Simple Preparation Processing of Spherical Tungsten Powder

In recent years, with the continuous development of science and technology, spherical tungsten powder is widely used in thermal spraying, porous materials and powder metallurgy industry. Its demand is also increasing. In the field of thermal spraying, spherical tungsten powder not only has good fluidity, but also has more uniform and dense coating so that the product has better abrasion resistance.

Compared with ordinary tungsten powder, spherical tungsten powder has two salient features. First, the appearance is spherical and the powder is of good fluidity. Second, high tap density. Preparation process of spherical tungsten powder at home and abroad mainly has the following ways:

First, the physical method.

1. Electric arc spraying gun method: DC arc welding machine is used in series for power supply. A simple SCDP-3 arc spraying gun arc torch is transmitted to control the arc fusion of two self-consumable tungsten wires, with compressed air atomized into spherical tungsten powder. The method has the advantages of simple equipment and low cost, but the production efficiency is low.

2. Microwave single-membrane cavity method: Put tungsten powder into the quartz glass tube and the tungsten powder is spheroidized in a single cavity sintering furnace under the action of microwave. This method is of simple-equipment, but the microwave heating of tungsten powder is of a certain limit. If tungsten powder particles are large, the spheroidization temperature is high. If tungsten powder particles are small, the spheroidization temperature is low, so that there will be sintering phenomenon during the spherical process of tungsten powder of different particle-size.

3. Plasma method: The ordinary tungsten powder added to the plasma jet fluid, so that the surface (or the whole) of tungsten powder particles melt to form droplets. The droplet shrinks to form a sphere due to the surface tension. The sphere is fixed by rapid cooling to obtain a spherical tungsten powder. In practice, the method requires a larger powder-cooling chamber and a large amount of argon gas to cool the heated tungsten powder, so the cost is high.

Second, the chemical method.

1. Vapor deposition method: A process of obtaining spherical tungsten powder with a large particle size (40-65 μm) from WF6 by vapor deposition. The process involves strong corrosive HF, and is of harsh working conditions and high demanding environmental requirements.

2. Tungsten powder reoxidation - reduction method: Tungsten powder by a certain temperature, oxidation time and air atmosphere to form a special tungsten oxide. Special tungsten oxide is reduced secondly by the same process. The method is China's main technology of preparation of spherical tungsten powder currently. The process is long and difficult to control.

3. Ammonium paratungstate redox method: Take the pure ammonium paratungstate as raw material and calcined it in argon into violet tungsten whose reduction of tungsten powder in the air is oxidized to tungsten trioxide. Tungsten trioxide is reduced in hydrogen. The method has long process-route, and is difficult to control and has great ammonia-pollution.

4. Ammonium tungstate ultrasonic stirring - drying - reduction method: The saturated solution of ammonium tungstate and dispersant ultrasonic-agitation is mixed evenly. Add concentrated sulfuric acid, the precipitate is produced during the stirring. The precipitate is filtered off, dried and crushed. After that it is placed to hydrogen reduction furnace to obtain spherical tungsten powder reduction. The method has long process-route and process-parameter is difficult to control, and the generated waste liquid has great pollution.

At present, the research and development of spherical tungsten powder is still developing. All of the above physical and chemical methods are representative at the present stage, but generally they have own deficiencies. In comparison, although the development of the plasma method which prepares spherical tungsten powder isn’t so mature, the spherical tungsten powder prepared by the method has the advantages of good quality, high purity and even size-distribution. Therefore, the plasma method is one of the main way for the development of spherical tungsten powder.

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