Sintered Material

any of the metal materials obtained by the methods of powder metallurgy. The production of sintered materials has increased in light of the numerous advantages these materials offer over metal materials obtained by melting. With melting, it is difficult, if not impossible, to produce metal materials with certain chemical compositions, for example, composites of metals and nonmetals and such pseudoalloys of metals and nonmetals that do not mix in the molten state as iron-lead and tungsten-copper. Only methods of powder metallurgy permit the production of certain materials with special physical characteristics and structure, for example, many porous metals. Sintered materials may be produced not only in the form of billets and semifinished items but also as finished parts not requiring further treatment by machining. Often sintered materials have better properties than the analogous materials produced by melting, for example, certain high-speed steels, heat-resistant alloys, and beryllium.

The first sintered materials—platinum items and semifinished items (medals, cups, crucibles, wire)—were prepared by P. G. Sobolevskii and V. V. Liubarskii in 1826 (the technology of that period could not provide temperatures above 1770°C required for the melting of platinum). At the turn of the 20th century, the first high-melting sintered materials, including materials of tungsten, with a melting point of 3400°C, were produced; at that time, such materials could not be obtained by melting. Industrial methods for the production of tungsten filaments for electric lamps were introduced in 1910 by Coolidge in the United States. Although modern technology (arc melting, electron-beam melting) permits the melting of any and all high-melting metals and alloys, most high-melting metals are produced by the methods of powder metallurgy.

The first composites made of sintered materials that can only be obtained by the methods of powder metallurgy (copper-graphite brushes for electric motors and generators) were produced about 1900. World War I saw the development of another important composite, namely, magnetodielectrics, which were based on ferromagnetic metal powders dispersed in a dielectric binder. The development of sintered hard alloys in the 1920’s by K. Schroter in Germany was a significant technological advance. Contacts for the electrical industry made from pseudoalloys and composites of sintered materials (tungsten-copper, silver-graphite) were first produced in the 1930’s. Composites of sintered materials based on copper with tin, lead (sometimes, zinc) and additives of nonmetallic components, usually silicon monoxide, have been used for friction wheels since 1932. The development of iron-base friction sintered materials began in the 1940’s. There is at present wide use of diamond-metal composites based on diamond powder, diamond fragments, and metal powders (copper and copper alloys, tungsten-cobalt hard alloys, alloys based on tungsten, copper, and nickel). The first patents for diamond-metal composites were issued in 1922. Composites based on sintered materials are produced on an industrial scale for various branches of modern industry. For example, SAP (sintered aluminum powder), a sintered material based on aluminum and aluminum oxide (6-20 percent), has a heat resistance at 300°-550°C that exceeds the resistance of aluminum alloys produced by melting.

An important group of sintered materials, which for all practical purposes can be produced only by the methods of powder metallurgy, comprises porous metals, alloys, and composites (based on iron, iron-graphite, bronze, and stainless steel). The pores in these materials usually occupy 15-30 percent of the volume. The production of porous sintered materials (for bearings and filters) was proposed in 1909 by Lovendal (British patent). The industrial production of porous sintered materials for bearings was begun in the mid-1920’s. The advantages of porous sintered materials include the presence of a lubricant in the pores (selflubrication) and the good adaptability of the materials under operating conditions owing to deformation of the pore spaces. There has been continuous progress in the production of porous sintered materials for various branches of technology; examples include metallic filters for fine purification of liquids and gases, artillery obturators of porous iron, which replaced lead obturators during World War II, and porous sintered materials for fuel elements and aircraft deicers. There has also been production of materials used to block the spread of flames in explosive atmospheres, materials made with metal powders or fibers for absorbing sound and vibrations, and porous elements used in carrying out chemical reactions and the transport of loose material in a fluidized bed, that is, in a suspended state. Heat-exchange tubes with a porous layer made from copper, nickel, and stainless steel powders were developed in the 1970’s.

