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Nano-powder and powder materials |
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Powders are manufactured by mechanical crushing or atomization of original molten metals, high-temperature oxide reduction and thermal decomposition of volatile compounds, electrolysis and using other techniques. Pieces are usually obtained by compression with further or parallel thermal, thermo-chemical processing without actual melting of a basic compound. The technologies of pulver metallurgy allow getting pieces from materials, otherwise difficult to be processed by any other method (for example, from non-fusible metals or from compositions of metals and non-metals) or economically not profitable. Materials with a higher melting point or solid materials and alloys, porous, frictional and other materials and corresponding goods are manufactured in the powder metallurgy.
The powder metallurgy is an area of technology, comprising a number of techniques to produce metal powders and metal-like compounds, semi finished materials and corresponding goods (or goods out of mixtures with non-metal powders). The technology comprises following steps: obtaining of initial metal powders with further producing of charges (mixtures) with a given chemical composition and technologic features; melting of elemental or alloy powders in dies with given shape and parameters (mostly through compacting); sintering, i.e. thermal die processing at just below specific metal melting points. Products after sintering are porous to a certain extent (from several to 30-40%, rarely to 60%).Additional chipless shaping (cold or hot) of sintered products is used to reduce porosity (or eliminate it wholly), concentrate mechanical qualities and finish a product according to exact shape parameters; occasionally additional thermal, thermo-chemical or thermo-mechanical processing is also applied.
The powder metallurgy has following advantages:
The ability to facilitate manufacture of complex or unique shapes which would be impractical or impossible with other metalworking processes. These metals are: some metals with a higher melting point (tungsten, tantalum); their alloys and composites (solid alloys based on tungsten, titanium und etc. carbides): metal compositions and so-called pseudo alloys, impossible to be mixed during melting, especially when they have considerably different melting points (for example, tungsten - copper); compositions of metals and non-metals (copper - mineral carbon, iron - plastic, aluminium - aluminium oxide, etc.); porous materials (for bearings, filters, sealers, heat exchangers) etc.
The ability to get some materials and products with higher technical and economic values. The powder metallurgy allows sparing metal amounts and considerably reducing production costs (for example, up to 60-80% of metal is sometimes wasted as feed-heads, cutting chips, etc. during producing the pieces though foundry casting work and cutting).
Using pure initial powders, we can obtain sintered materials with a lower ratio of impurities and exacter conformity with a given composition, than by typical cast alloys.
Sintered materials have in a number of cases better properties due to their specific structures with the same composition and hardness, than the cast materials. For example, a negative influence of a preferred orientation (texture) is lower, than that in a number of cast metals (for example, beryllium) due to the specific conditions of a liquid melt crystallization. A large drawback of some cast alloys (for example, of rapid and heat-resistant steels) is their sharp discontinuity of a localized composition due to liquation at indurations. The size and the form of structural elements of sintered materials can be easier controlled. We can obtain such types of grain positional relationship and form that are inapproachable in the case of cast metals. Due to there structure peculiarities, sintered metals are heat-faster, better endure impacts of tempretaure and strain cyclical changes, and also nuclear radiation, which is very important for modern technical materials.
Nano-powders are more and more often applied as filling components for new materials and composites, as components in many coatings, catalyst elements, materials used in electronics. back |
