The manufacture of magnets can be done with several different processes, though Powder Metallurgy is the most commonly utilized method. This process involves a suitable composition being pulverised into a fine powder, which is then compacted and heated up in order to cause densification through liquid phase sintering.
These magnets are therefore often simply referred to as sintered magnets, and this method is used in the creation of Ferrite, neodymium-iron-boron (neo) and Samarium Cobalt magnets. The rare earth magnets are actually metal alloys, though Ferrite is not. So strong magnets can vary.
The manufacture of neo and SmCo magnets
An induction melting furnace is used to melt the right raw materials under either an inert gas or a vacuum. The molten alloy is then poured in or onto either a chill plate or a mould or possibly processed with the use of a strip caster, which creates a continuous and thin metal strip.
The resultant chunks of cured metal are crushed into a fine powder that can be anything from three microns in diameter up to as much as seven. The powder is chemically reactive, able to spontaneously ignite in the air and thus needs to be prevented from being exposed to oxygen.
The powder can be compacted with a number of different methods, all of which entail the alignment of the particles so that the result will see the magnetic regions of the finished part all pointing in a particular direction.
Axial pressing is the first method, which involves the powder being put into a cavity in a particular tool that is situated on the press, with the tool then entered by punches in order to compress the powder. An aligning field will be applied just before compaction commences in order to have that alignment frozen in place.
With axial pressing, the aligning field will run parallel to that of the pressure of the compaction. With the very similar transverse pressing, the field will instead be perpendicular to that pressure.
This yields superior alignment and thus a better product in comparison to the use of mechanical or hydraulic presses to compact the powder, which means the shape is limited to the simpler cross-sections that the die cavity is able to push out.
This sees the power being placed in a flexible container which is then sealed and an aligning field applied before itself being put into an isostatic press. Pressure is applied to the exterior of the container with either fluid, water or hydraulic fluid so that it is equally compacted on every side.
The major benefit to the use of using isostatic pressing to manufacture strong magnets is that it can be used to make blocks that are especially large, and the equal pressure means the powder stays well aligned so that the highest energy product possible is the result.
Pressed parts are loaded inside a vacuum sintering furnace in “boats”, with the temperature used and the inert gas or vacuum resulting in a particular magnet grade and type being produced.
Magnets are used throughout society including on freezers and fridges, stereos, televisions, earphones and computers.