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
Fig.: Outline of Processes and Operations involved in making Powder Metallurgy parts.
The choice of a specific technique for powder production depends on particle size, shape, microstructure and chemistry of powder and also on the cost of the process.
Mechanical methods of powder production:
Chemical methods of powder production:
- Precipitation from solutions
- Thermal decomposition of compounds
- Hydride decomposition
- Electro- chemical methods
- Solid State Reduction
Physical methods of powder production:
- Water atomization
- Gas atomization
- Special atomization methods
A single powder may not have all the requisite properties and hence, powders of different materials are blended to form a final part with desired properties.
Blending is carried out for several purposes as follows:
- To imparts uniformity in the shapes of the powder particles.
- To facilitates mixing of different powder particles.
- To impart wide ranging physical and mechanical properties.
- To improve the flow characteristics of the powder particles reducing friction between particles and dies.
- To enhance green strength of parts by adding binders.
Blended powers are pressed in dies under high pressure to pressurize & bond the particles to form a cohesion among powder particles to impart. Required shape. • The work part after compaction is called a green compact or simply a green, (green means not yet fully processed.)
The compaction exercise imparts the following effects.
- Reduces voids and enhance density of consolidated powder.
- Improves green strength of powder particles.
- Facilitates plastic deformation of the powder particles to conform to the final desired shape of the part.
- Enhances the contact area among the powder particles and facilitates the subsequent sintering process.
Sintering bonds individual metallic particles, thereby increases strength and hardness of final part.
- Compressed metal powder is heated in a controlled- atmosphere furnace to a temperature (70% and 90% of Tm) below its melting point, but high enough to cause diffusion thereby bonding of neighbouring particles.
- Powder performs are heated in a controlled, inert or reducing atmosphere or in vacuum prevent oxidation.
- The primary driving force for sintering is not the fusion of material, but formation and growth of bonds between particles due to reduce of surface energy
- Part shrinkage occurs during sintering due to pore size reduction.
- Density increases due to filling up incipient holes and increasing area of contact among powder particles in compact perform.
Advantages, Limitations, and Applications
Powder metallurgy offers the following specific advantages.
- Parts can be produced from high melting point refractory metals with respectively less difficulty and at less cost.
- Production rates are high even for complex parts. This is primarily because of the use of automated equipment in the process.
- Near net shape components are produced. The dimensional tolerances on components are mostly such that no further machining is needed. Scrap is almost negligible.
- 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.
- Parts can be produced with impregnation and infiltration of other materials to obtain special characteristics needed for specific applications.
- Skilled machinists are not needed, so labour cost is low
- Parts with controlled porosity can be produced
- Bi-metallic products, sintered carbides and porous bearings can be produced only by this process.
- 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.
- High cost of tooling and equipment. This is particularly a limitation when production volumes are small.
- Large or complex shaped parts are difficult to produce by PM process.
- Parts have lower ductility and strength than those produced by forging.
- Uniformly high – density products are difficult to produce.
- Some powders (such as aluminum, magnesium, titanium and zirconium) in a finally divided state present fire hazard and risk of explosion.
- 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.
- Diamond Tools Industries
- Food Fortification
- Magnetic slot wedges
- Paints & Printing Inks
- Surface Coating
- Powder Metallurgy
- Soft Magnets
- Super Alloys
- Plasma Spray
- Welding electrodes
- Carbon Brushes
- 3D printing