Magnetic field orientation forming is the most commonly used method to obtain anisotropic magnets by using the interaction between magnetic powder and external magnetic field to arrange the easy magnetization direction of powder particles so that they are consistent with the final magnetization direction of magnets. The powder preparation process breaks the Nd-Fe-B alloy into single crystal particles, which in turn are uniaxial anisotropic, with each particle having only one magnetic axis -- the c axis of the main phase cell. The powder is loosely filled into the mold, and the filling density is about 25%-30% of the real density. Under the action of the external magnetic field above 0.8A/m, the powder particles change from multiple domains to single domains, and the direction of easy magnetization is adjusted to the direction of the external magnetic field by rotation or movement.
In the industrial production, the pressing forming methods are divided into one - time forming and two - time forming.
One forming can be used unidirectional press (pressure is generally 50-100MPa, the density of the billet is 55%-60% of the real density) or cold isostatic press (pressure is generally 200MPa, the density of the billet is 60% of the real density).
Two forming can use unidirectional press (pressure is generally 20-30MPa, compact density is 45% of the real density) plus cold isostatic press (pressure is generally 200MPa, compact density is 60% of the real density).
In the process of orientation forming, the alloy powder basically retains the state of C-axis orientation. After the completion of pressing, the blank is demagnetized (to eliminate the damage of the magnetic dipole interaction between magnetic powder particles on the orientation of adjacent particles), and then the mold is removed. A blank with good orientation in the direction of easy magnetization can be obtained.
The pressure up to 100 MPa will force the magnetic powder to obey the equilibrium condition of mechanical force and magnetic force, which will inevitably cause the movement or rotation of the magnetic powder particles, which may make its c axis deviate from the direction of the external magnetic field and reduce the orientation of the blank. Therefore, the forming process of magnetic field is to balance the relationship between magnetic field intensity and forming pressure reasonably under the premise of reaching blank density, so as to obtain as high orientation as possible.
