Electric separation technology is the key technology to remove non-metallic inclusions in powder. Its principle is to charge the powder particles through corona discharge phenomenon, and then use the difference in electrical properties between the metal powder and non-metallic inclusions to separate. Figure 6 is a schematic diagram of the principle of high voltage electrostatic separation.
The electrostatic separation device is composed of two electrodes, the negative electrode is a thin metal wire, and the positive electrode is a large diameter metal drum which is grounded and has a certain speed. When the potential difference between the two stages reaches a certain value, corona discharge occurs between the two poles. After the metal powder containing non-metallic inclusion falls into the corona electric field formed by the feeder, it meets the electrons and negative ions flying to the positive electrode, making it carry the negative charge of the roller. Due to the high electrical conductivity of the metal powder, the obtained negative charge is released quickly after contact with the roller, and falls into the finished powder collection area from the front of the roller under the action of gravity and centrifugal force. The non-metallic inclusion is not easy to lose charge due to its low conductivity, and is adsorbed on the roller under the action of coulomb force and electrostatic adsorption, and is brushed down by the roller when it rotates to the rear with the roller. The high voltage corona electrostatic separator imported from Germany Hellas was applied to the high-temperature alloy powder mixed with Al2O3 inclusion in Beijing Iron and Steel Research Institute. The results show that the roller speed and corona voltage are the main factors affecting the separation effect. The higher the corona voltage, the slower the roller speed and the better the separation effect. The process adopts the best parameters (corona electrode voltage 40 kV, roll speed 50 r/min), and the removal effect of 50 ~ 100μm non-metallic inclusions is remarkable, and the removal rate can reach 76.7%[30]. The powder triboelectric separation and inclusion detection equipment developed by Beijing Institute of Aeronautical Materials has a good effect on the removal of inclusion. The research results show that the separation rate is as high as more than 85% after electric separation by the equipment, and the inclusion in the powder is detected [31]. In addition, the electric separation process of the high-temperature alloy powder must be completed under the protection of high purity inert gas to prevent the oxidation of the powder and the adsorption of O and C elements on the powder surface. In summary, the electric separation technology has been applied in the production of high-temperature alloy powder in China, but the separation range is narrow, and can only separate non-metallic inclusions within a certain particle size range, and the separation efficiency and separation effect need to be further improved, and 100% removal of non-metallic inclusions cannot be guaranteed. In the future, the electroseparation technology of superalloy powder should be developed in two directions: removing large particle inclusions and improving the removal effect.
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