Advantages of Powder Metallurgy
  • Virtually unlimited choice of alloys and non metallics with associated properties
  • *A variety of metal or non metal powders can be used.
  • *Refractory materials are popularly processed by PM.
  • Can be very economical for mass production (100,000 parts).
  • Long term reliability through close control of dimensions and physical properties.
  • Very good material utilization - loss of material very less.
  • Minimization or elimination of Machining.
  • Very good surface finish can be easily obtained.
Limitation of Powder Metallurgy

*Initial Investment cost high

*Limited part size and complexity

 *High cost of powder material.

 *High cost of tooling.
*Less stronger than wrought ones.
* Fracture toughness may be low.
*Less well known process.
* Health hazard to the operator due to very fine powder being processed.
 
Applications of Powder Metallurgy
  • Only method for shaping super alloys and tungsten carbides.
  • Automobile parts- A commercial US car contains approximately 15Kg of PM parts.
  • Aerospace –A commercial aircraft engine is composed of 700-2000Kg PM parts.
  • Gears, Cams, Sprockets, Cutting tools, Piston rings, connecting rods and many house hold goods are produced by PM technique
 
The following steps are used in the powder metallurgy techniques:
1. Production of Metal Powder
2. Blending of metal powders + additives.
3. Compaction of the blend using cold compaction & hot compaction methods. (Isostatic compaction, rolling, extrusion, injection moulding etc.,).
* A die is used for the above process to get net shaped green component.
4) Sintering of the green component at very high temperature to impart necessary hardness and strength to the component.
5) The above component is subjected to secondary & finishing operations such as forging, machining, coining, plating etc.,
 

Step 1. Production of Metal powders
*Metal powders are produced by using a  variety of techniques, the size of the particles range from 0.1 to 1000μ m. *Bulk metals and alloys, ores, and other compounds are used.*The shape, porosity, purity, size and distribution of the particles depends on the type of process used. The methods  normally used for the production of metal powder are
a. Atomization.
b. Reduction
c. Electrolytic deposition
d. Pulverization
e. Mechanical alloying
f.  Others

     Atomization.
Here the liquid metal stream is produced through a small orifice and the stream is broken by a jet of inert gas/water/air. Finely divided particles are obtained. The size of the particle depends on the temperature of the metal, flow rate, nozzle size & jet characteristics. A continuous uniform production of metal powders can be obtained .

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As the molten metal is entering the nozzle inert gas is forced through the annular space of the nozzle. This will create a very fine spray of metal and fine particles are collected in the chamber.

 

Rotating Electrode Method
 In this method a consumable electrode(Metal rod) is rotated rapidly in a helium gas filled  chamber.The centrifugal force breaks up the molten tip of the electrode, producing  particles.  
Opposite to the spindle tip a non rotating electrode establishes an arc which heats the metal electrode which is rotating. Tiny droplets of metal are formed. Electrolytic deposition

Here Electrolytic method using an aqueous solution is used for fine deposition of powders on
the cathode. The powder thus produced will be in its purest form.


Pulverization
In this method metals (brittle & less brittle) are crushed in a ball mill to produce small particles.  
In a ball mill, using rotating hollow cylinders, partly filled with steel or white cast iron balls, the metals are crushed.Repeated rotation of hollow cylinder results in crushing of the metal.Brittle metals will produce particles of angular shapes. Ductile metal will produce flake particles. (Hence, not suitable for powder metallurgy application).

Mechanical Alloy  
In this method, powders of two or more metals are mixed in a ball mill.
Due to impact of hard balls, the powders undergo fracture and weld together by diffusion, forming alloy powders. Other methods
Some of the methods which are used in addition to the above are:
Precipitation from a chemical solution.
Production of fine metal chips by Machining &  Vapor deposition.

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Size of powders 0.1 um – 1 mm    Sieve size quoted as mesh  number
*Particle D = 15/mesh number(mm)    325 mesh◊45 μ m
Step 2. Blending Metal powders
Here powders of two or more metals having different size & shapes are mixed to get a uniform
mixture.  
The ideal mixture consists of Particles having uniform distribution. Powders of different metals are mixed to improve physical & mechanical properties as required.To improve the flow characteristics, lubricants are mixed with the metal powder. This will insure proper filling of the dies and improves the die life. About 0.25 to 5% by weight Zinc stearate or stearic acid are used as lubricants. Mixing of powder must be carried out under controlled conditions to prevent contamination. 3.Compaction of Powder
*In this step, the blended powders are pressed into shapes in dies using presses  activated by hydraulic & mechanical means. The pressure is around 70-800Mpa.
*By pressing, the required shape with the desired density, with good particle to particle contact can be obtained.The pressed powder is referred to as green compact. Pressing is generally done at room  temperature, but can also be carried out at elevated temperature.*The density of the green compact depends on the pressure applied.*As the compacting pressure is increased, the density approaches that of the theoretical density of the metal in the bulk form.*If the size of the particles are the same then there will always be some open  spaces between the particles. This space is referred to as porosity.*In general, the porosity will be around 24% by volume.*Introducing smaller particles will fill the spaces between the larger particles and thus result in a higher density of the compact.*Higher the density, higher will be the strength and elastic modulus of the part. Methods of compaction
Powders are compacted by any one of the methods:
*Using a punch and a die.
*By Rolling.
*By Extrusion.
*By Injection moulding.
*By Isostatic pressing.


Using punch and a Die
*Here a punch and a die assembly is used.*The metal powder mixture is filled in the die and
the punch is forced on it. The powder gets compacted.*The range of pressure used for compaction is shown in the table.

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Source: https://elearningatria.files.wordpress.com/2013/10/vtu-e-notes-mpiii-18.pdf