Rare Earth Magnet Manufacturing Process
There are several processes for making magnets, but the most common method is called Powder Metallurgy. In this process, a suitable composition is pulverized into fine powder, compacted and heated to cause densification via “liquid phase sintering”. Therefore, these magnets are most often called sintered magnets. Ferrite, Samarium Cobalt (SmCo) and neodymium-iron-boron (neo) magnets are all made by this method. Unlike ferrite, which is a ceramic material, all of the rare earth magnets are metal alloys.
How SmCo and Neo Magnets are Made
Suitable raw materials are melted under vacuum or inert gas in an induction melting furnace. The molten alloy is either poured into a mold, onto a chill plate, or processed in a strip caster – a device that forms a thin, continuous metal strip. These cured metal “chunks” are crushed and pulverized to form a fine powder ranging from 3 to 7 microns in diameter. This very fine powder is chemically reactive, capably of igniting spontaneously in air and therefore must be protected from exposure to oxygen. Rare Earth Magnet Manufacturing Process
There are several methods for compacting the powder and they all involve aligning the particles so that in the finished part all the magnetic regions are pointing in a prescribed direction. The first method is called axial or transverse pressing. This is where powder is placed into a cavity in a tool on the press and punches enter the tool to compress the powder. Just prior to compaction, an aligning field is applied. The compaction “freezes-in” this alignment. In axial (parallel) pressing, the aligning field is parallel to the direction of compaction. In transverse (perpendicular) pressing, the field is perpendicular to the compaction pressure. Because the small powder particles are elongated in the direction of magnetic alignment, transverse pressing yields better alignment, thus a higher energy product. Compacting powder in hydraulic or mechanical presses limits the shape to simple cross-sections that can be pushed out of the die cavity.
A second compaction method is called isostatic pressing wherein a flexible container is filled with powder, the container is sealed, an aligning field is applied, and the container is placed into the isostatic press. Using a fluid, either hydraulic fluid or water, pressure is applied to the outside of the sealed container, compacting it equally on all sides. The main advantages to making magnet blocks via isostatic pressing is that very large blocks can be made – frequently up to 100 x 100 x 250 mm and since pressure is applied equally on all sides, the powder remains in good alignment producing the highest possible energy product. Rare Earth Magnet Manufacturing Process
Pressed parts are packaged in “boats” for loading into a vacuum sintering furnace. The particular temperatures and presence of vacuum or inert gas is specific to the type and grade of magnet being produced. Both rare earth materials are heated to a sintering temperature and allowed to densify. SmCo has the additional requirement of a “solutionizing” treatment after sintering. After reaching room temperature, both materials are given a lower temperature tempering heat treatment. During sintering, the magnets shrink about 15-20% linearly. Completed magnets have a rough surface and only approximate dimensions. They also exhibit no external magnetic field.