Processing of discontinuously-reinforced metal matrix composites by rapid solidification

Abstract

The literature reviewed in this paper on rapid solidification processing techniques for discontinuously-reinforced metal matrix composites presents a cross-section of techniques and experimental results obtained, in recent years, by numerous investigators. The results, documented by researchers, from both academia and industry, have enriched our understanding of the interrelationships between processing-microstructural characteristics and macroscopic behavior, and suggest the existence of a potential for rapid improvement in processing techniques in the years to come. The primary limitations that place constraints on the rapid advancement of the field of processing lies in the development of techniques that facilitate wettability between the metal matrix and ceramic reinforcement coupled with high speed compaction techniques that would result in minimum exposure of the composite to elevated temperatures. Although shock consolidation has emerged as a viable technique that overcomes many of the problems and shortcomings associated with thermal compaction processes, it is limited by its ability to control the homogeneous distribution of the shock wave during consolidation. The foregoing description of the various processing techniques used for discontinuously-reinforced metal matrix composites demonstrates the feasibility of spray atomization and deposition as a viable route for the production of large quantities of metal matrix composites, since it circumvents many of the difficulties associated with particulate handling. Materials produced both on a laboratory- and industry-scale level have demonstrated the feasibility of producing a spectrum of metal matrix composites based on the systems of: (1) aluminum/silicon carbide particulates; (2) aluminum/silicon carbide whiskers; (3) nickel/silicon carbide particulates; and (4) magnesium/silicon carbide particulates, using the spray deposition technique. The spray deposition technique offers advantages over the competitive techniques in terms of microstructural refinement, property enhancement and overall efficiency of the process. The results reported in this paper on properties of discontinuously-reinforced MMCs show an attractive combination of density, strength, modulus, ductility, fracture toughness, cyclic fatigue and fracture resistance. Furthermore, the combination of properties in the final heat treated product can be controlled to a large extent by the selection of an appropriate heat treatment schedule. The attractive combination of properties exhibited by spray atomized and deposited composite materials stems from an integrity of the metal-ceramic interfaces coupled with the elimination of oxide contamination. At the present time, it has been demonstrated that the processing of these materials based on aluminum, titanium, magnesium and intermetallic matrices can be carried out using existing commercial presses and rolling mills, thus obviating the need for investment in special purpose equipment. However, machining these materials is far more difficult than the conventional un-reinforced alloys, resulting in exorbitant costs. As a result, rapid utilization of composite materials produced using rapid solidification techniques is dictated by the competing influences of: 1. (1) economics of production of the final product; and 2. (2) a lack of reproducibility due to limited understanding of the wettability characteristics of the reinforcement by the matrix, and the metal-ceramic interfaces. As processing advances occur, it is expected that the reliability will be enhanced in certain families of composites. Continuous efforts at modeling several of the composite material systems coupled with innovations in the development of intelligent manufacturing systems are also expected to be areas that will rapidly accelerate the economics of rapid solidification processing of these materials. It is the opinion of the authors that most of the research on processing of these newer generation materials is still at an embryonic stage and concurrent improvements in both the processing and manufacturing of these materials are bound to have a significant impact on this technology in the years to come. A major application of metal matrix composite components is summarized in Table 33.