How are neodymium magnets made?
Short answer: neodymium magnets are made by powder metallurgy. An alloy of neodymium, iron and boron is melted, milled into a powder of 3 – 5 micrometres, pressed in a strong magnetic field so that all grains point the same way, and sintered at approx. 1,050 – 1,100 °C. The magnet is then ground to size, coated and only magnetised as the very last step.
The production process in 9 steps
- Melting the alloy: neodymium (often with praseodymium), iron, boron and small additives are melted under vacuum or argon and rapidly cooled into thin flakes (strip casting).
- Hydrogen treatment: the flakes absorb hydrogen and break up into coarse powder (hydrogen decrepitation).
- Fine milling: a jet mill grinds the powder into grains of 3 – 5 µm, under nitrogen, because the powder would otherwise oxidise or ignite.
- Pressing in a magnetic field: the powder is pressed in a mould while a field of approx. 2 Tesla turns all grains in one direction. That direction later becomes the direction of magnetisation (axial, diametrical or through the thickness).
- Sintering: in a vacuum furnace at 1,050 – 1,100 °C the grains fuse into a dense block. It shrinks by about 15 – 20% in the process.
- Heat treatment: a second heating at approx. 500 – 600 °C optimises the coercivity.
- Machining: the hard, brittle block is cut and ground to size with diamond tools or wire EDM, to within a few hundredths of a millimetre. Holes and countersunk holes are made at this stage.
- Coating: usually electroplated nickel-copper-nickel (NiCuNi, 10 – 20 µm), or epoxy, zinc, gold or a plastic casing. See why do neodymium magnets have a coating?
- Magnetising: a short current pulse through a coil produces a field of often more than 3 Tesla. Only then is it a magnet. This is followed by checks on dimensions, coating, flux and holding force.
Why is magnetising done at the end?
A magnetised magnet attracts grinding dust and tools and is difficult to handle and coat. In addition, the heat of sintering would remove the magnetism again (above the Curie point). That is why magnets are machined and coated unmagnetised.
Sintered or bonded?
| Sintered NdFeB | Bonded NdFeB | |
|---|---|---|
| Production | pressing and sintering | powder in plastic, injection moulding or pressing |
| BHmax | 33 – 53 MGOe | approx. 5 – 12 MGOe |
| Shapes | discs, blocks, rings, arcs | complex shapes, thin rings |
| Use | virtually all strong magnets | small motors, sensors |
Our neodymium disc, block, rod and pot magnets are sintered NdFeB.
Axially or diametrically magnetised?
The direction of magnetisation is already fixed during pressing. An axially magnetised disc has its poles on the flat sides, a diametrically magnetised disc or rod on the curved sides. More in axially or diametrically magnetised: what is the difference?
Frequently asked questions
Why are neodymium magnets so fragile?
Sintered NdFeB is a ceramic-like, brittle material. It is hard (approx. 600 Vickers), but does not withstand impacts and bending well. That is why magnets shatter when they snap together.
Can I drill or saw a neodymium magnet myself?
Better not: the material breaks easily, the dust is flammable and the heat can make the magnet lose its strength. Choose a magnet with a hole or countersunk hole.
Where are neodymium magnets made?
About 90% come from China. There are also factories in Japan, Vietnam and, recently, in Europe and the United States.
Read also
- What is neodymium and where does it come from?
- What are rare earth metals?
- Neodymium, ferrite or samarium cobalt?
Need help choosing? Use the Magnet Advisor or contact us.
