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, nuclear magnetic resonance imaging, electric vehicles, wind power generation, industrial permanent magnet motors, consumer electronics (CD, DVD, cell phones, audio, copiers, scanners, video cameras, cameras, refrigerators, TV sets, air conditioners, etc.) and magnetic machinery, magnetic levitation technology, magnetic transmission and other industries.

In the past 30 years, the global permanent magnet material industry has been booming since 1985, when the industry started to be industrialized in Japan, China, Europe and the United States, and the magnetic properties have been setting new records and increasing the number of material varieties and grades. Along with the expansion of the market, the manufacturers are also increasing, and many customers are inevitably caught in this confusion, how to judge the merits of the product? The most comprehensive way to judge: first, magnet performance; second, magnet size; third, magnet coating.

First, the guarantee of magnet performance comes from the control of the production process of raw materials

1、According to the requirements of the enterprise manufacturing high-grade or mid-grade or low-grade sintered NdFeB, the raw material composition according to the national standard to purchase raw materials.

2、The advanced production process directly determines the performance quality of the magnet. At present, the most advanced technologies are Scaled Ingot Casting (SC) technology, Hydrogen Crushing (HD) technology and Airflow 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Small capacity vacuum induction smelting furnaces (10kg, 25kg, 50kg) have been replaced by large capacity (100kg, 200kg, 600kg, 800kg) vacuum induction furnaces, SC (StripCasting) technology has gradually replaced large ingots (ingots with thickness greater than 20-40mm in the cooling direction), HD (Hydrogen Crushing) technology and gas flow mill (JM) instead of jaw crusher, disc mill, ball mill (wet powder making), to ensure the uniformity of the powder, and is conducive to liquid phase sintering and grain refinement.

3、On magnetic field orientation, China is the only country in the world that adopts two-step press molding, with small pressure vertical molding for orientation and quasi-isostatic molding at the end, which is one of the most important features of China’s sintered NdFeB industry.

Secondly, the guarantee of magnet size depends on the processing strength of the factory

The actual application of NdFeB permanent magnets has various shapes, such as round, cylindrical, cylindrical (with inner hole); square, square, square column; tile, fan, trapezoid, polygon and various irregular shapes.

Each shape of permanent magnets has different sizes, and the production process is difficult to be formed in one go. The general production process is: Mr. output large (large size) blanks, after sintering and tempering treatment, then through mechanical processing (including cutting, punching) and grinding, surface plating (coating) processing, and then magnet performance, surface quality and dimensional accuracy testing, and then magnetization, packaging and factory.

1, mechanical processing is divided into three categories: (1) cutting processing: cutting cylindrical, square-shaped magnets into round, square-shaped, (2) shape processing: processing round, square magnets into fan-shaped, tile-shaped or with grooves or other complex shapes of magnets, (3) punching processing: processing round, square bar-shaped magnets into cylindrical or square-shaped magnets. The processing methods are: grinding and slicing processing, EDM cutting processing and laser processing.

2、The surface of sintered NdFeB permanent magnet components generally requires smoothness and certain precision, and the surface of the magnet delivered in blank needs surface grinding processing. The common grinding methods for square NdFeB permanent magnet alloy are plane grinding, double end grinding, internal grinding, external grinding, etc. Cylindrical commonly used coreless grinding, double end grinding, etc. For tile, fan and VCM magnets, multi-station grinding is used.

A qualified magnet not only needs to meet the performance standard, but also the dimensional tolerance control directly affects its application. The dimensional guarantee directly depends on the processing strength of the factory. The processing equipment is constantly updated with the economic and market demand, and the trend of more efficient equipment and industrial automation is not only to meet the growing demand of customers for product accuracy, but also to save manpower and cost, making it more competitive in the market.

Again, the quality of magnet plating directly determines the application life of the product

Experimentally, a 1cm3 sintered NdFeB magnet will be corroded by oxidation if it is left in the air at 150℃ for 51 days. In weak acid solution, it is more likely to be corroded. In order to make NdFeB permanent magnets durable, it is required to have a service life of 20-30 years.

It must be treated with anti-corrosion treatment to resist the corrosion of the magnet by corrosive media. At present, the sintered NdFeB magnets are generally coated with metal plating, electroplating + chemical plating, electrophoretic coating and phosphate treatment to prevent the magnet from the corrosive medium.

1, generally galvanized, nickel + copper + nickel plating, nickel + copper + chemical nickel plating three processes, other metal plating requirements, are generally applied after nickel plating and then other metal plating.

2, in some special circumstances will also use phosphating: (1) in the NdFeB magnet products because of the turnover, preservation of the time is too long and not clear when the subsequent surface treatment method, the use of phosphating simple and easy; (2) when the magnet needs epoxy glue bonding, painting, etc., glue, paint and other epoxy organic adhesion requires a good infiltration performance of the substrate. Phosphating process can improve the surface of the magnet’s ability to infiltrate.

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

Customers can choose the coating according to their product working requirements. With the expansion of motor application field, customers have higher requirements for corrosion resistance of NdFeB. The HAST test (also called PCT test) is to test the corrosion resistance of sintered NdFeB permanent magnets under humid and high temperature environment.

And how can the customer judge whether the plating meets the requirements or not? The purpose of 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 test, the sample is taken out from the test chamber, dried, and observed with eyes or magnifying glass to see whether there are spots on the surface of the sample, the size of the spot area box color change.

Post time: Jan-06-2023