Production of Magnet - Sintered NdFeB

Magnet, in general, is a key component in modern industry to transform one type of energy/information to another type, such as motor, hard disk, sensor, medical instrument. Among these applications, sintered NdFeB is the mostly used magnetic material. Therefore, to have a brief idea about how it is produced, can be very helpful for the user to purchase the right magnet.

A magnet might be sintered, machined and coated by complete different factories. Or even within one batch, the sintered material can be sourced from different suppliers. Therefore for the users who have a high-quality demand should establish a sequence of quality control processes.

The exact combination of various elements is normally like a recipe of each factory. But the basic idea is that some other lanthanide elements improve the properties of magnet dramatically, such as Dysprosium.

However, the recent development of Dy-free magnet can realize the products as strong as the conventional ones by improving manufacture process. Therefore, most of the factories are advertising themselves as Dy-free magnet supplier, in order to emphasize their advanced technology.

Tips for final product: for high-end users, once the quality against demagnetization (coercivity) and corrosion (HAST) is accepted for the inspected samples, any change of "recipe" or switch of supplier for processes 1-4 can be potential risk of quality between batches.

Inspection: hysteresis, ingredients, HAST

Dysprosium sample (Wikipedia)

There are two types of production at this stage:

  • casting, crushing and jet milling;
  • strip casting, hydrogen decrepitation and jet milling.

Either way minimizes the particle size which determines the grain size of the final product. However, the producer tries to control the particle size in a suitable range (neither too big, nor too small) by inspecting the average and width of its distribution.

Tips for final product: it is very nice to see the particle size, but there is a no practical reason for the user to pay attention to this.

Inspection: no need.

Illustration of NdFeB Powder in different sizes. S. Namkung, D.H. Kim and T.S. Jang, Adv. Mater. Sci. 28 (2011) 185-189, Effect of Particle Size Distribution on The Microstructure and Magnetic Properties of Sintered NdFeB Magnets

The material is still in powder form, and microscopically each particle has its own magnetic orientation. Hence, in order to produce a strong magnet, the particles are aligned in an external magnetic field and pressed tightly. Here the producer shall use the customized pressing tool for different applications, the arrangements of pressing direction and magnetic field direction are in general classified as:

  • pressing of parts in axial field (AP);
  • pressing of parts in transverse field (TP);
  • isostatic Pressing of blocks (HR).

Tips for final product: the quality of this process can be critical for many properties, such as homogeneity, magnetic moment, and axis deviation. The inhomogeneity of material is one reason for crack and discontinuity of magnetic field, which leads to a weakened magnetic moment. On the other hand, the misalignment or inhomogeneity of external field will lead an unpredictable magnetic axis deviation. For sensor application, the user shall consider the practical tolerance of magnetic axis. However due to the practical reason the hysteresis is unlikely to reveal the quality inside of the pressed block.

Inspection: magnetic moment/flux and axis deviation.

Illustration of a magnetic dipole model. (Wikipedia)

By keeping the pressed block in the sintering furnace, the material densifies and the shape shrinks gradually. After this process, the powder particles turn into grains. The producers will normally pick several blocks from the different positions in the furnace and cut some small pieces for small samples out of them. The hysteresis tests are done with these samples, in order to confirm the homogeneity of quality.

Tips for final product: Even though the ingredient contributes to the most part of magnetic properties, the inhomogeneous heat treatment within the furnace and between different batches can lead to a fluctuation of remanence and coercivity. The hysteresis is in some cases not enough for the user to control the quality.

Inspection: hysteresis, magnetic moment/flux, temperature coefficient α and β.

Microcrystalline grains within a piece of NdFeB made visible with a Kerr microscope.(Wikipedia)

cutting, grinding, polishing are typical machining processes. For some sensor application which regulates the magnetic axis within a tolerance, the producer shall not only carefully prepare the pressed block in process 3, but also ensure the machining according to the particle alignment.

Tips for final product: tighter tolerances depends on the shape and size of the magnet and may not be achievable. Since the Neodymium magnet is very hard, only diamond cutting tools (CNC diamond grinding wheels, diamond drills, etc) and wire cutting machines (EDM) are possible to perform large material removing process.

Inspection: dimension, magnetic axis.

 

Once a magnet starts to corrode, it breaks into pieces gradually. HAST- low weight loss material can slow down the corrosion process, but technically coating is the first protection of magnet against corrosion which shall be by all means avoided. The typical coatings are:

  • Nickel-Copper-Nickel;
  • Ni-Cu-Ni plus Epoxy;
  • Gold.

Certainly there are many other types of coatings which shall be chosen carefully according to the applied environment.

Tips for final product: NiCuNi coating is sufficient for many applications. But when the environment is so harsh that a NiCuNi+Epoxy is unable to protect the magnet, the user shall consider other material like SmCo.

Inspection: salt spray test and HAST test.

Black epoxy coating

A pulse magnetization is applied to switch the random magnetic domain to a paralleled state. Theoretically the magnetic field strength shall be at least 1.5-2 times of coercivity for NdFeB.

Tips for final product: many users prefer a magnetized shipment state, however in some cases the magnet is not fully magnetized. For the users, the measured magnetic field strength density is too weak to be used.

Inspection: magnetic moment/flux, surface strength density.

Magnetizer