Define Powder Metallurgy

Powder Metallurgy
Powder metallurgy is used for manufacturing products or articles from powdered metals by placing these powders in molds and are compacting the same using heavy compressive force.

Typical examples of such article or products are grinding wheels, filament wire, magnets, welding rods, tungsten carbide cutting tools, self-lubricating bearings electrical contacts and turbines blades having high temperature strength.

The manufacture of parts by powder metallurgy process involves the manufacture of powders, blending, compacting, profiteering, sintering and a number of secondary operations such as sizing, coining, machining, impregnation, infiltration, plating, and heat treatment.

The compressed articles are then heated to temperatures much below their melting points to bind the particles together and improve their strength and other properties.

Few non-metallic materials can also be added to the metallic powders to provide adequate bond or impart some the needed properties. The products made through this process are very costly on account of the high cost of metal powders as well as of the dies used.

The powders of almost all metals and a large quantity of alloys, and nonmetals may be used. The application of powder metallurgy process is economically feasible only for high mass production.

Parts made by powder metallurgy process exhibit properties, which cannot be produced by conventional methods. Simple shaped parts can be made to size with high precision without waste, and completely or almost ready for installation.

Powder Metallurgy Process:-

The powder metallurgy process consists of the following basic steps:
  • Formation of metallic powders.
  • Mixing or blending of the metallic powders in required proportions.
  • Compressing and compacting the powders into desired shapes and sizes in form of articles.
  • Sintering the compacted articles in a controlled furnace atmosphere.
  • Subjecting the sintered articles to secondary processing if needed so.

Production of Metal Powders

Metallic powders possessing different properties can be produced easily. The most commonly used powders are copper-base and iron-base materials. But titanium, chromium, nickel, and stainless steel metal powders are also used.

In the majority of powders, the size of the particle varies from several microns to 0.5 mm. The most common particle size of powders falls into a range of 10 to 40 microns.

The chemical and physical properties of metals depend upon the size and shape of the powder particles. There are various methods of manufacturing powders.

The commonly used powder making processes are given as under.
  1. Atomization
  2. Chemical reduction
  3. Electrolytic process
  4. Crushing
  5. Milling
  6. Condensation of metal vapors
  7. Hydride and carbonyl processes.

1. Atomization

In this process, the molten metal is forced through an orifice and as it emerges, a high pressure stream of gas or liquid impinges on it causing it to atomize into fine particles. The inert gas is then employed in order to improve the purity of the powder. It is used mostly for low melting point metals such as tin, zinc, lead, aluminium, cadmium etc., because of the corrosive action of the metal on the orifice (or nozzle) at high temperatures. Alloy powders are also produced by this method.

2. Chemical Reduction Process

In this process, the compounds of metals such as iron oxides are reduced with CO or H2 at temperatures below the melting point of the metal in an atmosphere controlled furnace. The reduced product is then crushed and ground.

3. Electrolytic Process

Electrolysis process is quite similar to electroplating and is principally employed for the production of extremely pure, powders of copper and iron. For making copper powder, copper plates are placed as anodes in a tank of electrolyte, whereas, aluminium plates are placed in to the electrolyte to act as cathodes. High amperage produces a powdery deposit of anode metal on the cathodes. After a definite time period, the cathode plates are taken out from the tank, rinsed to remove electrolyte and are then dried. The copper deposited on the cathode plates is then scraped off and pulverized to produce copper powder of the desired grain size. The electrolytic powder is quite resistant to oxidation.

4. Crushing Process

The crushing process requires equipments such as stamps, crushers or gyratory crushes. Various ferrous and non-ferrous alloys can be heat-treated in order to obtain a sufficiently brittle material which can be easily crushed into powder form.

