Art of Sintering
Crafting High-Performance
magnets
advanced
sintering process
The sintering process is a sophisticated multi-stage procedure essential in producing high-performance rare earth magnets such as Nd-Fe-B (Neodymium-Iron-Boron) and Sm-Co (Samarium-Cobalt). This report provides a comprehensive overview of each phase in the sintering process, from the initial separation and refinement of rare earth elements to the final inspection of magnetic properties
The process begins with the extraction of rare earth elements from raw ore, which are then purified to obtain high purity rare earth oxides. This separation and refinement stage is critical, as the purity of the rare earth elements directly affects the quality and performance of the final magnet. Advanced techniques are used to ensure the highest possible level of purity, isolating specific elements necessary for the composition of powerful magnetic alloys. The refined oxides serve as the foundation for the subsequent stages in magnet production.
Once the rare earth oxides are purified, they are combined with other elements such as iron, cobalt, and specific rare earth metals to achieve the desired magnetic composition. Each element is carefully measured to ensure a precise mixture. The blend of materials is then melted in a vacuum induction furnace, which prevents contamination by removing oxygen and other reactive gases. This careful composition and control over environmental conditions are necessary to produce a magnet with consistent and predictable properties.
In the melting stage, the raw material mixture is heated to high temperatures in the induction furnace, using high frequency currents to liquefy the metals and oxides. This process is essential for creating a homogenous alloy, ensuring that the various components are thoroughly mixed on a molecular level. The result is a molten alloy with the exact chemical composition required to produce a high-quality magnetic material. After melting, the molten alloy is cooled to form ingots, which will undergo further processing.
Following melting, the ingots are subjected to a pulverization process, which breaks them down into fine particles, typically several microns in size. Pulverization allows the material to be shaped and compacted in subsequent steps. Because rare earth metals are highly susceptible to oxidation, the particles are protected by an inert atmosphere, typically nitrogen or argon, which prevents exposure to oxygen. This step preserves the material’s magnetic properties and prepares it for the compaction phase.
The finely pulverized particles are then aligned in a magnetic field and compacted to create magnetic anisotropy, which enhances the magnetic strength of the final product. A jig holds the particles in place while the magnetic field is applied, allowing the particles to align according to the field’s direction. Two compaction methods are used: perpendicular pressing, where the particles are pressed in a perpendicular magnetic field, and parallel pressing, where they are compacted in a parallel field. Perpendicular pressing yields a higher-performance magnet for most applications, while the parallel method is typically used for ring magnets, where specific shape constraints apply.
Once compacted, the magnets undergo the sintering process, where they are heat-treated in a furnace to increase their density and strength. Initially, the density of the pressed magnets is only about 50% of their final value, but after sintering, they reach nearly 100% of their true density. This significant increase in density, along with a reduction in volume by roughly 50%, is essential to achieving the desired magnetic properties. The sintering process also results in a 70-80% reduction in size. Following sintering, the magnets undergo an aging process to further refine their metal properties, which optimizes their stability and performance.
After sintering and aging, the magnets are inspected for key magnetic properties, including remnant flux density, coercivity, and maximum energy product. These properties determine the strength, resistance to demagnetization, and overall efficiency of the magnet. Advanced testing equipment is used to measure these properties precisely, ensuring that each magnet meets rigorous quality standards. Only those magnets that meet the required specifications proceed to further machining, plating, or assembly.