Spherical powder with a special performance which the ordinary powder can not replaced, such as good mobility and high bulk density, so it is more widely used in the field of thermal spraying, injection molding and gel injection molding forming.
Because of the plasma with high temperature, high enthalpy, high chemical reactivity, and have controllability to the reaction atmosphere and the reaction temperature and other characteristics, display unique advantages in the synthesis of the powder material preparation and ball handling, cause people’s attention.
Currently, the plasma powder processing technology already has a considerable size production capacity. RF plasma technique has achieved the tungsten powder, molybdenum powder, rhenium powder, tantalum powder, nickel powder, copper powder and other spheroidizing treatment of metal powder.
From the 1980s, China has began to explore the use of plasma spheroidized tungsten powder, but there has been a slight oxidation phenomena on the spherical tungsten powder surface, in order to prepare pure spherical tungsten powder, it also need to conduct hydrogen reduction on the spheroidization processed powder, so that the production efficiency is low, cost more, difficult to achieve the requirements of industrial production.
Recently, scientists have developed the first plasma powder processing system in China, the system uses a water-cooled quartz plasma torch greatly reduce the amount of inert cooling gas, successfully avoid the oxidation of tungsten metal powder in spheroidization process. Use irregular shape tungsten powder as the raw material, the particle size in the range of 5.5 ~ 26.5¼m, through plasma nodulizing handling to obtain a smooth surface, good sphericity powder, its spheroidization rate reached 100%, and remains single phase tungsten powder after spheroidizing.
The bulk density of 26.5¼m particle size tungsten powder after spheroidizing was improved from 6.80g/cm3 to 11.5g/cm3, the powder liquidity also improved markedly. The studies show that such a radio-frequency plasma processing method can prepared micron single phase spherical tungsten powder, does not exist powder oxidation problem, high production efficiency, low cost, suitable for industrialized production requirements. They also studied the influence of process parameters to tungsten powder spheroidized, found the reasonable feed rate is an important factor to ensure the powder spheroidization rate. Excessive feed rate can not satisfy the need of tungsten powder absorb heat, melting and spheroidizing, also lead to part of the powder in the plasma running track deviate the plasma high-temperature region, make the powder absorbing not enough heat, resulting the processed powder reduce spheroidization rate. In addition, when the feeding rate is constant, the spheroidization rate reduced with the initial powder particle size increased. The smaller the initial powder particle size, the larger the special surface area, when the plasma absorb more energy in the crossing, it will more conducive to powder spheroidization. Generally, as powder particle size decreased, the melting point reduced, absorb the same amount of energy is easier to spheroidizing. Therefore, under the same process parameters, smaller particle size of the powder with higher spheroidization rate.
Use RF plasma spheroidizing other metals such as non-ferrous metal powder. RF induction thermal plasma prepared submicron copper powder, when copper powder through plasma, appears gasification, nucleation, condensation. However, the low melting point metal in the plasma sphere process inevitably occur evaporation and gasification, the prepared metal powder include a lot of fine spherical powder obtained by evaporation, coagulation, particle size range of the product distribution is broader. This is a problem to be solved.
Source:http://www.mhcmp.com
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