Spray atomization and deposition is a process in which a stream of molten metal is atomized into fine droplets (300 ^m or less) using a high velocity inert gas, usually argon or nitrogen, which is then deposited on a mold or substrate. When the particles impact the substrate, they flatten and weld together to form a high density preform, which can be subsequently forged to form a fully dense product.10 The production of MMCs by spray deposition can be accomplished by introducing particulates into the metal spray, leading to codeposition with the atomized metal onto the substrate. The process is a rapid solidification process because the metal experiences a rapid transition through the liquidus to the solidus, followed by slower cooling from the solidus to room temperature. Inherent in spray processes are composites with minimal reinforcement degradation, little segregation, and fine grained matrices. The critical process parameters are the initial temperature, size distribution, and velocity of the metal droplets; the velocity, temperature, and feed rate of the reinforcement; and the temperature of the substrate.8 In general, spray deposition methods are characterized by significant porosity levels. Careful control of the atomizing and particulate feeding conditions are required to insure that a uniform distribution of particulates is produced within a typically 90-98% dense aluminum matrix. A number of aluminum alloys containing SiCp have been produced by spray deposition, including aluminum-silicon casting alloys and the 2XXX, 6XXX, and 7XXX wrought alloys.
Spray deposition was developed commercially by Osprey, Ltd as a method of building-up bulk material by atomizing a molten stream of metal with jets of cold gas. It has been adapted to particulate MMC production by injection of ceramic powder into the spray. The four stages in the Osprey method, shown in Fig. 9.10, are (1) melting and dispensing, (2) gas atomization, (3) deposition, and (4) collector manipulation.10 Induction heating is used to produce the melt which flows into a gas atomizer. Melting and dispensing is carried out in a vacuum chamber. The atomized stream of metal is collected on a substrate placed in the line of flight. Overspray is separated by a cyclone and collected. Among the notable microstructural features of Osprey MMC material are strong interfacial bonds, little or no interfacial reaction layers, and very low oxide contents. A major attraction of the process is its high rate of metal deposition.
The advantages of spray deposition processes include fine grain sizes and minimal reinforcement degradation. Disadvantages include high porosity levels and the resulting need to further process these materials to achieve full consolidation. In general, spray processes are more expensive than casting due to the longer processing times, the high costs of the gases used, and the large amount of waste powder that results from spraying.
9.7 Powder Metallurgy Methods
When higher strength discontinuous MMCs are required, PM processes are often used because segregation, brittle reaction products, and high residual stresses from solidification shrinkage can be minimized. In addition, with the advent of rapid solidification and mechanical alloying technology, the matrix alloy can be produced as a prealloyed powder, rather than starting with elemental blends. PM processing can be used to make aluminum MMCs with both SiC particulates and whiskers, although Al2O3 particles and Si3N4 whiskers have also been employed. Processing, as shown in Fig. 9.11, involves (1) blending of the gas atomized matrix alloy and reinforcement in powder form; (2) compacting (cold pressing) the homogeneous blend to roughly 75-80% density; (3) degassing the preform (which has an open interconnected pore structure) to remove volatile contaminants (lubricants and mixing and blending additives), water vapor, and gases; and (4) consolidation by vacuum hot pressing or hot isostatic pressing. Then, the consolidated billets are normally extruded, rolled, or forged.
The matrix and reinforcement powders are blended to produce a homogeneous distribution. Prealloyed atomized matrix alloy powder is mixed with the particulate or whisker reinforcement and thoroughly blended. Since the metal powder is usually 25-30 ^m and the ceramic particulates are often much smaller
Cold Isostatic Compaction
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