Powder metallurgy
The technology developed for the fabrication of metallic parts and for the development of alloy components is known as powder metallurgy. It may be defined as the technique of manufacturing metal powders, consolidating them and fabricating desired shapes o articles with little or no melting. This technology is highly useful in making desired shapes of articles with little or no melting. This technology is highly useful in making high melting alloy metals, which may be used in machine parts, self lubricant.
Powder metallurgy involves the following operations in sequence:
1.PREPARATION OF METAL POWDER
2.MIXING
3.COMPACTING
4.SINTERING
1. PREPARATION OF METAL POWDER:-There are mainly four chip processes:
(i)Mechanical pulverization:-Mechanical pulverization is done by the use of ball mills, counter rotating blades etc. Metal powders are also produced by shotting. In this method, molten metal is passed through an orifice and cooled by dropping it into water. The small spherical metal parts so obtained can be powdered further by pulverization. Magnesium powder is prepared by pulverization while aluminum powder involves shotting and pulverization methods.
(ii)Atomization:-This process is generally employed in metals with low melting points e.g. tin(232°C), Lead (327°C), Zinc(419°C). In this method, liquid is made to pass through a small orifice with the help of compressed air and then cooled in to powder. In this process, factors such as temperature and pressure are maintained appropriately. In atomization process, powders of different degrees of fineness can be obtained.
(iii)Reduction of metal oxide:-Metal powder can be obtained by reduction of metallic oxides. Tungsten metal can be obtained in powdered form by passing a stream of hydrogen and maintaining the temperature at 1200°C.
The powdered particles so obtained are sponge like and are suitable for cold processing. Spongy iron is produced by heating iron ore with charcoal at a constant temperature. Metallic powders of metals with high melting points are obtained in this method.
(iv) Electric deposition:-In this method, pure metallic powder of copper, iron, thorium etc., are obtained in the powdered form gets deposited on the cathode and can be scraped off.
Thorium powder is obtained by electrolysis of potassium thorium fluroxide fused with a mixture of equal parts of KCl and NaCl using graphite crucible as anode and molybdenum as the cathode.
2. MIXING OR BLENDING OF POWDER:-Uniformity of the finished powder is obtained by proper blending or mixing of the alloying powder. During mixing, lubricants and volatilizing agents are added to obtain the required porosity to get the blended powders. To ensure proper mixing electrically operated double cone mixers are used. The metal powder and other agents are mixed in exact proportions to get the specified product. Mixing can be either dry or wet mixing. In wet mixing water or some organic solvent is used to ensure proper mixing. Lubricants like graphite, lithium sterate metal soap and other long chain fatty acids are used to reduce friction during mixing or blending.
3. COMPACTING THE POWDER:-Articles of required shapes are produced when the metallic or blended metallic powder with non-metallic admixtures is pressured in to shape using suitable die in the compacting process. The die cavity is filled with powder. The pressure required for the process depends on the nature of the material. Required porosity and strength of finished powder can be obtained during the compaction process. There are 2 groups of compacting methods.
A. Pressure technique
a) Die compacting
b) Isotactic compaction
c) Vibrating method
d) Continuous method
B. Non-pressure techniques
a) Slip casing
b) Gravity method
4. SINTERING:-Heating of the compacted material in a furnace under controlled condition is called sintering. The process is carried out at a temperature below the highest melting point of the constituent of the compound powder mixture. Sintering is the process of bonding of powdered particles by atomic forces. This process of bonding should not contain free hydrogen. A dry hydrogen atmosphere is used in sintering of refractory carbides. The temperature and strength o the materials obtained are proportional to the time involved in heating.
Following three stages involved in sintering:
a.In the first stage, the newly formed bond areas grow in size and the pore shrinks.
b.In the second stage, neck formation takes place.
c.In the third stages, there is rounding of the central pore.
USES OF POWDER METALLUGY
1.It is a time, cost and labour saving technique in mass production.
2.High melting point alloys of varied proportions can be obtained using this technique.
3.Products of required characteristics can be obtained by mixing metals and non-metals of varied proportions.
4.Articles of required characteristics can be produced which are used for lubricating purpose.
5.Powder metallurgy is used to produce magnetic and electrical contact material, super hard cutting tools etc.
APPLICATIONS OF POWDER METALLURGY
1.Powder metallurgy techniques are used to produce self lubricating process alloy bearings eg. Bronze (10% Sn and 90% Cu).
2.Oil pump gears for automobiles are used to produce by mixing iron powder with graphite and then compacting and sintering in an electric furnace under and inert atmosphere of hydrogen.
3.Tungsten filament is produced from tungsten powder which is used for production of electric arc.
4.High melting metal powder of tungsten, tantalum, titanium, molybdenum etc., with carbon powder and bonding agents produce metallic carbides which are used to make cutting tools.
5.Refractory composites are produced when high melting metal powders are mixed with ceramics oxides, carbides, nitrides etc.
6.Permanent magnetic alloys of Al, Ni, Cu, Fe. Can easily produce by metallurgy techniques. Soft magnetic alloys, NI-Zn ferrite alloy etc can also be produced by same technique.
7.Powder metallurgy techniques are used in surgical implants which require replacement of bony skeleton of the human body with alloy metal structures.
LIMITATIONS F POWDER METALLURGY
1.It is cost effective only for mass production.
2.The machine parts so produced are not strong enough or as tough as the metals produced by other techniques such as casting, forging etc.
3.Big machineries cannot be produced by powder metallurgy techniques.
4.In many machine parts, final polishing, extraplating etc, has to be done.