Production of Metallic Powder Machinery's Handbook, 31st Edition
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Drive Rollers Fig. 8. Method of Mechanical Comminution to Obtain Fine Particles
Mechanical Alloying: Mechanical alloying is a process for the production of new materi als. It takes place in a high-energy mill where two powders of different origin are mixed. By the conversion of mechanical into chemical energy, nanocrystalline structures emerge that cannot be produced by conventional melting methods. In general, the process can be viewed as a means of assembling metal constituents with a controlled microstructure. If two metals will form a solid solution, mechanical alloying can be used to achieve this state without the need for an excursion into high temperatures. Conversely, if the two metals are insoluble in the liquid or solid state, an extremely fine dispersion of one of the metals in the other can be accomplished. The dispersed phase can result in strengthening of the particles or can impart special electrical or magnetic properties to the powder. The process of mechanical alloying was originally developed as a means of overcoming the disadvantages associated with using powder metallurgy to alloy elements that are difficult to combine. Some oxides are insoluble in molten metals. Mechanical alloying provides a means of dispersing these oxides into the metals. Examples are nickel-based superalloys strength ened with dispersed thorium oxide (ThO 2 ) or yttrium oxide (Y 2 O 3 ). These superalloys have excellent strength and corrosion resistance at elevated temperatures, making them attractive candidate materials for use in applications such as jet engine turbine blades, vanes, and combustors. Chemical Processes.— There are numerous chemical methods for producing powders. Generally, chemical methods result in very fine powder particle sizes. The most common chemical powder treatments involve oxide reduction, precipita- tion from solutions, and thermal decomposition. Chemical reduction involves a variety of chemical reactions that reduce the metal into elemental powders. A common process involves liberating metals from their oxides by the use of reducing agents, which attach to the oxygen in the oxide and render metal powders. The powders produced in this way have great variation in properties and yet have closely controlled particle sizes and shapes. Oxide-reduced powders are often characterized as “spongy,” due to pores present within individual particles. Solution-precipitated powders can provide narrow particle size distributions and high purity. Thermal decomposition is most often used to process car- bonyls. These powders, once milled and annealed, exceed 99.5 percent purity. Mill scale and oxidized metallic products are annealed to reduce both the oxygen and carbon contents. FeO, Fe 2 O 3 , or Fe 3 O 4 , are reduced in the presence of a reducing atmo- sphere. In addition, the carbon within the particles is removed via the formation of CO and CO 2 . Hydrometallurgical manufacturing followed by thermal decomposition constitutes an- other chemical method. Precipitation of a metal from a solution can be accomplished by using electrolysis, cementation, or chemical reduction. This is done either from a solution containing an ore, or by means of precipitation of a metal hydroxide followed by heating which results in decomposition and reduction.
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