Powder Metallurgy(redirected from Greenform)
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powder metallurgy[′pau̇d·ər ′med·əl‚ər·jē]
a field of technology involved with the production of metal powders and metallic compounds. Powder metallurgy also manufactures objects and semifinished products from these powders and compounds, or from mixtures of these powders and compounds with nonmetallic powders, without melting the principal constituent.
Powder metallurgy involves (1) producing metallic powders and preparing a mixture from them with a given chemical composition and specific processing properties; (2) molding powders or mixtures made with them into semifinished products of a given shape and size, chiefly by pressing; and (3) sintering, or heating the semifinished products at a temperature below the melting point of the metal as a whole or of its principal component.
Sintered articles usually possess a certain degree of porosity, from several percent to 30–40 percent and occasionally up to 60 percent. The sintered articles are subject to additional hot or cold compression to reduce porosity and sometimes eliminate it, to improve mechanical properties, and to achieve precise dimensions. Sometimes additional heat, thermochemical, or thermo-mechanical treatment is also used. Molding is sometimes eliminated: in this case, powders are poured into molds and then sintered. Pressing and sintering are often combined in a single operation called hot pressing, or compacting of powders under heat.
Production of powders. Metals are pulverized in ring-roller pulverizers and in tumbling and ball mills. A more advanced method is the spraying of molten metals, which offers the following advantages: effective elimination of many impurities from the melt, high productivity, and economy. Powders are produced from iron, copper, tungsten, and molybdenum by gas reduction of oxides of these metals with carbon or hydrogen. Hydrometal-lurgical methods are now used to reduce solutions of compounds of these metals with hydrogen. Copper powders are usually produced by electrolytic precipitation. Less common methods of obtaining metal powders include electrolysis of melts and thermal dissociation of volatile compounds (carbonyl method).
Molding. The chief method of molding metallic powders is pressing in tempered-steel dies on high-speed automatic presses (up to 20 pressings/min) at a pressure of 200–1,000 meganewtons/m2 (20–100 kilograms-force/mm2). The pressed objects have a shape, size, and density that may be altered during sintering and other subsequent operations. Such new cold molding methods as isostatic pressing, rolling, and extrusion are becoming increasingly important.
Sintering. Sintering takes place in a controlled environment that must have hydrogen; an atmosphere that includes carbon compounds; a vacuum; controlled charges; and a temperature approximately 70–85 percent below the absolute melting point. For multicomponent melts, the temperature is somewhat higher than the melting point of the constituent with the lowest melting point. The environment should ensure the reduction of oxides, prevent the formation of such undesirable impurities as soot, carbides, and nitrides, prevent the burning out of such components as carbon in solid melts, and ensure the safety of the sintering process itself.
Furnaces designed for sintering should provide for both the heating and cooling of products in a controlled environment. Sintering gives finished articles the required density, size, and properties and provides semifinished products with the properties necessary for further operations. Hot pressing, or pressure sintering, and in particular isostatic pressing are becoming widespread.
Powder metallurgy has the following advantages, which have determined the method’s development.
(1) Powder metallurgy permits the manufacture of materials difficult or impossible to produce by other methods. These materials include such high-melting metals as tungsten and tantalum, alloys and compositions based on high-melting compounds, that is, alloys based on such compounds as carbides of tungsten and titanium, and compositions and pseudoalloys of metals that do not coalesce in molten form, especially given a substantial difference in melting points: an example is a tungsten-copper combination. Other materials suitable for manufacture by powder metallurgy are compositions made of metals and nonmetals, including copper-graphite, iron-plastic, and aluminum-aluminum oxide, as well as porous materials for bearings, filters, gaskets, and heat exchangers.
(2) The production of some materials and articles by powder metallurgy has technical and economic advantages. The method permits savings in the amount of metal used and a marked reduction in net cost: for example, sometimes up to 60–80 percent of the metal used in the casting and cutting of metal parts is wasted in gates or shavings.
(3) The use of pure base powders makes it possible to produce sintered materials with fewer impurities and closer adherence to desired specifications than in ordinary cast fusions.
(4) Given identical composition and density, sintered materials, because of their characteristic structure, are superior to smelted materials. In particular, the adverse effect of the preferred orientation (texture) in sintered materials is less evident than in a number of cast metals, such as beryllium, because of the specific conditions for solidifying the melt. A major disadvantage of cast alloys, for example, high-speed steels and some heat-resistant steels, is the marked heterogeneity of the local composition, caused by liquidation upon solidification. It is easier to control the size and shape of structural elements in sintered materials. Most importantly, it is possible to obtain variations in the distribution and shape of particles that cannot be achieved in the case of smelted metal. Because of these structural characteristics, sintered materials are more resistant to heat, to cyclical fluctuations in temperature and stress, and to nuclear irradiation; resistance to the last is very important in the case of new technological materials.
