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Orientation and Magnetization of Sintered NdFeB Magnets

Magnetic materials fall into two categories: isotropic magnets and anisotropic magnets.

  1. Isotropic magnets exhibit uniform magnetic properties regardless of direction and can be magnetized in any orientation.
  2. Conversely, anisotropic magnets possess varying magnetic properties across different directions, with one specific direction offering optimal magnetic performance known as the magnet’s orientation direction.Prominent examples of anisotropic magnets include sintered NdFeB and sintered SmCo hard magnetic materials.

Orientation is an important process in the production of sintered neodymium-iron-boron magnets

The magnetism of a magnet originates from magnetic ordering (aligning the magnetic domains in one direction). Sintered neodymium-iron-boron is made by pressing magnetic powder into a mold. The magnetic powder is placed in a mold and given a shape, while an electromagnet applies a strong magnetic field and the press applies a certain pressure to the powder, causing the easy magnetization axes of the powder particles to align. After pressing, the green compact is demagnetized, then removed from the mold to obtain a green compact with good easy magnetization axis orientation. Subsequently, it is cut into the specified size according to the user’s needs to produce the magnet.

Powder orientation is the key process for preparing high-performance neodymium-iron-boron permanent magnets. The quality of the magnet orientation in the green compact stage is affected by many factors, including: the intensity of the orientation magnetic field, the shape and size of the powder particles, the molding method, the relative direction of the orientation field and the molding pressure, and the loose packing density of the orientation powder, etc.

The magnetic bias angle caused by post-processing has a certain impact on the magnetic field distribution of the magnet

The magnetic bias angle refers to the angle between the direction of the magnetic force lines of the magnet and the orientation plane of the magnet. The ideal state of the magnetic bias angle is perpendicular to the orientation plane, but in the post-processing process, due to the operation of the binder and the processing methods during cutting, a certain angle will be created between the cutting direction and the pole face, resulting in a lower magnetic field strength on the orientation plane than normal after subsequent magnetization.

Magnetization is the last step for sintered neodymium-iron-boron magnets to acquire magnetism

After the magnet green compact is cut to the required size for the user, and undergoes anti-corrosion treatment such as electroplating to become the final magnet product, the magnet itself does not yet exhibit magnetism externally. It needs to go through the magnetization process to “magnetize” the magnet.

The equipment we use to magnetize the magnets is called a magnetizer or magnetizing machine. The magnetizer first charges a capacitor with a high DC voltage (i.e., energy storage), and then discharges through a coil with an extremely low resistance (the magnetizing fixture). The peak value of the discharge pulse current is very high, reaching tens of thousands of amperes. This current pulse generates a strong magnetic field inside the magnetizing fixture, which can permanently magnetize the magnet placed in the fixture.

Accidents can also occur during the magnetization process, such as incomplete magnetization, explosion of the magnetizer pole tips, or magnet fracture.

  • Incomplete magnetization is mainly caused by insufficient magnetizing voltage, where the magnetic field generated by the coil does not reach 1.5~2 times the saturation magnetization intensity of the magnet.
  • For multi-pole magnetization, magnets with a relatively thick orientation direction are also difficult to magnetize to saturation because the distance between the upper and lower pole tips of the magnetizer is too large. The magnetic field strength generated by the pole tips is not sufficient to form a normal closed magnetic circuit, so the magnetic field cannot penetrate the magnet, leading to magnetic pole disorder and insufficient magnetic field strength.
  • Explosion of the magnetizing pole tips is mainly caused by setting the voltage too high, exceeding the safe voltage of the magnetizer.

Magnets that are not fully saturated or have been demagnetized will be more difficult to fully saturate. Because the initial state of the magnetic domains is disordered and does not exhibit magnetism externally. To fully saturate, it is only necessary to overcome the resistance of the displacement and rotation of the magnetic domains themselves. However, if the magnet is not fully saturated, or if it has been partially demagnetized, there will be regions with a reverse magnetic field inside.

Whether magnetizing in the forward or reverse direction, there will be some magnetized regions that need to be reverse magnetized, requiring an additional magnetic field stronger than the theoretical magnetizing field to overcome the inherent coercivity of the reverse magnetic field regions.