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18 PROCESSING

Figure 1.10. Tridimensional n twork of silicon carbide fo minfiltrated with copper. Courtesy o T Fitqeruld and AMortensen,hUTwhen cost is not relevant, whereas particles or chopped-fiber reinforced alloys have foun d larger-scale com mer-cial applications (Don om oto e t al. 19 83; Ebisawa et al.1991; Hoover 1991; Hoover 1990; Khmovicz 1990).Efforts are still needed to improve the fabrication ofhigh-quality fibers and preforms. Conversely, inexpen-sive reinforcem ents that m ay no t prod uce excellentstructural materials, but feature low w eight, good wearresistance, or noise damping, have been proposed. nexample of such m aterial is red m ud, a by-product ofalum inum fabrication (Sing Solanki et al. 1991 ).

1 5 2 ManixPorosity may be found in the m atrix of materials pro-duced by any of the liquid-metal processes, but can beminimized in several cases by a better understanding ofthe processes. Application of a vacuum and redu ction ofthe vortex during mixing, for example, reduces theamo unt of residual porosity in dispersion processes. Ininfiltration, heterogeneity of the m atrix composition ormicrostructure in infiltrated materials may also be re-duced through control of the processing parameters.For example, high-speed infiltration of a low-temperature Saffil (Imperial Chemical Industries,RUNCORN, U.K.) alumina preform by moltenAI-Cu alloys with very little superheat results in afine-grained structure and concentration of macro-

segregation in a small region (Michaud andMortensen 199 2; Mortensen and Michaud 1990).New alloys specifically designed for use in MMCfabrication have also been developed to minimizechemical reaction between matrix and reinforcementor to modify the matrix microstructure.

1 5 3 Process evelopmentTh e four basic metho ds presented in this chapter havebeen modified and combined to create new processes.Plasma spray and in-situ processes have been com binedto form metal-ceramic composite by reaction of themetal with the surrounding g s during flight (Mathur etal. 1989 a). Com binations of infiltration and in-situ pro-cessing have also been investigated (Fukunaga et al.1990), s well s com bination of in filtration and disper-sion (Klier et al. 1990; Tank 1990). Oth er liquid-metalroutes have been investigated , including laser m elting ofa metal surface to locally incorporate a reinforcingphase, or methods closely related to powder metallurgyprocesses, such as liquid-phase hot pressing and sinter-ing. The processes investigated for MMC productionare generally batch processes, w hich lim it the p roduc tssize and fabrication speed. Various methods for contin-uous casting of MMCs have been proposed, adaptingsqueeze casting (Atsushei 1985) or pressure casting(Clifford and Coo k 19 89 ), although both are o ftenimpractical because of the need for a pressure vessel.Infiltration using electromagnetic bo dy forces mightbe used for continuous casting (Andrews andMortensen 1991 .Spray processes could conceivablyalso be adapted for semicontinuous processes(Mathur et al. 1989a).In ord er to minimize the composites cost, emphasishas been put o n the development of net-shape or near-net-shape processes such s squeeze casting to reinforcemetals with fibers or whiskers. Conversely, low-cost,particle-reinforced alum inum alloy ingots have becomecom merc ially available; they can be remelted and recastusing investment-casting or die-casting processes. nadvantage of these m aterials during die casting is theirincreased viscosity in the molten s tate as compa red withunreinforced alloys, which reduces the turbulence dur-ing mold filling and thus the porosity in the casting(Hoover 1991).

1 6 ummaryDuring the last 10 years, a large research effort hasfocused on developing and understanding variousliquid-state processes for the produ ction of MMCs. As