Powder metallurgy also has certain disadvantages that hinder its development: (1) the cost of metal powders is relatively high; (2) the necessity of sintering in a controlled environment increases costs; (3) it is sometimes difficult to manufacture articles and semifinished products of a large size; (4) it is difficult to produce compact, nonporous metals and alloys; and (5) pure metal powders must be used to produce pure metals.
The disadvantages and some of the advantages of powder metallurgy are not constant factors; to a great extent they depend on the state and development of the process itself and of other branches of industry as well. As technology develops, powder metallurgy may cease to be used in some fields and be adopted in others. Powder metallurgy was first developed in 1826 by P. G. Sobolevskii and V. V. Liubarskii for the manufacture of platinum coins. The method was used for this purpose because it was impossible at that time to reach the melting point of platinum (1769°C). The industrial use of powder metallurgy was discontinued in the mid-19th century owing to the development of high temperature engineering.
Powder metallurgy was restored abroad at the turn of the 20th century as a means of manufacturing filaments for electric bulbs from high-melting metals. However, developments in arc, electron-beam, and plasma smelting as well as in electric-pulse heating made it possible to achieve previously unattainable temperatures. As a result, there was a slight drop in the manufacture of these metals by the method of powder metallurgy. In addition, advances in high-temperature technology eliminated a number of powder metallurgy’s disadvantages that had hindered its development. One of these was the difficulty of producing powders from pure metals and from alloys; the spraying method effectively discharges into the slag the admixtures and impurities contained in the metal to be smelted. The introduction of high-temperature isostatic methods also resolved the difficulties involved in the manufacture of large nonporous semifinished products.
At the same time, the many advantages of powder metallurgy act as a constant factor that will probably remain important even as technology develops further.
REFERENCESFedorchenko, I. M., and R. A. Andrievskii. Osnovy poroshkovoi metallurgii Kiev, 1961.
Bal’shin, M. Iu. Nauchnye osnovy poroshkovoi metallurgii i metallurgii volokna. Moscow, 1972.
Kiparisov, S. S., and G. A. Libenson. Poroshkovaia metallurgiia. Moscow, 1972.
M. IU. BAL’SHIN
A metalworking process used to fabricate parts of simple or complex shape from a wide variety of metals and alloys in the form of powders. The process involves shaping of the powder and subsequent bonding of its individual particles by heating or mechanical working. Powder metallurgy is a highly flexible and automated process that is environmentally friendly, with a low relative energy consumption and a high level of materials utilization. Thus it is possible to fabricate high-quality parts to close tolerance at low cost. Powder metallurgy processing encompasses an extensive range of ferrous and nonferrous alloy powders, ceramic powders, and mixes of metallic and ceramic powders (composite powders). See Metallurgy
Regardless of the processing route, all powder metallurgy methods of part fabrication start with the raw material in the form of a powder. A powder is a finely divided solid, smaller than about 1 mm (0.04 in.) in its maximum dimension. There are four major methods used to produce metal powders, involving mechanical comminution, chemical reactions, electrolytic deposition, and liquid-metal atomization. Metal powders exhibit a diversity of shapes ranging from spherical to acicular. Particle shape is an important property, since it influences the surface area of the powder, its permeability and flow, and its density after compaction. Chemical composition and purity also affect the compaction behavior of powders.
Powder metallurgy processes include pressing and sintering, powder injection molding, and full-density processing. See Sintering
Normally, parts made by pressing and sintering require no further treatment. However, properties, tolerances, and surface finish can be enhanced by secondary operations such as repressing, resintering, machining, heat treatment, and various surface treatments.
Powder injection molding is a process that builds on established injection molding technology used to fabricate plastics into complex shapes at low cost. It produces parts which have the shape and precision of injection-molded plastics but which exhibit superior mechanical properties such as strength, toughness, and ductility.
Parts fabricated by pressing and sintering are used in many applications. However, their performance is limited because of the presence of porosity. In order to increase properties and performance and to better compete with products manufactured by other metalworking methods (such as casting and forging), several powder metallurgy techniques have been developed that result in fully dense materials; that is, all porosity is eliminated. Examples of full-density processing are hot isostatic pressing, powder forging, and spray forming.
Powder metallurgy competes with several more conventional metalworking methods in the fabrication of parts, including casting, machining, and stamping. Characteristic advantages of powder metallurgy are close tolerances, low cost, net shaping, high production rates, and controlled properties. Other attractive features include compositional flexibility, low tooling costs, available shape complexity, and a relatively small number of steps in most powder metallurgy production operations.
Metal powders can be thermally unstable in the presence of oxygen. Very fine metal powders can burn in air (pyrophoricity) and are potentially explosive. Some respirable fine powders pose a health concern and can cause disease or lung dysfunction. Control is exercised by the use of protective equipment and safe handling systems such as glove boxes. See Industrial health and safety