The main manufacturing processes for neodymium-iron-boron magnets

Neodymium-iron-boron magnets (NdFeB) are one of the most widely used high-performance rare earth permanent magnet materials today, offering advantages such as high remanence, high coercivity, and maximum magnetic energy product. Their production process is relatively complex and primarily includes the following main steps:

1.Raw Material Preparation and Blending

The first step in producing neodymium iron boron magnets is raw material preparation. The primary raw materials include neodymium metal, iron alloys (such as pure iron or Fe-B intermediate alloys), boron iron alloys, and potentially added heavy rare earth elements (such as dysprosium or terbium) to enhance high-temperature performance. These materials are weighed and mixed according to specific chemical stoichiometric ratios based on product performance requirements.

2.Melting and ingot casting

The prepared raw materials are placed in a vacuum induction furnace and melted at high temperatures under an inert gas atmosphere to form a uniform Nd-Fe-B ternary alloy. After melting, the alloy is cooled into ingots. The cooling rate must be controlled during this process to achieve an appropriate grain structure.

3.Hydrogen Decrepitation (HD)

Hydrogen decrepitation is a pre-grinding process that uses hydrogen gas to cause the alloy to become brittle and fracture. In a hydrogen atmosphere, the alloy absorbs hydrogen gas, causing volume expansion, increased internal stress, and fracture, resulting in coarse-grained powder. This step improves the efficiency of subsequent grinding.

4.Jet Milling

The hydrogen-fractured powder is fed into a jet milling machine, where it undergoes impact grinding under high-pressure gas to further refine the particle size to approximately 3–5 microns. The fine powder enhances sintering density and Magnetic Properties.

5.Magnetic Field Pressing

The powder is loaded into a mould and pressed under an oriented magnetic field. The direction of the magnetic field determines the easy magnetisation axis of the magnet, so the magnetic field direction and intensity must be precisely controlled. Pressing methods typically include axial pressing or cold isostatic pressing (CIP) to obtain dense and well-oriented green bodies.

6.Sintering

The pressed green compact is sintered at high temperatures (typically 1050–1150°C) in a protective atmosphere (e.g., argon or vacuum). During this process, solid-state diffusion occurs between powder particles, forming a dense polycrystalline structure while improving the magnetic material's microstructure and enhancing magnetic properties.

7.Tempering/Aging

Following sintering, a two-stage tempering process is typically performed: the first stage involves rapid cooling to a medium-temperature zone (e.g., 500–600°C) followed by holding at that temperature for a period of time; the second stage involves cooling to a lower temperature (e.g., 300–400°C) and holding again. This process facilitates the precipitation of Nd-rich phases, optimises grain boundary structure, and thereby enhances coercivity.

8.Mechanical Processing

The shape and dimensions of sintered magnets often do not meet practical application requirements and require mechanical processing such as cutting, grinding, and chamfering. Due to the material’s high hardness and brittleness, diamond tools are typically used for processing.

9.Surface Treatment

Neodymium-iron-boron magnets have active chemical properties and are prone to oxidation and corrosion. Therefore, surface protection treatment is essential. Common methods include electroplating (nickel, zinc, copper), chemical plating, epoxy resin coating, and phosphating. The choice of method depends on the usage environment and Corrosion Resistance requirements.

10.Magnetisation and Testing  

After processing and surface treatment, the magnets must undergo magnetisation to achieve a saturated magnetic state. Subsequently, various performance tests are conducted, including open-circuit flux, remanence, coercivity, and maximum magnetic energy product, to ensure compliance with standards or customer requirements.

11.Packaging and Shipping

Finally, products that pass quality inspection are packaged in moisture-proof packaging and labelled and shipped according to customer requirements.

Summary

The manufacturing of neodymium-iron-boron magnets is a process involving multiple precision processes, from raw material melting to final magnetisation testing, with each step significantly impacting the final product's performance. With technological advancements, certain processes such as hot pressing/hot deformation methods have also been applied to the production of isotropic magnets to enhance density and magnetic performance. Understanding these processes not only helps grasp the manufacturing principles of neodymium-iron-boron magnets but also provides a foundational basis for material research, process optimisation, and quality control.