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In the previous article we learnt about the basics of power metallurgy and we will continue our study of the topic by going into the details of various processes involved in the powder metallurgy process.
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Metallic Powder – How to prepare it?
Since power metallurgy is all about working with metallic powders, the first question that would arise in the mind of the reader is that where does this powder come from, or how is it manufactured? There are several methods for preparation of the powder and some of them are listed as follows
- Pulverization – as you would expect a simple method of creating metal powders is to crush or pulverize solid metal pieces. But remember we are not talking about a solid piece of salt but metals hence this method requires the application of a lot of force and is only suitable for brittle materials such as Antimony. Pulverization can be carried out using various types of machines such as hammering machines or counter rotating plates and ball milling. Another interesting method is to pour melted metal through a sieve and letting it drop into water so that the small grains immediately loose heat and get solidified and thus get converted to powder which can be dried to remove water. The exact method depends on the type of metal to be pulverized and the grain size of the powder also depends on the method used.
- Atomization – this process is somewhat similar to the water sieve process described above, and the only difference is that instead of dropping molten metal into water, it is broken down by use of a stream of compressed air. This process requires special nozzles and carefully controlled conditions of temperature and pressure. One drawback of this method is the oxidization of the molten metal upon coming in contact with air and hence it is used for such metals like Zinc, Cadmium etc.
- Electrolysis – this is the normal electrolytic process where a salt of the metal under consideration is made to go through the electrolytic process so that the metal gets deposited at the electrodes in powder form which is then removed, washed and dried to get the desired powder. Mainly used for metals such as Silver, Zinc and Tin this method has the advantages of producing oxidation resistant powders.
There are several other processes as well such as chemical reduction which are used for suitable materials. The process of powder manufacturing does not end here but the powder so obtained is again crushed to a finer quality and annealed so that any work hardening is removed. The powder is then graded depending on the fineness of the final powder which is done via screening, and then the various grades are labeled according to their specification.