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INTRODUCTION
Powder metallurgy is concerned with the production of metal powders and converting them to useful shapes. It is a material processing technique in which particulate material are consolidate to semi finished and finished products. Generally the emphasis is on the metallic material but the principal of the process apply with little modification to ceramic, polymers and a variety of composite materials composed of metallic and non metallic phases. Nowadays powder metallic techniques are increasingly used to provide exceptional properties that are required in highly sophisticated aerospace electronic and nuclear energy industries. However an automobiles industry is the major consumer of powder metallurgy product today. There are two important reasons to use powder metallurgy by industries. Products like tungsten filament, tungsten carbide, porous self lubricating bearings etc. are either difficult or impossible to make by other methods. The other reason is that powder metallurgy process of manufacturing structural components competes with other manufacturing products such as casting machining and forging. Powder metallurgy process minimizes or eliminates the machining, and scrap losses at the same time is suited to high volume production of components. The process offers economy, savings in energy and raw materials along with mass production of quality precision components.
HISTORY OF POWDER METALLURGY
Powder metallurgy principle of shaping metallic objects without melting from powdered materials can be traced back to the early civilizations. These include the ancient Egyptian iron implants which date from at least 3000 B.C. In Greece the manufacture of iron components were widespread in 800-600 B.C. The manufacture of large objects were known to Indians as early as 300A.D. and the famous Delhi iron pillar weighing more then six tons is a typical master piece indeed . These are processed by direct reduction of iron oxide without melting, since the technology to obtain temperature high enough to melt pure iron was not available until about 1800. The significant development in the use of the powder metallurgy principle took place during the early part of nineteenth century for processing platinum and the credit to this is to be given to Wollaston in England and sobolevskiy in Russia. These developments ultimately led to the modern renaissance of powder metallurgy in the beginning of twentieth century with the manufacture of tungsten filaments for the incandescent lamp industry. The invention of electric lamp by Thomas Edison and Swan a century ago has contributed substantially to the rapid progress of this field. Powder metallurgy emerged as a new dimension in materials technology in twentieth century particularly during the world war period and subsequent years. Today the technology is used advantageously to process advanced material for the nuclear, electronics and aerospace industries. But in modern India the progress made in this field is mainly during the past two decades.
The Powder Metallurgy Process.
METAL POWDER TREATMENT
Annealing

It is customary that the powder producer delivers the powder to the fabricator ready for mixing. The aims of annealing are:
1) To soften the powder
2) To reduce the residual amount of oxygen, carbon and/or nitrogen from
the powder.
The annealing operation may be done in an atmosphere furnace or a vacuum furnace. The former may be of batch or continuous type. Annealing temperatures are kept as low as possible to minimize sintering.
Powder Mixing
The term ‘blending’ is strictly applied to a one component operation, whereas mixing involves more than one type of powder, e.g. mixing of solid lubricant with a metal powder or powders of several other metals. Sometimes the additive acts as lubricant as well as alloying addition, e.g. graphite in iron powder.
Various variables in the powder mixing process have been highlighted
by Hausner.1 They are:
1. Type of mixer
2. Volume of the mixer
3. Geometry of the mixer
4. Inner surface area of the mixer
5. Constructional material and surface finish of the mixer
6. Volume of the powder in the mixer before mixing
7. Volume of the powder in the mixer after mixing
8. Volume ratio of component powders
9. Volume ratio of mixer to powder
10. Characteristics of component powders
11. Type, location and number of loading and emptying devices
12. Rotational speed of mixer
13. Mixing time
14. Mixing temperature
15. Mixing medium (gaseous or liquid)
16. Humidity, when mixing in air.
Mixing efficiency is best when the powder volume is about 50% to 60% of the mixer volume. Optimum mixing time may be from between 5 to 30 minutes but this can be determined only by experience with a given mixture in a particular mixer. The aim is to mix the powders only as long as necessary to achieve a thorough mix and to fix a uniform apparent density of the mix from batch to batch. The apparent density of the mix tends to increase with mixing time.
