Manufacturing Process of Sintered Rare Earth Magnets

What is a Sintered Rare Earth Magnet

Sintered rare earth magnets, commonly known as neodymium magnets, are currently the strongest permanent magnets. They are primarily made from an alloy of neodymium (Nd), iron (Fe), and boron (B), and are used in various industries from mobile phones and computers to wind turbines. Sintered rare earth magnets are powerful yet inconspicuous magnets. Compared to other magnets, the smaller volume of sintered rare earth magnets provides greater magnetic strength, making them a versatile choice, especially suitable for solutions that require discretion or applications where space is at a premium.

The Manufacturing Process of Sintered Rare Earth Magnets

Melting and Grinding

Once all the components are obtained, they are typically melted using electric currents to form ingots or strips. These ingots are then ground into powder by airflow milling or other grinding methods and mixed to prepare for pressing. Airflow milling allows the production of particles of a specific size. The composition and mixture of the alloy determine the magnet's strength, grade, and other properties. For the production of neodymium magnets, the particle size is typically around 3 microns.

Pressing and Magnetization

After the grinding process, the particles are pressed together. The method used depends on the grade of magnet being produced and the manufacturer's process. The three main pressing methods are axial, transverse, and isostatic pressing. During pressing, an external magnetic field is applied to align the magnetic domains of the particles in one direction, known as the direction of magnetization. After pressing, the material undergoes demagnetization before sintering. This involves heating the material to an extremely high temperature in an oxygen-free environment but below the material's melting point. By applying a magnetic field during pressing, the manufacturer sets the preferred direction of magnetization, and after sintering, a block is obtained with enhanced magnetic properties.

Sintering

After pressing, the magnet is still not very strong. Sintering helps lock the magnetic particles in place by heating the alloy mixture to a temperature high enough to cause adhesion but low enough to avoid liquefaction. Sintering fuses the already pressed particles into a solid block. A complete magnet is achieved through a process called quenching, which quickly cools the material to maximize magnetic properties and minimize the formation of less magnetic variants of the alloy that may occur at temperatures below the sintering temperature.

Machining

Sintering usually causes the magnet to shrink. Specific sizes and shapes are often required for their respective applications, necessitating a process called machining to define the shape and tolerances. After cooling, the magnet is machined into the desired shape using wire-cut EDM or diamond cutting tools and then cleaned and dried before plating to prevent corrosion.