Methods of powders production
1-Physical Methods
1-1- Machining, Grinding and Filling:
Relatively coarse and bulky powders entirely free from fine particles are obtained by this method, and the powder particles are of irregular shape . This method has a limited application and employed as a raw material in the production of powder by other methods.
1-2-Milling:
Milling is used in the past to produce powders of the required grade of fineness and the powder particles are of irregular shape, employed as a raw material in the production of powder by other methods. It involves the application of impact force on the material being comminuted. The milling action is carried out by the use of wide variety of equipment such as ball mill, rod mill, impact mill, disk mill, eddy mill(produce a semi-spherical shape particles), vortex mill, etc. Figure 3 shows a schematic diagram of simple ball mill.
1-3- Shotting:
This method consists essentially in pouring a fine stream of molten metal through a vibrating screen into air or neutral atmosphere then it is fallen in water . In this way , molten metal stream is disintegrating into a large number of droplets which solidify as spherical particles during its free fall in air or into feathery particles if it solidify in water .
1-4- Condensation:
This method is used especially to produce a fine powder of Zinc of performed by evaporating and condensing of the respective material in a vacuum cylinder filled with a low pressure of an inert gass, the particle fog condenses preferably on cooled substrates that are implemented into a container. The particles build up a spherical powder layer on the cooling wall and can be removed continuously by a scraper.
1-5-Atomization
ATOMIZATION is the dominant method for producing metal powders with low melting temperature from aluminum, brass, zinc, cadmium, lead. Atomization is simply the breakup of a liquid into fine droplets. Any material available in liquid form can be atomized. The general types of atomization processes include: Water, gas and centrifugal. As shown in figures 4,5 & 6. An oxide film is formed in the case of air atomization and that film can be avoided by using an inert gas. Atomization can give a wide range of particle size (that make this method the most important method used for the production of powders) by controlling the following factors:
1- Diameter of the atomizing nozzle which the metal pass through.
2- Liquid metal superheat or viscosity.
3- Velocity, temperature and type of atomizing agent (air or inert gas).