5. Milling Process

The milling process is commonly used for production of metallic powder. It is carried out by using equipments such as ball mill, impact mill, eddy mill, disk mill, vortex mill, etc. Milling and grinding process can easily be employed for brittle, tougher, malleable, ductile and harder metals to pulverize them. A ball mill is a horizontal barrel shaped container holding a quantity of balls, which, being free to tumble about as the container rotates, crush and abrade any powder particles that are introduced into the container. Generally, a large mass to be powdered, first of all, goes through heavy crushing machines, then through crushing rolls and finally through a ball mill to produce successively finer grades of powder.

6. Condensation of Metal Powders

This process can be applied in case of metals, such as Zn, Cd and Mg, which can be boiled and the vapors are condensed in a powder form. Generally a rod of metal say Zn is fed into a high temperature flame and vaporized droplets of metal are then allowed to condense on to a cool surface of a material to which they will not adhere. This method is not highly suitable for large scale production of powder.

7. Hydride and Carbonyl Processes

High hardness oriented metals such as tantalum, niobium and zirconium are made to combine with hydrogen form hydrides that are stable at room temperature, but to begin to dissociate into hydrogen and the pure metal when heated to about 350°C. Similarly nickel and iron can be made to combine with CO to form volatile carbonyls. The carbonyl vapor is then decomposed in a cooled chamber so that almost spherical particles of very pure metals are deposited.

Advantages of Powder Metallurgy:-

  • The processes of powder metallurgy are quite and clean.
  • Articles of any intricate or complicated shape can be manufactured.
  • The dimensional accuracy and surface finish obtainable are much better for many applications and hence machining can be eliminated.
  • Unlike casting, press forming machining, no material is being wasted as scrap and the process makes utilizes full raw material
  • Hard to process materials such as diamond can be converted into usable components and tools through this process.
  • High production rates can be easily achieved.
  • The phase diagram constraints, which do not allow an alloy formation between mutually insoluble constituents in liquid state, such as in case of copper and lead are removed in this process and mixtures of such metal powders can be easily processed and shaped through this process.
  • This process facilitates production of many such parts, which cannot be produced through other methods, such as sintered carbides and self-lubricating bearings.
  • The process enables an effective control over several properties such as purity, density, porosity, particle size, etc., in the parts produced through this process.
  • The components produced by this process are highly pure and bears longer life.
  • It enables production of parts from such alloys, which possess poor cast ability.
  • It is possible to ensure uniformity of composition, since exact proportions of constituent metal powders can be used.
  • The preparation and processing of powdered iron and nonferrous parts made in this way exhibit good properties, which cannot be produced in any other way.
  • Simple shaped parts can be made to size with 100 micron accuracy without waste.
  • Porous parts can be produced that could not be made in any other way.
  • Parts with wide variations in compositions and materials can be produced.
  • Structure and properties can be controlled more closely than in other fabricating processes.
  • Highly qualified or skilled labor is not required. in powder metallurgy process
  • Super-hard cutting tool bits, which are impossible to produce by other manufacturing processes, can be easily manufactured using this process.
  • Components shapes obtained possess excellent reproducibility.
  • Control of grain size, relatively much uniform structure and defect such voids and blowholes in structure can be eliminated.

Limitations of Powder Metallurgy:- 

  • Powder metallurgy process is not economical for small-scale production.
  • The cost of tool and die of powder metallurgical set-up is relatively high
  • The size of products as compared to casting is limited because of the requirement of large presses and expensive tools which would be required for compacting.
  • Metal powders are expensive and in some cases difficult to store without some deterioration.
  • Intricate or complex shapes produced by casting cannot be made by powder metallurgy because metallic powders lack the ability to flow to the extent of molten metals.
  • Articles made by powder metallurgy in most cases do not have as good physical properties as wrought or cast parts.
  • It may be difficult sometimes to obtain particular alloy powders
  • Parts pressed from the top tend to be less dense at the bottom.
  • A completely deep structure cannot be produced through this process.
  • The process is not found economical for small-scale production.
  • It is not easy to convert brass, bronze and a numbers of steels into powdered form.