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Introduction To Powder Metallurgy | Principles of Powder Metallurgy Process

What is Powder Metallurgy Process | Advantages and Disadvantages Of Powder Metallurgy 

Introduction Of Powder Metallurgy
The process of manufacturing of shaped components or semi-finished products such as bar and sheet from metal powder is called as Powder metallurgy. 
The technique of powder metallurgy combines unique technical features with cost effectiveness and generally used to produce sintered hard metals known as ‘carbides’ or ‘tungsten carbides’. 
This technique deals with the production of metal and non metal powders and manufacture of components. 
Powder metallurgy is generally used for iron based components.  
The powders used as raw material can be elemental, pre-alloyed, or partially alloyed. 
Elemental powders like iron and copper are more compressible and produce pressed compacts with good strength.  
Pre-alloyed powders are harder but less compressible therefore require higher pressing loads to produce high density compacts.
Powder metallurgy technique has many advantage as well as limitation. 
Two main techniques used to form and consolidate the powder are sintering and metal injection molding. Recent developments have made it possible to use rapid manufacturing techniques which use the metal powder for the products. Because with this technique the powder is melted and not sintered, better mechanical strength can be accomplished.

Principles of Powder Metallurgy Process

Powder metallurgy is the process of blending fine powdered materials, compacting the same into a desired shape or form inside a mould followed by heating of the compacted powder in a controlled atmosphere, referred to as sintering to facilitate the formation of bonding of the powder particles to form the final part. Thus, the powder metallurgy process generally consists of four basic steps, as indicated in Figure 

(1) powder manufacture, 
(2) blending of powders, 
(3) compacting of powders in a mould or die, and
(4) sintering. 

Compacting is generally performed at room temperature and at high pressure. Sintering is usually done at elevated temperature and at atmospheric pressure. Often, compacting and sintering are combined. Optional secondary processing often follows to obtain special properties or enhanced dimensional precision. Powder Metallurgy route is very suitable for parts that are required to be manufactured from a single or multiple materials (in powder form) with very high strength and melting temperature that pose challenge for the application of casting or deformation processes.

Steps In Powder Metallurgy
Steps In Powder Metallurgy
Advantages Of Powder Metallurgy:

 Metal in powder form is costlier than in solid form. Further, expensive dies and equipment needed to adapt this process implies that the process is justified by the unusual properties obtained in the products. Powder metallurgy offers the following specific advantages.
  1. Parts can be produced from high melting point refractory metals with respectively less difficulty and at less cost.
  2. Production rates are high even for complex parts. This is primarily because of the use of automated equipment in the process.
  3. Near net shape components are produced. The dimensional tolerances on components are mostly such that no further machining is needed. Scrap is almost negligible.
  4. Parts can be made from a great variety of compositions. It is therefore much easy to have parts of desired mechanical and physical properties like density, hardness toughness, stiffness, damping, and specific electrical or magnetic properties.
  5. Parts can be produced with impregnation and infiltration of other materials to obtain special characteristics needed for specific applications.
  6. Skilled machinists are not needed, so labour cost is low
  7. Parts with controlled porosity can be produced
  8. Bi-metallic products, sintered carbides and porous bearings can be produced only by this process.
Limitations Of Powder Metallurgy :

 Powder metallurgy has the following limitations.
  1. High cost of metal powders compared to the cost of raw material used for casting or forging a component. A few powders are even difficult to store without some deterioration.
  2. High cost of tooling and equipment. This is particularly a limitation when production volumes are small.
  3. Large or complex shaped parts are difficult to produce by PM process.
  4. Parts have lower ductility and strength than those produced by forging.
  5. Uniformly high – density products are difficult to produce.
  6. Some powders (such as aluminum, magnesium, titanium and zirconium) in a finally divided state present fire hazard and risk of explosion.
  7. Low melting point metal powders (such as of zinc, tin, cadmium) give thermal difficulties during sintering operation, as most oxides of these metals cannot be reduced at temperatures below the melting point.
Applications of Powder Metallurgy:

          There is a great variety of machine components that are produced from metal powders, many of these are put to use without any machining operation carried out on them. Following are some of the prominent PM Products.

Filters:
 Permanent metal powder filters have greater strength and shock resistance than ceramic filters. 

Cutting Tools and Dies. 
Cemented carbide cutting tool inserts find extensive applications in machine shops. These are produced by PM from tungsten carbide powder mixed with cobalt binder.

Machinery Parts. 
Several machinery parts including gears, bushes and bearings, sprockets, rotors are made from metal powders mixed with sufficient graphite to give to product the desired carbon content. The parts have nearly 20 percent porosity. The pores of the parts which are to rub against another surface in their use, are impregnated with oil to promote quiet operation.

Bearing and Bushes. 
Bearing and bushes to be used with rotating parts are made from copper powder mixed with graphite. In small quantities, lead or tin may also be added for obtaining better wear resistance. After sintering, the bearings are sized and then impregnated with oil by vacuum treatment. Porosity in the bearings may be as high as 40 percent of the volume. Other machinery parts made by PM include clutch plates, brake drums, ball retainers and welding rods.

Magnets. 
Small magnets produced from different compositions of powders of iron, aluminum, nickel and cobalt have shown excellent performance, far superior to those cast.

Electrical Parts. 
The possibility of combining several metal powders and maintaining some characteristics of each has promoted PM for production of electric contact parts. These parts are required to have excellent electrical conductivity, be wear resistant, and somewhat refractory. Several combinations such as copper – tungsten, cobalt – tungsten, silver – tungsten, copper-nickel, and silver – molybdenum have been used for production of these parts.

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