How to judge the quality of sintered NdFeB magnets?
Sintered neodymium iron boron permanent magnets, as one of the materials that promote contemporary technology and social progress, are widely used in the following fields: computer hard drives, nuclear magnetic resonance imaging, electric vehicles, wind power generation, industrial permanent magnet motors, consumer electronics (CD, DVD) , Mobile phones, audio, copiers, scanners, cameras, cameras, refrigerators, televisions, air conditioners, etc.) and magnetic machinery, magnetic levitation technology, magnetic transmission and other industries.
In the past 30 years, the permanent magnet material industry has developed vigorously, with its magnetic properties constantly breaking new records, and the variety and grades of materials have been increasing. With the expansion of the market, its manufacturers are increasing day by day, and many customers will inevitably fall into such confusion. How to judge the pros and cons of the products?
Comprehensive judgment method: 1. Magnet performance; 2. Magnet size; 3. 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 high-grade or mid-grade or low-grade sintered NdFeB, the raw materials are selected according to the raw material composition stipulated by national standards.
2. Whether the production process is advanced or not directly determines the performance and quality of the magnet. The current 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 Small-capacity vacuum induction smelting furnaces (10kg, 25kg, 50kg) have been replaced by large-capacity (100kg, 200kg, 600kg, 800kg) vacuum induction furnaces. SC (StripCasting) rapid-setting cast sheet technology has gradually replaced large ingots (ingots with a thickness greater than 20-40mm in the cooling direction), and hydrogen crushing (HD) technology and jet mill (JM) have replaced jaw crushers and disc mills. The machine and ball mill (wet milling) ensure the uniformity of the powder and are beneficial to liquid phase sintering and grain refinement.
3. In terms of magnetic field orientation, my country is a country that adopts two-step compression molding. It uses low pressure vertical compression molding during orientation, and then adopts quasi-isostatic compression molding. This is one of the characteristics of my country's sintered NdFeB industry.
4. In addition, 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. 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 buy? The Institute of Metrology has developed a variety of models of permanent magnetic material technology magnetic parameter measuring instruments. The 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 electric vehicle field and large permanent magnet motors.
Customers can select the NdFeB grades they need according to the magnet parameters Br (remanence), Hcb (coercivity), Hcj (intrinsic coercivity), (BH)max (large magnetic energy product), and these four The parameter is the standard for judging whether the product is produced in accordance with the customer's requirements.
Secondly, the guarantee of magnet size depends on the processing strength of the factory
The practically used neodymium-iron-boron permanent magnets have various shapes, such as discs, cylinders, cylinders (with internal holes); square plates, squares, square columns; tile-shaped, sector-shaped, trapezoidal, polygonal and various shapes. Irregular shapes, etc. The permanent magnets of each shape have different sizes, and it is difficult to form them at one time during the production process. The general production process is: Mr. produces large (large-size) blanks, after sintering and tempering, and then through mechanical processing (including cutting, punching) and grinding, surface coating (coating) treatment, and then proceed Magnet performance, surface quality and dimensional accuracy are tested, then magnetized, packaged and shipped out of the factory.
1. Mechanical processing is divided into three categories:
(1) Cutting process: cutting cylindrical and square columnar magnets into discs and squares;
(2) Shape processing: processing round and square magnets into fan-shaped, tile-shaped or grooved or other complex-shaped magnets;
(3) Punching: processing round 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-grinded. Common grinding methods for square NdFeB permanent magnet alloys include surface grinding, double-end grinding, internal grinding, and external grinding. Cylinders are commonly used for coreless grinding, double-end grinding, etc. Multi-station grinders are used for tile, sector and VCM magnets.
A qualified magnet 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. The trend of new equipment and industrial automation not only meets the increasing demand of customers for product accuracy, but also saves The manpower and cost are reduced, making it more competitive in the market.
Again, the quality of the magnet coating directly determines the application life of the product
Experiments have shown that 1cm3 of sintered neodymium iron boron permanent magnets will be completely oxidized and corroded if they are placed 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 be subjected to surface anti-corrosion treatment to resist the corrosion of the corrosive medium to the magnet. At present, the manufacturing industry of sintered NdFeB permanent magnets generally uses methods such as electroplating metal, electroplating + electroless metal plating, electrophoretic coating, and phosphating treatment. An additional spacer is plated on the surface of the magnet to isolate the surface of the magnet from the corrosive medium. , In order to prevent the media from harming the magnet.
1, generally three processes are mainly zinc plating, nickel plating + copper + nickel, nickel plating + copper + electroless nickel plating, and other metal plating requirements are generally applied to other metal plating after nickel plating.
2. Phosphating is also used in some special cases: (1) When the neodymium iron boron magnet product is turned over and stored for 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 adhesive bonding, painting, etc., the bonding force of epoxy organic compounds such as glue and paint requires good wettability of the matrix. 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 excellent corrosion resistance. However, compared with spray coating, its resistance to humidity and heat is poor.
Customers can choose the plating layer according to their product work requirements. With the expansion of the application field of motors, customers have 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.
And how do customers judge whether the coating meets the requirements? The purpose of the salt spray test is to do a quick anti-corrosion test on the sintered NdFeB magnets whose surface has been treated with anti-corrosion coating. At the end of the experiment, the sample is taken out of the test box and dried. Use your eyes or a magnifying glass to observe whether there are spots on the surface of the sample, and the change in color of the size box of the spot area.
To sum up, only by understanding the production process and the requirements of the product can customers correctly judge the eligibility of the product. In short, 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 (magnetic energy product) and demagnetization curve can be tested by performance; dimensional tolerances can be measured by vernier calipers Its accuracy; on the coating, the color and brightness of the coating can be observed with the naked eye, and the bonding force, salt spray experiment and other detection methods; the overall appearance, mainly with the naked eye or a magnifying glass, or an optical microscope (for products with a line degree less than 0.2mm), a magnet If the surface is smooth, there are no visible particles and foreign bodies, no spots, no edges or corners, the appearance is qualified.
