Spherical Tungsten Powder Decomposition by RF Plasma Technology

Spherical powders have special properties which can not be replaced by ordinary powders, such as good fluidity and high bulk density. Therefore, spherical powders are widely used in thermal spraying, injection molding and gel casting. Because of its high temperature, high enthalpy, high chemical reactivity, controllable reaction atmosphere and temperature, plasma has shown unique advantages in the synthesis and spheroidization of powders and attracted people's attention.

Radio frequency plasma technology has the advantages of high temperature, large volume of plasma torch, high energy density, fast heat transfer and cooling speed. It is a good way to prepare spherical powder with uniform composition, high sphericity, few defects and good fluidity. Plasma as heat source has great technical advantages in refractory metal spheroidization. However, due to the defects of the existing technology, the spheroidization rate of powders after plasma treatment is low, which makes the bulk density and fluidity of powders not reach the optimal state. Therefore, there is much room for improvement in the existing technology.

In order to solve the above technical problems, some scholars have improved the radio frequency plasma technology. The specific contents are as follows:

The raw material powder is selected as irregular tungsten powder, sifted 200 meshes, purity > 99.9%. The raw material powder is put into air grinding to grind, and the powder with good dispersion and uniform particle size distribution is obtained. The separating wheel speed of air grinding is 2500-7000 rpm, the grinding chamber pressure is 0.1 MPa, and the feeding rate is 1-15 kg/min. The raw material tungsten powder is fully dried and loaded into the powder supply chamber, and the air is isolated from the atmosphere. The RF plasma generator first evacuates, then passes in the working gas argon (Ar), starts the RF plasma generator, and establishes a stable argon plasma torch.

When the argon plasma torch runs steadily, the input velocity of the reaction gas argon is 20 L/min, the pressure of the reaction gas argon is 65 kPa, and the flow velocity of the protective gas is 85 L/min. The pressure of protecting gas is 65 kPa, and the power loaded on induction coil is 75 kW. Tungsten powder is heated in the high temperature zone of argon plasma torch with hydrogen gas (H₂) loaded. The flow rate of loading gas is 4 L/min, and the flow rate of tungsten powder is 5 g/min.

The droplets of tungsten powder formed by heating condense rapidly into spherical tungsten powder in the cooling chamber. The spherical tungsten powder is collected and dried in the drying chamber, then slightly reduced.

Because of the optimization of plasma working parameters and powder feeding speed, tungsten powder can fully absorb heat and get melting and spheroidizing when it passes through the high temperature zone of plasma. After plasma spheroidization, the powder with smooth surface and good sphericity can be obtained. The spheroidizing rate reaches 100%, and the spheroidizing rate is 100%. It is still single-phase tungsten powder. Because of the good sphericity, the contact surface of these spherical particles is small, the bridging phenomenon is reduced, the voids between particles are small, and the packing density is increased. The higher the sphericity of powder, the larger the bulk density is. Therefore, the spherical tungsten powder has the characteristics of high bulk density and high fluidity.

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