A Brief Introduction to Radially Oriented NdFeB Ring Magnets

Many magnet users are tending to confuse radially magnetization with diametrical magnetization. As the name implies, the magnetization direction of radially magnetized ring magnets is along the radial vector. For sintered NdFeB magnets, radially magnetization is basing on radially orientation, but radially oriented NdFeB ring magnet is serving more as the basis to obtain multipole NdFeB ring magnets.

Besides conventional powder metallurgy process, radially oriented NdFeB ring magnets can be also manufactured via hot-deformed process. Powder metallurgy process is not easy to manufacture magnet with the small diameter or high height owing to the anisotropy of young’s modulus. In the meanwhile, it is also difficult to obtain high orientation degree and magnetic performance due to the relatively complicated orientation field design. Hot-deformed process utilizes nanocrystalline NdFeB powder as raw material and further compress it into dense blank under a certain temperature, then eventually obtained full-density ring magnet through hot deformation process.

Powder Metallurgy Process

Magnetic field orientation during molding process utilizes interactions between NdFeB powder and the external magnetic field to order the easy magnetization direction of powder and make it consistent with the final magnetization direction. The mainstream generation mode of radially orientation field includes regular repulsive orientation technology and Chinese unique rotating orientation technology.

Repulsive Orientation Technology

Magnetic circuit of repulse orientation technology is composed of electric coils and mold. More specially, mold mandrel and mounting sleeve of female mold adopt magnetic conductive material. Punch and female mold is made of non-magnetic conductive material. Electric coils are placed at the ends of mold mandrel. The current direction of two coils is opposite and thus repulsive magnetic field will form the radially magnetic field to orient powder. Repulsive orientation technology has excellent axial symmetry and thus magnetic performance uniformity along the circumference can be guaranteed. However, magnetic force line of height direction will deviate from the horizontal plane and then restrict magnet’s height.

Rotating Orientation Technology

Rotating orientation technology use electric coil, fan-shaped yoke iron, and mold mandrel to form a fan-shaped magnetic field, then powder at different angles is oriented in succession due to the rotation of the female mold. Rotating orientation technology can effectively reduce orientation field area and improve magnetic field strength. However, mechanical fitting accuracy of rotation mechanism will affect concentricity of ring magnet and magnetic performance uniformity along the circumference.

Hot-deformed Process

Nd₂Fe₁₄B main phase has tetragonal structure and elastic modulus of the easy magnetization axis is relatively low. For isotropic nanocrystalline NdFeB magnets, its easy magnetization direction will form preferred orientation along pressure direction during hot deformation process. The most noteworthy feature of hot-deformed process is that it does not need the magnetic field to orient powder. Hot-deformed process is well suitable for high L/D ratio and thin-wall ring magnets.