is poor.

Tags: neodymium magnets NdFeB magnetsŁ¬neodymium magnet Smco magnets

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