Controlling internal defects in bonded NdFeB magnet production requires coordinated management across multiple dimensions, including raw materials, processes, equipment, environment, and testing. Raw material purity is fundamental to defect control. The core components of NdFeB magnets include neodymium, iron, boron, and other additive elements, with impurity content directly affecting magnetic properties and internal structure. For example, excessive oxygen content can lead to oxidation of the neodymium-rich phase, forming intergranular corrosion channels and causing cracks or pulverization; impurities such as sulfur and phosphorus reduce material toughness and increase the risk of cold cracking. Therefore, production requires strict screening of raw material suppliers, using high-purity metal raw materials, and using equipment such as ICP atomic emission spectrometers to detect component proportions and ensure that each element meets the formulation requirements.
The melting process is a crucial step in controlling internal defects. Vacuum melting furnaces must be used for high-temperature melting under argon protection to prevent gas contamination that could lead to porosity. If the furnace is not sufficiently sealed during melting, oxygen or water vapor from the air may enter the solution, forming tiny needle-like or circular pores. These defects are often hidden on the magnet's cross-section and are difficult to detect through surface inspection. To reduce such problems, the furnace seal must be checked regularly, and solvent isolation or vacuum technology should be used to prevent gas ingress. Simultaneously, melting parameters, such as temperature profiles and stirring speed, should be optimized to promote compositional uniformity and prevent localized segregation.
Hydrogen explosion (HD) and powder preparation processes significantly affect the microstructure of magnets. The hydrogen explosion process utilizes the hydrogen absorption properties of rare earth intermetallic compounds to cause the alloy to crack along the neodymium-rich phase layer, forming a loose powder. If hydrogen explosion parameters are not properly controlled, such as excessively high hydrogen pressure or insufficient holding time, uneven grain fragmentation may occur, leaving large-sized particles that affect subsequent pressing and sintering density. During air jet milling, powder particles are pulverized through high-speed collisions. If the equipment is worn or contaminated, impurities may be introduced, forming hard particle inclusions. Therefore, the hydrogen explosion furnace and air jet mill must be maintained regularly, using non-contaminating grinding media, and the powder particle size distribution should be monitored using a laser particle size analyzer to ensure compliance with molding requirements.
The molding and sintering processes are the last line of defense for defect control. During the forming process, the secondary forming technology of magnetic field press and isostatic press can improve the orientation degree. However, if the pressure fluctuates or the holding time is insufficient, it may lead to uneven density of the green blank, resulting in voids or shrinkage cracks after sintering. Strict temperature and time control are required during the sintering process to avoid α-Fe phase precipitation or localized over-burning. For example, excessively high sintering temperatures can lead to grain coarsening and reduced coercivity; excessively low temperatures may cause under-burning, resulting in insufficient magnet density. Real-time monitoring of furnace temperature uniformity can be achieved through a dual-path temperature control system and manual recording, reducing defects caused by temperature fluctuations. Surface treatment and post-processing also require attention to defect control. Due to its hard and brittle texture, bonded ndfeb is prone to chipping or micro-cracks during slicing, drilling, and other mechanical processing. Using non-contact processes such as wire cutting and laser cutting can reduce mechanical stress; optimizing tool materials (such as diamond wheels) and coolants (such as rust-resistant types) can reduce processing damage. Surface coatings (such as nickel plating and epoxy resin plating) must undergo cross-cut adhesion testing and thermal shock testing to check adhesion and prevent corrosion failure due to coating peeling.
A quality inspection system is a closed-loop guarantee for defect control. During production, X-ray inspection equipment is used to observe the internal structure and identify hidden defects such as porosity and cracks; demagnetization curves are analyzed using a hysteresis loop meter to determine the uniformity of the internal structure; and salt spray testing and constant temperature and humidity chambers are used to simulate harsh environments and verify corrosion resistance and temperature stability. For example, one company once experienced insufficient coating adhesion due to excessive concentration of degreasing agent in the electroplating bath. This problem was successfully avoided by increasing the coating thickness and conducting adhesion tests.
Production environment control is crucial for defect prevention. NdFeB powder is easily oxidized; the processing workshop must be kept dry, with humidity controlled within a reasonable range, and nitrogen protection or dry processing technology should be used to reduce the risk of oxidation. Operators must wear dustproof clothing and anti-magnetic gloves to avoid human contamination or iron filings attracted by magnets, which can cause surface damage. By continuously auditing and improving the 5M1E (Man, Machine, Material, Method, Environment, Measurement), the defect rate can be systematically reduced and product consistency improved.