In the mid-1930’s, mass production began of iron- and copper-base sintered materials in the form of precision parts not requiring further machining. The parts have been used in machine building, including the building of automotive vehicles, tractors, agricultural machinery, household appliances, and machine tools. Among the parts are gears, gear wheels, sprockets, cams, levers, catches of door locks, switch parts, parts of electrical machinery (commutator bars, direct-current and alternating-current magnetic circuits made with soft magnetic sintered materials), and permanent magnets made with sintered materials based on iron-nickel-aluminum alloys (alni alloys) and iron-nickel-aluminum-cobalt alloys (Alnico alloys).

The last, but not least, group of sintered materials in the form of billets, semifinished pieces, and finished parts is that of high-quality sintered materials. These materials are superior in such properties as strength, heat resistance, and wear resistance to metals and alloys of analogous composition and use obtained by melting. The mechanical properties of a series of cast alloys are poor owing to a coarse-grained structure and liquation. The high-quality group includes magnetic alloys of the alni and Alnico type mentioned above. High-quality sintered materials have been produced since the 1940’s by the methods of powder metallurgy and have been used not only in mass-produced magnetic parts but also in those cases where increased strength is required. Since the 1950’s, beryllium for the nuclear industry has been produced primarily by the methods of powder metallurgy because of the poor mechanical properties and coarse-grained structure of the cast metal. Sintered materials were used in the late 1960’s to produce high-speed steel and in the 1970’s to produce heat-resistant nickel-base superalloys; certain characteristics of these materials are better than those of cast alloys of analogous composition. The production of sintered materials has experienced a more rapid development than the production of metal materials through melting. Thus, from 1964 to 1972, the annual production of sintered materials in the United States rose by a factor of 2.5 (from 47,000 to 118,000 tons) and by a factor of approximately 4 (from 4,000 to 17,000 tons) in Japan.

For both cast materials and deformable materials obtained by conventional methods, it is undesirable to have components, additives, and impurities that facilitate the formation of a significant temperature gap between the liquidus and solidus lines or the appearance of a liquid phase at temperatures below the meltinghardening temperatures of the major mass of metal. On the other hand, the introduction of such elements into sintered materials increases the strength and facilitates production, thereby allowing a decrease in the sintering temperature. Thus, in iron-base cast alloys, phosphorus is an undesirable admixture and must not be present in quantities exceeding 0.1 percent. In iron-base sintered materials, however, phosphorus is intentionally added in quantities of 0.3-0.6 percent to improve the mechanical properties and lower the unit cost of finished parts (owing to the formation of a liquid phase and a decrease in the sintering temperature). Copper (1-20 percent), another additive of iron-base sintered materials, both improves the properties of sintered items and lowers the cost of the sintering process through the formation of a liquid phase during sintering.

Compact (nonporous) sintered materials usually have the same physical and mechanical properties as cast (deformed and annealed) metals. Table 1 gives the values attainable, depending on porosity, for the properties of sintered materials (elasticity modulus E, Poisson ratio v, tensile strength σ, electrical conductivity λ, and thermal conductivity λ_T) expressed as a ratio of these properties to the corresponding properties of the compact metal (E_c, v_c, σ_c, λ_c, and λ_Tc).

In comparison with all other methods for the production of finished parts (casting, pressure treatment, machining), production from sintered materials requires the least expenditure of working time, factory space, and equipment.

There are, however, certain disadvantages to using sintered materials. Economically, it is most advantageous to use sintered materials when large-scale production is contemplated because of the necessity of manufacturing a die or mold for each type of part. This limitation, however, may be eliminated with the development of new methods for forming sintered materials. Another factor to be considered with sintered materials is the high cost of the starting powders. However, this drawback may also be eliminated as powders are produced on a large scale and production methods are improved. A third factor is the requirement that the starting powders possess high purity, especially when powders of iron and iron alloys are involved, because impurities in the starting materials cannot be effectively removed from sintered materials after production. This drawback is gradually being overcome with the large-scale production of pure powder through the atomization of molten iron.

Specific measures for the preservation and storage of finished parts and intermediate products (impregnation with oil or paraffin) are necessary only for porous sintered materials.