Preparation Process of Sintered NdFeB Magnets - Orientation Forming(2)

The orientation of powder is also affected by the friction force within the powder, which is especially serious when the bulk density is large. In actual production, organic lubricant is used to reduce the internal friction force, but the lubricant must be completely removed before the sintering reaction (usually around 200℃) to prevent the oxidation or carbonization of the lubricant from reducing the performance of the magnet.

The forming process in actual production usually has the following three kinds:

· vertical pressure (horizontal) system of Transverse Direction Pressing, TDP

· parallel pressure (Axial) system of Axial Direction Pressing, ADP

· Isostatic Pressing Isostatic Pressing, IP (such as static pressure usually USES the liquid medium, and use the Isostatic Pressing of Rubber for medium called Rubber moulded Rubber Isostatic Pressing, RIP)

One of the most common is vertical pressure, as the name implies that the magnetic field direction H is perpendicular to the compression direction P; Parallel pressure means that the magnetic field direction is parallel to the forming pressure; Isostatic pressure is applied evenly to the magnetic powder in all directions through a medium such as liquid or rubber mold. Under the same process parameters, such as magnetic particle filling, magnetic field strength and forming pressure, the performance of magnets obtained by isostatic pressure method is the highest, followed by vertical pressure and the lowest by parallel pressure. If the orientation is measured by the ratio of remanent magnetization to saturated magnetization, RIP is as high as 94%~96%, TDP is 90%~93%, and ADP is only 86%~88%. The (BH) max difference among the three can be 16~40kJ/m3 (2~5MGOe). This difference typically reflects the competitive relationship between mechanical pressure, magnetic dipole interaction, and internal and external friction.

Cold isostatic pressure is also often used for secondary pressure of unidirectional pressing blank. In the case of limited orientation magnetic field, lower pressure is used to obtain the proper orientation, and then isostatic pressure is used to further improve the density of blank without damaging the existing orientation level.