Magnetic field sintering of ferromagnetic metal powder

To improve the properties of the alloy by controlling the grain boundary microstructure technology has received increasing attention, it is widely studied thermomechanical processing techniques used to control the grain size (grain boundary density), the grain boundary character distribution (GBCD) and grain boundary connectivity Wait. In addition, techniques for applying potential energy (such as magnetic fields, electric fields, ultrasonic vibrations, and temperature gradients) are also employed. Among them, the application of external magnetic fields has attracted more attention in the material processing industry because it can control the microstructure more precisely. So far, it has been found that the applied magnetic field for recrystallization of ferromagnetic materials, and other metallurgical phenomena affect the phase transition behavior of precipitation are very large. Therefore, researchers at Tohoku University in Japan have done a lot of research in this area. This time, the sintering behavior of iron powder and cobalt powder under external magnetic field was studied. The original materials used were 99.9% pure carbonyl iron powder and 99.5% pure carbonyl cobalt powder. Their average particle size was 2.3μm. And 0.8 μm, the shape of the iron powder is spherical, and the cobalt powder is a polyhedral shape. These metal powders were subjected to a deoxidation treatment of 673 K × 3.6 ks in an argon gas flow before the investigation to remove oxides adhering to the surface. A compact having a diameter of 10 mm × a height of 3 mm was pressed by a pressure of 200 MPa, and sintered in an infrared sintering furnace. During the sintering process, an external magnetic field is applied in a direction parallel to the axis of the cylindrical sample, followed by temperature rise. The applied DC magnetic field is gradually increased to 1.2 MA/m (15 kOe). The iron powder compact is magnetically sintered at a ferromagnetic temperature range of 873 to 973 K under a vacuum of 5×10-3 Pa, and also sintered at 1,123 K paramagnetic temperature for 5, 20, 50 and 100 h; the cobalt powder compact is at 1173 K ferromagnetic Sintered at temperature for 5, 20, 50 h.
The research results show that the magnetic field sintering can effectively increase the density of iron powder and promote grain growth. The stronger the magnetic field, the higher the degree of densification, especially at the intermediate stage of sintering. It is considered that the magnetic field has an effect of enhancing the driving force for grain boundary migration, so it plays an important role in densification at the time of sintering. Compared with the iron powder compact, the magnetic field plays a role in suppressing the densification of the cobalt powder compact.

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