Spheroidization of iron powder in a microwave and hybrid plasma torches

The process of porous iron powder spheroidization in microwave discharge and combined microwave and DC discharge modes in nitrogen and helium plasma was studied with powder particle sizes ranging from 45 to 85 μm. The powder was obtained by air spraying and subsequent hydrogen annealing. Plasma spraying produced hollow spheroidized particles with a wall thickness from 1 to 10 μm. The share of spheroidized powder particles in their total volume was determined. It was found that microwave power rising from 1.5 to 5 kW leads to a linear increase in the spheroidization degree of iron powder particles. When working in the hybrid plasmatron mode, microwave radiation conditions are combined with a DC discharge and make it possible to increase the plasma temperature. When the ratio of microwave and DC discharge power is 1 : 1, virtually 100 % iron powder spheroidization is obtained. The metallographic study of spheroidized particles showed that their final size differs from the initial one by about 10 times. It was found that iron powder oxidation occurs regardless of the spheroidization mode. This is due to the insufficient purification degree of plasma gases. The structure of particle surfaces when using nitrogen or helium as a plasma gas is different. Experiments showed that the use of helium is more preferable, since the particles have only a slight roughness in comparison with the particle structure during nitrogen spheroidization.

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