A basic information regarding POWDER METALLURGY

1.  Powder Metallurgy is the process of blending
fine powdered materials, pressing them into a
desired shape or form (compacting), and then
heating the compressed material in a controlled
atmosphere to bond the material (sintering and
presintering).
 The final component usually does not require
further machining processes.

2. There are 2 main requirements of any material
for it to be processed by powder metallurgy:
1. The metal in the powder form must be able to
respond to solid phase welding.
2. The metal powder must be capable of
sufficiently close packing under pressure to
permit welding to take place and In case of
alloys, be capable of being sufficiently and
intimately mixed.

3.  The early Egyptians made metal products by application
of powder metallurgy principles as early as 3000 B.C.
 In 1829, an Englishman made ductile platinum by cold
pressing and sintering.
 In 1870, Auervan Welsbach prepared osmium filament
by using powder metallurgy.
 In 1916,the first commercial tungsten wire was
produced by this technique.
 Powder metallurgy was used in Germany for producing
tungsten carbide tool tips after the first world war.
 Powder metallurgy became widely recognised after
1945 and today, parts which cannot be formed using
other processes are made using this process.

4.  Porous products like bearings and filters.
 Refractory parts made from tungsten and molybdenum used in
electric bulbs and radio valves, x ray tubes ,etc
 Products of complex shapes like gears which need complex
machining when made by other processes.
 Automotive components like electrical contacts
, crankshafts,camshafts, piston rings , brackets, brake
linings, connecting rods ,etc.
 Products from materials that are very difficult to machine like
tungsten carbide , etc Components are gauges , wire drawing
dies, wire guides , deep drawing, stamping and blanking tools
stone hammers, rock drilling bits.
 Tungsten parts used in high temperature operations like nozzles
for rockets and missiles.
 Grinding wheels, clocks and other timing devices ,

5.  The performance of metal powders during processing
and the properties of the product are highly dependent
on the characteristics of the metal powders that are used.
 The most important characteristics of metal powders are :
1. Purity
2. Chemical composition
3. Particle size
4. Particle shape
5. Size distribution
6. Particle microstructure
7. Apparent Density
8. Flow rate

7. Al-Si
microstructure

10.  The parts made by powder metallurgy are
usually brittle and difficult to handle before
they are sintered.
 To overcome this, the compacted parts are first
heated at a temperature lower than the actual
sintering operation temperature.
 This gives the parts sufficient hardness and
strength to be handled and machined as
required.
 Presintering also removes lubricants and
binders added to the powders earlier.

11.  After being compressed into the required
shape, the required components are sintered.
 Sintering is done to achieve the maximum possible
hardness and strength needed in the final product.
 Sintering is usually done at 70- 80% of the metal’s
melting temperature in an inert atmosphere of
hydrogen, ammonia or other hydrocarbons.
 The sintering time varies from thirty minutes to
several hours depending on the metal used.
 Sintering causes the bonding of the solid particles
within the component. Once cooled, the powder
bonds to form a solid piece.

13.  Metal powders are contained in an enclosure e.g. a rubber
membrane or a metallic can that is subjected to isostatic,(that is
uniform in all directions,) external pressure. As the pressure is
isostatic, the as-pressed component is of uniform density. This will
then be sintered in a suitable atmosphere to yield the required
product.
 Normally this technique is only used for semi-fabricated products
such as bars, billets, sheet, and roughly shaped components, all of
which require considerable secondary operations to produce the
final, accurately dimensioned component. Again, at economical
working pressures, products are not fully dense and usually need
additional working such as hot extrusion, hot rolling or forging to
fully densify the material.

15. 1. Graphite: Refractory graphite, Control rods for
nuclear reactors.
2. Ferrites: Parts like permanent
magnets, computer memories ,electronic
components
3. Ceramics : Tubes, tiles, nozzles and linings.
4. Metal powders : High speed tool
steels, superalloys, stainless steel, titanium
alloys , berylium , etc.

16.  Uniform Density of Compacted objects
 High strength and good handling properties of
compacted body
 Reduction in internal stresses
 Possibility of combining powders without
additional binding compounds
 Low tool costs
 Low material and finishing costs

17.  Dimensional control of the compacted objects is
a little less precise than other methods.
 The surfaces of pressed components are not
very smooth.
 This process is generally a low production
process and is thus used rarely.
 The flexible molds used in CIP dies usually
have a short life.

18.  The dimensional accuracy and the desired surface finish
can be maintained.
 Cleaner and quieter operation.
 Longer life of components due to enhanced
microstructure and lack of defects.
 High production rates even with complex shapes.
 Highly qualified labor not required for operation.
 99.9% utilization of material, negligible amount of scrap.
 Certain mixtures or alloys can be obtained through this
process. (for eg. It is very difficult to produce lead and
copper alloys in the liquid form. However, the problem of
immiscibility can be averted by mixing both elements in
the powder form and then shaping it by powder
metallurgy.

19.  It is difficult to produce certain shapes which require
the flow of metal which is not possible in powder form.
 The parts made by powder metallurgy may be brittle if
not sintered properly.
 The dies for this method are quite expensive.
 The size of the products that can be manufactured is
limited due to the high costs of large dies and material
handling.
 Powder metallurgy is un-economical for small scale
production.
 Some metals are difficult or impossible to compress
due to their tendency of sticking to the walls of the die
under high pressures.
Disadvantages of Powder
metallurgy over other processes