Powder-metallurgy process: 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.
- Chemical reduction
- Electrolytic process
- Condensation of metal vapors
- Hydride and carbonyl processes.
The above mentioned metallic powder making techniques are discussed briefly as under.
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.
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. Iron powder is produced in this way
Fe3O4 4C = 3Fe 4CO
Fe3O4 4CO = 3Fe 4CO2
Copper powder is also produced by the same procedure by heating copper oxide in a stream of hydrogen.
Cu2 H2 = 2Cu H2O
Powders of W, Mo, Ni and CO can easily be produced or manufactured by reduction process because it is convenient, economical and flexible technique and perhaps the largest volume of metallurgy powders is made by the process of oxide reduction.
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 cathoden plates is then scraped off and pulverized to produce copper powder of the desired grain size. The electrolytic powder is quite resistant to oxidation.
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.
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.
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.
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.