Metallurgy powder

Powder metallurgical production methods for high-tech materials are very clean operations and do not generate any waste or emissions. Internal and external scrap recycling is excellently organized worldwide. 

For information about hazardous substances in the production of tungsten alloys, such as thoriated tungsten, cobalt, or nickel-containing cemented carbides, see Chapter 14. 

ENVIRONMENTAL CONSIDERATIONS ABOUT THE SUBSTfrUTION OF HIGH SPEED STEEL BY HARDMETALS [12.5] 

in Proc. 7th Tungsten Symposium, Goslar, pp. 13-23, ITIA, London (1996). 

Powder metallurgy (P/M) is the forming of precision shapes and components from metal or nonmetal powders, or mixtures of the two. Components can be impregnated with oil or plastic, heat treated, plated, machined or forged. P/M 

Powder metallurgy is particularly suitable when the constituents are chopped fiber and a particulate matrix. This technique has been used for Al, Sn, Pb-Sn, Cu and Ni. The constituents are mixed well in an inert atmosphere, placed in a mold, degassed, pressed and heated. 

As a general rule, it is necessary to use as high a temperature as possible, but too much pressure will damage the fiber and it is normal to limit the fiber content to 25%. To obtain uniform pressure, it is preferable to place the materials in a can, which is sealed by electron beam welding, and can then be processed in an isostatic press. 

Extrusion and forging are other routes for processing material in particulate form, whilst coextrusion and drawing can be used for coated fiber (wire). In extrusion, a powdered metal and chopped fiber are dispersed, placed in cans and then hot extruded to give a solid bar with aligned fibers. 

Yih and Chung [132,133] used coated fillers to make MMC by powder metallurgy. 

The low cost route, using powder metallurgy for a Ti matrix composite has been recorded [134]. 

Metal and alloy powders may be produced through the following routes, with the last three accounting for the most common methods currently employed  

Grinding and pulverization of a metallic solid or oxide-based ore 

Ingots, Billets, Blooms, Beam Blanks, Rounds 

Structural Shapes, Rails, Pipe, Tubing, Blooms, and Billets 

Cold-Rolled Sheet, Strip, Plate, and Pipe 

Chemical precipitation of metal from a solution of a soluble salt may also be used to form metallic powders. In this procedure, a reducing agent such as sodium borohydride is added to an aqueous metal salt, MX (Eq. 14). A mixture of aqueous products will be produced in addition to the reduced metal, since sodium borohydride also reacts exothermically with water to yield borax, Na2B207. As we will see in Chapter 6, this is the most widely used procedure for the synthesis of nanoparticulate metals, from the reduction of metal salts confined within nanosized entrainer molecules. 

By varying parameters such as jet design, pressure and volume of the atomizing fluid, and density of the hquid metal stream, it is possible to control the overall particle size and shape. In principle, atomization is applicable to all metals that can be melted, and is commercially used for the production of iron, steels, alloy steels, copper, brass, bronze, and other low-melting-point metals such as aluminum, tin, lead, zinc, and cadmium. 

Atomization is particularly useful for the production of homogeneous powdered alloys, since the constituent metals are intimately mixed in the molten state. Further, 

In conventional steels, too much phosphorus causes embrittlement by grain boundary segregation, and its concentration is therefore nsnaUy limited to 0.05%. Powder metallurgy, on the other hand, allows larger amounts to be nsed in sintering techniques which can optimise characteristics such as tensile strength. 

In ferrous powder metallurgy, up to 0.8% of P can be added in the form of FejP to improve the magnetic properties of certain alloys. Powdered ferrophosphorus, which is mainly FcjP -i- Fc2P, is often used as the source of P, but not red P, which is liable to ignite. 

Addition of red P, however, is reported to be more beneficial to desired properties than adding the element as ferrophosphorus [14], It would be interesting to ascertain the effect on properties of adding the element in the form of the black allotrope [8]. 

Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition) 

In sintering, the green compact is placed on a wide-mesh belt and slowly moves through a controlled atmosphere furnace (Fig. 3). The parts are heated to below the melting point of the base metal, held at the sintering temperature, and cooled. Basically a solid-state process, sintering transforms mechanical bonds, ie, contact points, between the powder particles in the compact into metallurgical bonds which provide the primary functional properties of the part. 

By the 1950s P/M parts were used in postage meters and home appHances. In the 1970s P/M superaHoys for aerospace appHcations were used, followed by steel P/M forgings. The 1980s opened the way for rapid soHdification processing, P/M tool steels, and metal injection molded parts. 

Metal injection mol ding (MIM) holds great promise for producing complex shapes in large quantities. Spray forming, a single-step gas atomization and deposition process, produces near-net shape products. In this process droplets of molten metal are coUected and soHdifted onto a substrate. Potential appHcations include tool steel end mills, superalloy tubes, and aerospace turbine disks (6,7). 

Sintering green compact Sizing Method of part production 

Cold die compaction performed at room temperature. Gives high porosity and low strength. 

Hot forging deformation of reheated sintered compact to final density and shape. 

Continuous compaction for strip or sheet product. Slower than conventional rolling. 

Isostatic compaction (hot or cold) compaction of powder in a membrane using pressurized fluid (oil, water) or gas. Permits more uniform compaction and near-net shapes. Undercuts and reverse tapers possible, but not transverse holes. Used for ceramics mainly. 

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Herring C 1949 Surface tension as a motivation for sintering The Physics of Powder Metallurgy ed W E Kingston (New York McGraw-Hiii) pp 143-79... 

The metal is isolated commercially by a complex chemical process, the final stage of which is the hydrogen reduction of ammonium ruthenium chloride, which yields a powder. The powder is consolidated by powder metallurgy techniques or by argon-arc welding. 

E. G. Visser, M. T. Johnson and P. J. van der Zaag, Proceedings of the 6th International Conference on Ferrites (ICF-6), The Japanese Society of Powder and Powder Metallurgy, Tokyo, 1992. 

A. R. E. Singer and A. D. Roche, ia E. N. Aqua and C. I. Whitman, eds.. Modem Developments in Powder Metallurgy Metal Powder Industries... 

G. H. Gessiager, Powder Metallurgy of Superalloys, Butterworths, London, 1984, p. 282. 

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