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.
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.
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.
Permanent magnets suffer extremely low loss except when driven beyond certain critical parameters at which point we say they have “irreversible loss.” If re-magnetization restores all of the original magnetic output, then they have not suffered irrecoverable or structural loss. When only irreversible loss is present and it results in unacceptable differences between initial and subsequent operation, it is almost always possible to “precondition” the magnets so that little to no additional loss is observed in operation. (This assumes that the optimal material has been selected and extraordinary methods must be applied).
If the loss during use is expected to come from elevated temperature, then a Thermal Stabilization is recommended. If the loss is due to demagnetizing stress, then a Magnetic Stabilization should be performed.
Variation in magnetic output (flux output) is unavoidable. Most applications can tolerate normal variations in flux output. For those that require extremely tight tolerances, magnets can be “calibrated” to a set output by partial knockdown. This is usually a magnetic calibration procedure, though occasionally thermal calibration is used – where uniformity at elevated temperature is essential.
Arnold has extensive experience in performing both thermal and magnetic procedures. For example, one consideration is that thermal conditioning can be performed either in open circuit or fixed load line.
For applications requiring stable output over a wide temperature range, many customers consider RECOMA® STAB , a grade of SmCo with less than 50 ppm change in output per ºC from -40 to 200 ºC.