MAKING OF METAL POWDERS - POWDER METALLURGY

The powders of almost all metals and of a large quantity of alloys are used at the present times. The powders most commonly used are copper-base and iron-base materials. But stainless steel, titanium, nickel, chromium, metal powders are also used. Amongst powder properties, composition, size, form and structure of particle, specific surface, porosity and volume characteristics, fluidity, strength, hardness, permeability, regarding liquids and gases, electric conductivity, compressibility and sinterability are of great interest in powder metallurgy.

The particle size of powders fall into a range of 0.1u to several millimeters (1u = 10^-6 mm). In the majority of powders , the size of the particles varies from the several microns to 0.5 mm. There are various methods of producing powders of those size. The most commonly used methods are mechanical, atomization, reduction, electrolysis, and shotting.

Mechanical, In this method metals are disintegrated to produce the size of the powder required by crushing, rolling and milling. In the final stage the crushed metal is finally ground in a ball mill in which many steel balls impinge upon the powder to grind it to the needed size.

Atomization is the process, molten metal is forced through a nozzle into a stream of water or air. The air is usually supplied at a pressure if about 2 to 3 * 10^-2 kgf/mm2 (2 to 3* 10N/m2). Upon contact with the stream, the molten metal is solidified into particles by the air pressure, nozzle size and metal flow rate.

Reduction, This is a process adapted for some of the refractory metals. For example, pulverized tungsten oxide is heated in a current of hydrogen to produce a fine tungsten powder.

Iron powder is also produced by reducing iron chloride in hydrogen, Copper is another metal for which chemical reduction can be used.

Electrolysis, In this process the metal plates are placed in a tank of electrolyte which is an acid solution. The plates acts as a nodes, while other plates are placed into the tank to act as cathodes. High electric current produces a powdery deposit on the cathodes. The cathodes are removed from the tank, rinsed to removed the electrolyte solution and then dried. After a drying period, the deposit is scraped off and pulverized to produce powder of the desired size.

Shotting, In this method the molten metal is made to pass through a sieve and the particles are dropped in a liquid (water or kerosene).

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Mechinopedia For Mechanical Engineers

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