Types of Mixers
Among various types of mixers available, the following are most common for metal powders (Fig.1):
Double Cone Mixer:
This consists of vertical cylinders with conical ends, which rotate about a horizontal axis. This rotation imparts a continuous rolling motion which spreads and folds the powders as they move in and out of the conical area. This action thoroughly mixes the powders with little or no change in the size and shape of the individual particles.
V-Mixer:
This is constructed by joining two cylinders of equal length into a ‘V’. As the ‘V’ rotates about its horizontal axis, the powder charge splits and refolds.
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INTRODUCTION
Powder Metallurgy is the Science of producing
metal powder and using them to make useful
objects which will be having some outstanding
characteristics than those produced by
conventional methods. In this metal powders of
different properties are subjected to a sufficiently
a high temp and press and compacted so that a
bonding takes place between them.
STEPS IN POWDER METALLURGY
Production of Metal powders.
Conditioning of powders.
Compaction.
Presintering.
Sintering.
Secondary operations.
Production of Metal powders
Mechanical :
Atomization.
Shafting & Graining.
Milling & Grinding.
Physico-chemical:
Reduction.
Electrolysis.
Shafting & Graining : In this process the molten metal is poured in a vibrating sieve screen. The molten metal will disintegrate into small droplets and solidifies in an conditioned environment.
Milling & Grinding : In this process the pulverisation of metal which are brittle in nature are crushed by using various types of crushers, rotary mills and grinders, In order to break down the metals by crushing and impact.
AUTOMIZATION
Reduction :

This is a method of obtaining powder of metals present in their oxide form. The oxides of these metals in their powdered form are reduced with carbon monoxide or hydrogen and the reduced powder is subsequently ground.
Ex Fe3o4 + 4 Co -------> 3 Fe + 4 Co2
Cu2o + H2 -------> 2 Cu + H2o
Electrolysis is similar to Electroplating technique. In this process the metal plates are placed in an electrolytic solution with one plate as anode and another plate as cathode. The conventional electrolysis process will be carried out.
Conditioning of powders
The Conditioning of powders involve blending and mixing of metal powders. Different quantities of powder of metals or even non-metals are mixed thoroughly in this stage to obtain the desired properties like heat resistant, wear resistant, toughness …etc.
Compacting ( Briquetting ) :
This is an important stage in powder metallurgy where metal powders are pressed into objects of desired shape and approximately to the final dimensions. Powders are compacted by using high pressures. Compacting is also designed to impart the desired level of porosity and to provide adequate strength for handling.
Different techniques of Compacting powders into various shapes are
1.Die pressing 2. Roll pressing 3. Extrusion
Presintering : Presintering is the process of heating the compacted object to a temperature upto 0.5Tm of the base metal. Presintering is carried out on those components which need some machining after compaction.
Sintering : Sintering is the process of heating the green compact or presintered compact to an elevated temperature usually in the range of 0.7 to 0.9 times of the base metal, with out applying the external pressure.
Secondary Operations :
Sizing.
Coining.
Impregnation.
Infiltration.
Plating.
Heat treatment.
Factors governs the quality of powder metallurgy components
Size of powder particles and distribution of particles.
Shape of metal powder.
Fineness.
Flowability.
Compactibility.
Advantages of Powder Metallurgy technique
Close dimensional tolerances and surface finish may be obtained.
There is no unwanted loss of material during fabrication.
Non-metallic substances can be introduced as required and in any proportion to get the desired properties.
A wide range of properties can be obtained.
High melting point metals and their carbides
can be easily produced by this process.
Porous objects like filters, oil impregnated bearings can be conveniently produced by this process.
Applications of Powder Metallurgy.
Porous and Self - Lubricating Bearings
Refractory parts such as filaments, cathodes, anodes used in electric bulbs x-ray tubes etc..
Toothed components like gears
Welding rods and electrodes
Carbide tipped tools