Magnetic field orientation forming uses the interaction between magnetic powder and external magnetic field to arrange the easy magnetization directions of the powder particles so that they are 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. 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. The filling density is about 25%-30% of the actual density. Under the action of an external magnetic field above 0.8A/m, these powder particles change from multi-domain to single-domain and are rotated or moved. The easy magnetization direction is adjusted to the direction of the external magnetic field.
In industrial production, 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 (pressure is generally 50-100MPa, green compact density is 55%-60% solid density) or cold isostatic press (pressure is generally 200MPa, green compact density is 60% solid density) ).
The two-step forming can use a one-way press (pressure is generally 20-30MPa, green compact density is 45% solid density) plus a cold isostatic press (pressure is generally 200MPa, green compact density is 60% solid density) .
During the orientation forming process, the alloy powder basically retains the c-axis orientation arrangement. After the pressing is completed, the blank is demagnetized (to eliminate the damage to the orientation of adjacent particles caused by the magnetic dipole interaction between magnetic powder particles), and then demolded. A blank with good orientation in the easy magnetization direction can be obtained.
Pressures as high as 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 cause the c-axis to deviate from the direction of the external magnetic field and reduce the orientation of the blank. Therefore, the magnetic field formation process is to reasonably balance the relationship between the magnetic field intensity and the forming pressure on the premise of achieving the density of the blank to obtain the highest possible orientation degree.
The degree of powder orientation is also affected by the internal friction of the powder. The impact is particularly serious when the bulk density is large. In actual production, organic lubricants are used to reduce the internal friction, but it must be removed before the sintering reaction occurs (usually around 200 degree ). The lubricant is completely released to prevent oxidation or carbonization of the lubricant from reducing the performance of the magnet.
The most common of them are vertical pressing, which means that the direction of the magnetic field H is perpendicular to the pressing direction P; parallel pressing means that the direction of the magnetic field is parallel to the forming pressure; and isostatic pressing applies pressure evenly to the magnetic powder in all directions through a medium such as liquid or rubber mold. . When the process parameters such as magnetic powder filling, magnetic field intensity, and forming pressure are the same, the performance of the magnet obtained by isostatic pressing is the highest, followed by vertical pressing, and parallel pressing is the lowest. If the degree of orientation is measured by the ratio of remanence and saturation magnetization, RIP is as high as 94%~96%, TDP is 90%~93%, and ADP is only 86%~88%, between the three (BH) max can differ by 16~40kJ/m3 (2~5MGOe). This difference typically reflects the competitive relationship between mechanical pressure, magnetic dipole interaction and internal and external friction forces.
