Sintered NdFeB permanent magnets, as one of the important materials to promote modern technology and social progress, are widely used in the following fields: computer hard disk, magnetic resonance imaging, electric vehicle, wind power generation, industrial permanent magnet motor, consumer electronics (CD, DVD, cell phone, stereo, duplicator, scanner, camera, camera, camera, camera, camera, camera, camera, camera, camera, camera, camera, camera, camera, camera, camera, camera, camera, camera, camera, camera, camera, camera, camera, camera, camera, and camera The refrigerator, TV sets, air conditioners, etc.) and magnetic machinery, magnetic levitation technology, magnetic drive and other industries.
Since 1985, industrialization has been started in Japan, China and Europe and the United States. In the past 30 years, the global permanent magnetic materials industry has developed rapidly. The magnetic energy is constantly refreshed and the variety and brand of the materials are increasing. With the expansion of the market, its manufacturers are also increasing, and many customers are inevitably trapped in such confusion. How can we judge the pros and cons of the product?
The most comprehensive method of judgement: first, magnet performance; second, magnet size; third, magnet coating.
First of all, the protection of magnet performance comes from the control of raw material production process.
1. According to the requirements of manufacturing high-grade or medium grade or low grade sintered NdFeB, enterprises should choose raw materials according to the national standards.
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 ingot (SC) technology, HD technology and JM technology. The small capacity vacuum induction smelting furnace (10kg, 25kg, 50kg) has been replaced by large capacity (100kg, 200kg, 600kg, 800kg) vacuum induction furnace. SC (StripCasting) fast casting technology has gradually replaced the large cast ingot (the casting ingot with the thickness of the cooling direction more than 20-40mm). The hydrogen crushing (HD) technology and the airflow mill (JM) have replaced the jaw crusher, disc mill and ball mill (wet milling) to ensure the uniformity of the powder, and it is beneficial to the liquid phase sintering and grain refinement.
3. In the orientation of the magnetic field, China is the only country in the world which uses two steps to suppress the molding. It is formed by the vertical compression molding with small pressure in the orientation. Finally, the quasi isostatic pressing is used. This is one of the most important features of the NdFeB industry in China.
4. And monitoring the quality of the production process is very important. It can be controlled by SC thickness measurement and JM particle size distribution. The quality of the product depends on the control of the production process, but the customer will be puzzled, how to judge the performance of the products I purchased? China Institute of Metrology and research has developed a variety of models of permanent magnetic material technology magnetic parameters measuring instruments. Pulsed magnetic field magnetometer (PFM) is a test instrument for measuring high coercive force permanent magnets. It is mainly to adapt to the high coercive permanent magnets required by the electric vehicle and the large permanent magnet motor.
The customer can select the NdFeB brand according to the magnetite parameters Br (remanence), Hcb (coercive force), Hcj (intrinsic coercive force) and (BH) max (maximum magnetic energy product). At the same time, these four parameters are the criteria to determine whether the product is produced according to the customer's requirements.
Secondly, the protection of magnet size depends on the processing strength of the factory.
The shape of the Nd-Fe-B permanent magnet is varied, such as circular, cylindrical, cylindrical (with inner hole), square, square, square column, tile, fan, trapezoid, polygon, and various irregular shapes, etc. Each shape of the permanent magnet has different sizes, and the production process is difficult to achieve once. The general production process is: the bulk (large size) of the blank, after the sintering and tempering treatment, then through the machining (including cutting, drilling) and grinding, surface coating (coating) treatment, and then the magnetic properties, surface quality and dimensional accuracy testing, and then magnetized, packaged and out of the factory.
1. Mechanical processing is divided into three categories:
(1) cutting process: cutting cylindrical and square columnar magnets into round and square sheets.
(2) contour processing: making circular or square magnets into fans, tiles or magnets with grooves or other complex shapes.
(3) punching processing: round rods or square bar magnets are processed into cylindrical or square cylindrical magnets. The processing methods include grinding, cutting, EDM and laser processing.
2. The surface of sintered Nd-Fe-B permanent magnets generally requires smooth and certain accuracy. The surface of the magnet for rough delivery needs surface grinding. The common grinding methods of square NdFeB permanent magnet alloys are planar grinding, double face grinding, internal grinding and cylindrical grinding. The cylinder is often used for non core grinding and double end grinding. Watt, sector and VCM magnets are grinded with multi-station grinders.
A qualified magnet does not only need to meet the standard of performance, but also the control of dimensional tolerance directly affects its application. The guarantee of size depends directly on the processing strength of the factory. The processing equipment is constantly updated with the demand of the economy and the market. The trend of more efficient equipment and industrial automation is not only to meet the demand for the increasing precision of the product, but also saves the manpower and cost, making it more competitive in the market.
Thirdly, the quality of the magnet coating directly determines the life of the product.
The experiment shows that the sintered NdFeB permanent magnet of 1cm3 stays in the air at 150 C for 51 days, and all of them will be oxidized and eroded. It is more easily corroded in weak acid solution. In order to make Nd-Fe-B permanent magnet durable, it requires 20-30 years of service life, and it must withstand surface corrosion resistance, in order to resist corrosion of corrosion medium to magnets. At present, the manufacturing industry of sintered NdFeB permanent magnets generally adopts electroplating metal, electroplating + electroless metal, electrophoretic coating and phosphating treatment. A layer of additional isolation is deposited on the surface of the magnet to separate the surface of the magnet from the corrosive medium, so as to prevent the invasion of the magnet.
1. Mainly zinc plating, nickel plating + copper + nickel, nickel plating + copper + electroless nickel plating technology, other metal plating requirements, generally in the nickel plating after other metal plating.
2. Phosphating is also used in some special cases: (1) the use of phosphating is easy to use when the Nd-Fe-B magnet products are long and not clear to the subsequent surface treatment. (2) when the magnets need epoxy adhesive bonding, paint and so on, the bonding force of epoxy organic compounds, such as glue and lacquer, requires a very good matrix. Good embellish property. Phosphating process can improve the invasive ability of magnet surface.
3. Electrophoretic coating has become one of the widely used anticorrosive surface treatment technologies. Because it not only has good adhesion to the surface of porous magnets, but also has corrosion resistance to salt fog, acid and alkali resistance. But compared with the sprayed coating, its moisture and heat resistance is poor.
Customers can choose coatings according to their product 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 PCT test) is specially designed to test the corrosion resistance of sintered NdFeB permanent magnets under humid and high temperature conditions.
The purpose of the salt spray experiment is to make a rapid corrosion prevention experiment on the sintered Nd-Fe-B magnets, which have been treated with anticorrosive coating. At the end of the experiment, the samples are removed and dried from the test box, and the size box of the sample surface with spots and spots is observed with eyes or magnifying glasses. The change in color.
To sum up, only by understanding the production process and understanding the requirements of the product can customers correctly judge the product's qualification. To sum up briefly, it is the grasp of performance, the control of dimensional tolerance, the detection of coating and the evaluation of appearance.
Performance can be measured by performance of Br (remanence), Hcb (coercive force), Hcj (intrinsic coercion force), (BH) max (maximum magnetic energy product) and demagnetization curve; dimensional tolerance can be measured by a vernier caliper; on the coating, the color and lustre brightness of the coating can be observed by the naked eye, and the testing side through binding force and salt fog experiment can be observed by the naked eye. Method; the overall appearance, mainly using the naked eye or magnifying glass, or optical microscope (for the product below 0.2mm), the surface of the magnet is clean, no visible particles and foreign objects, no spots, no edge drop angle, the appearance is qualified.