Preparation process of sintered NdFeB magnets (3) – orientation forming
Magnetic field orientation forming is to use the interaction between magnetic powder and external magnetic field to arrange the easy magnetization direction of powder particles so that it is consistent with the final magnetization direction of the magnet. This is the most commonly used method to obtain anisotropic magnets. The powder preparation process breaks the Nd-Fe-B alloy into single crystal particles, and they are uniaxially anisotropic, and each particle has only one easy magnetic axis—the c-axis of the main phase unit cell. The powder is loosely filled into the mold, and the filling density is about 25%-30% of the solid density. Under the action of an external magnetic field above 0.8A/m, these powder particles change from multi-domain to single-domain, and rotate or move. The easy magnetization direction is adjusted to the direction of the external magnetic field.
In industrial production, the current press forming methods are divided into two categories: one-time forming and two-time forming.
One-time forming can use a one-way press (the pressure is generally 50-100MPa, the compact density is 55%-60% solid density) or cold isostatic press (the pressure is generally 200MPa, the compact density is 60% solid density) ).
One-way press (generally 20-30MPa pressure, 45% compact density) plus cold isostatic press (generally 200MPa pressure, 60% compact density) can be used for the two formings. .
During the orientation forming process, the alloy powder basically retains the state of the c-axis orientation alignment. After the pressing is completed, the blank is demagnetized (to eliminate the damage of the magnetic dipole interaction between the magnetic powder particles to the orientation of adjacent particles), and then demolded. A blank with a good orientation in the direction of easy magnetization can be obtained. NdFeB magnets orientation forming
The pressure up to 100 MPa will force the magnetic powder to obey the balance conditions of mechanical force and magnetic force, which will inevitably cause the movement or rotation of the magnetic powder particles, which may make the c-axis deviate from the direction of the external magnetic field and reduce the orientation degree of the blank. Therefore, the magnetic field formation process is to reasonably balance the relationship between the magnetic field strength and the forming pressure under the premise of reaching the blank density, so as to obtain the highest degree of orientation as possible.
The degree of powder orientation is also affected by the internal friction of the powder, especially when the bulk density is high. In actual production, an organic lubricant is used to reduce the internal friction, but it must be sintered before the sintering reaction occurs (usually around 200°C). Lubricant is completely removed to prevent lubricant oxidation or carbonization from reducing the performance of the magnet.
The forming process in actual production usually has the following three types:
Vertical pressing (horizontal pressing) system Transverse Direction Pressing, TDP
Axial Direction Pressing, ADP
Isostatic Pressing, IP (Isostatic pressing usually uses a liquid medium, and isostatic pressing using rubber as a medium is called Rubber Isostatic Pressing, RIP)
The most common one is vertical pressing. As the name implies, the magnetic field direction H is perpendicular to the pressing direction P; parallel pressing means that the magnetic field direction is parallel to the forming pressure; and isostatic pressing is to apply pressure evenly to the magnetic powder in all directions through a medium such as liquid or rubber mold. . In the case of the same process parameters such as magnetic powder filling, magnetic field strength, and forming pressure, the magnet obtained by isostatic pressing has the highest performance, followed by vertical pressing, and parallel pressing is the lowest. If the degree of orientation is measured by the ratio of remanence to saturation magnetization, RIP is as high as 94%~96%, TDP is 90%~93%, and ADP is only 86%~88%. The ratio between the three (BH) max can differ by 16~40kJ/m3 (2~5MGOe), this difference typically reflects the competition between mechanical pressure, magnetic dipole interaction and internal and external friction. NdFeB magnets orientation forming
Cold isostatic pressing is also often used for secondary pressing of unidirectional compacted blanks. In the case of limited orientation magnetic field, a lower pressure is used to obtain an appropriate degree of orientation, and then isostatic pressing is used to further increase the density of the green compact. Without destroying the existing orientation level. NdFeB magnets orientation forming