Q.1 Explain powder metallurgy process. Also give its advantages and applications. (AKTU. 2010-11)
Ans. Power metallurgy is a manufacturing process in which the desired shape and size can be obtained from metal powders by compacting it in a die. The metal powder is forced into the die. In this process, the metal powder is compacted under high pressure in a die which has the shape of the object to be produced and then is sintered. Sintering is basically a heating process without melting the metal. Smaller products like balls used in ball point pens can be produced by this process.
Powder metallurgy is a mass production process which involves high initial investments in dies, presses and furnaces for sintering. Products manufactured by powder metallurgy are gears, tungsten carbide tool tips, self lubricating bearings, bulb filaments, etc.
Steps in Powder Metallurgical Process: -
Following are some important steps used in powder metallurgy process :
(a) Preparing the Metal Powder: -
Metal powders can be produced by grinding, chemical reduction, pulverization,etc. Metal powder can be produced in a ball mill by pulverizing small size pieces of metal. A ball mill consists of rotating drum which contains hardened steel balls. The metal to be pulverized is broken into tiny particles by continuous impact of hard steel balls, and is ultimately converted into a fine powder.
(b) Bleanding (Mixing of Fine Powder): -
Powders of different metals and non-metals are mixed together to obtain the desired properties in the final product.
(c) Compaction: -
During this process, the metal powder is filled in a rigid die. A high pressure is applied by means of movable punch. The tinely divided particles of the powder are forced together to constitute a strong mechanical bond. Pressure required for compacting the metal powder depends upon the desired porosity, type of powders used and shape and size of the product. Metal powders can also be compacted by pressing the steam of powder in between two rolls rotating in opposite direction. Roll compaction process is very similar to rolling process used in metal forming operations.
Roll compaction process is also called briquetting. The product after compaction is called green compact.
(d) Sintering: -
Compacted powder is brittle in nature and has lower strength. To improve the strength of compacted powder (green compact), sintering is employed. The briquette or green compact is heated below its melting point for a specified time period. The heating temperature is kept below the melting point of metallic constituents so that the pressed shape may be retained. Sintering time depends on the type of material used.
Sintering imparts strength to the green compact because the bond between the metal particles are increased by solid state diffusion. During the sintering process parts are protected from oxidation by using controlled atmosphere.
(e) Finishing Operations: -
These are the post-sintering operations in which produced parts are given required accuracy and surface finished. Operations like drilling, machining, grinding, etc may be performed on the sintered parts.
Schematic diagram of Powder metallurgy proces :
Applications: -
A variety of products can be manufactured by powder metallurgy process. For example-porous products like bearing, filters; ganges, wire drawing dies, wire guides; gears, electric motor bushes; can shaft, piston rings, rocker shaft, etc.
Advantages of Powder Metallurgy: -
(a) Production rate is high.
(b) Required dimensional accuracy and surface finish can be obtained.
(c) Complicated parts can be produced.
(d) Porous parts can be produced.
(e) Product life is long.
(f) Wastage of raw material is minimum.
(g) Metals and non-metal scan be mixed in required proportion.
Disadvantages: -
(a) Products produced by powder metallurgy may not possess good physical properties like cast parts.
(b) Cost of producing powder, compacting and sintering is high.
(c) Initial investment in the die is high.
(d) Large sized products can not be produced.
(e) Products have poor ductility.
(f) Machining of components is very difficult.
(g) Metallic powders occupy greater volume, and are difficult to store and may get oxidized while in store.
