How to Judge the Quality of Sintered NdFeB Magnets?

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, cameras, cameras, refrigerators, televisions, air conditioners, etc.) and magnetic machinery, magnetic levitation technology, magnetic transmission and other industries.

Since 1985, industrialization has begun in Japan, China and Europe and the United States. In the past 30 years, the global permanent magnet materials industry has flourished, and its magnetic properties have been continuously recorded. The variety of materials and grades has been increasing. 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?

How to Judge the Quality of Sintered NdFeB Magnets

How to Judge the Quality of Sintered NdFeB Magnets

The most comprehensive judgment method: A, magnet performance; B, magnet size; C, magnet plating.

First, 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 The small-capacity vacuum induction smelting furnace (10kg, 25kg, 50kg) has been replaced by a large capacity (100kg, 200kg, 600kg, 800kg) vacuum induction furnace. SC (StripCasting) rapid solidification casting 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 grinding Machine and ball mill (wet milling) ensure the uniformity of the powder and facilitate 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. Moreover, the monitoring of the quality of the production process is very important, and can be controlled by the thickness measurement of the SC piece and the particle size distribution of the JM powder. 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? China Institute of Metrology has developed a variety of magnetic material measuring instruments for permanent magnet materials. Pulsed Magnetic Field Magnetometer (PFM) is a test instrument for testing ultra-high-coercive permanent magnets, mainly for the high-coercive permanent magnets required in the electric vehicle field and large permanent magnet motors.

Customers can choose the NdFeB grades they need based on the magnet parameters Br (remanence), Hcb (coercive force), Hcj (inner coercivity), and (BH)max (maximum magnetic energy product). The parameter is the standard that determines 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 Irregular shapes, etc. Each shape of the permanent magnets has different sizes, and it is difficult to make one molding in the production process. The general production process is: the production of large (large size) billets, after sintering and tempering, and then through mechanical processing (including cutting, drilling) and grinding, surface coating (coating) treatment, and then Magnet performance, surface quality and dimensional accuracy are measured and then magnetized, packaged and shipped.

1. Mechanical processing is divided into three categories:

(1) Cutting processing: cutting cylindrical and square columnar magnets into a disk shape or a square piece shape;

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

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

2. The surface of the sintered NdFeB permanent magnet component is generally required to be smooth and achieve a certain precision, and the surface of the magnet for the delivery of the blank needs to be surface-ground. Commonly used grinding methods for the block NdFeB permanent magnet alloy include plane grinding, double end grinding, internal round grinding, and external grinding. Cylindrical grinding, double-face grinding, etc. are commonly used for cylinders. Tile, sector and VCM magnets use multi-station grinders.

A qualified magnet not only requires performance compliance, but the control of dimensional tolerance also 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 a 1 cm3 sintered NdFeB permanent magnet is 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 the NdFeB 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 magnet by the corrosive medium. At present, the sintered NdFeB permanent magnet manufacturing industry generally adopts electroplating metal, electroplating + electroless metal plating, electrophoretic coating and phosphating treatment, and an additional spacer is plated on the surface of the magnet to isolate the surface of the magnet from the corrosive medium. To prevent the media 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 because the 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 is excellent in 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.

How does the customer judge whether the coating meets the requirements? The purpose of the salt spray experiment is to conduct a quick 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 and dried. Use an eye or a magnifying glass to observe the change in the color of the box on the surface of the sample with or without spots and spots.

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

In terms of performance, it can be tested for Br (remanence), Hcb (coercivity), Hcj (intrinsic coercivity), (BH)max (maximum energy product) and demagnetization curve; dimensional tolerance can be performed by vernier caliper Measure the accuracy; 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, etc.; the overall appearance, mainly with the naked eye or magnifying glass, or optical microscope (for products with linearity less than 0.2mm), The surface of the magnet is smooth, free of visible particles and foreign matter, no spots, no falling corners, and the appearance is acceptable.

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