Evolution of interaction domain microstructure during spray deposition
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INTRODUCTION
SPRAY atomization and deposition generally refers to a material synthesis approach that involves the energetic disintegration of a molten material by gas jets into micron-size droplets (atomization), followed by the immediate deposition of the mixture of solid, liquid, and partially solidified droplets on a surface (deposition) to form a coherent, highly dense material. Over the last 2 decades, this processing approach has received considerable attentiont~-41 as a viable processing alternative for the synthesis of highly reactive alloys, such as those based on Mg and A1;r5-81 for the synthesis of hightemperature and high-performance materials, such as those based on Ni, Fe, Cu, Ti, Ni3A1, and TiAI intermetallics;t9-~3J and more recently, for the synthesis of discontinuously reinforced metal-matrix composites. [14-17] Modifications to the processing methodology, originally studied by Singer, I~8"~9jhave resulted in the development of the Osprey process, ~9,12,2~ controlled spray deposition (CSD). t231 spray forming or casting, t4,1~ liquid dynamic compaction (LDC), t5,7-8] and variable codeposition of multiphase materials, 115,~6~ although the general principle is strictly the same for these processes. Moreover, it also has become increasingly evident that the mechanisms that govern the evolution of the microstructure during spray atomization and deposition may not be unique to this synthesis approach, but rather may be generalized to most discrete droplet processes, such as plasma deposition and thermal spraying. [24,25} X. LIANG, Assistant Research Specialist, and E.J. LAVERNIA, Associate Professor, are with the Department of Materials Science and Engineering, School of Engineering, the University of California, Irvine, CA 92717. Manuscript submitted December 2, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
Discrete droplet processes, such as spray atomization and deposition, offer a potentially attractive manufacturing route for several reasons, t3,26j First, the highly efficient heat convection that is present during atomization ensures the maintenance of a relatively low processing temperature, which limits large-scale segregation and coarsening phenomena. Second, the inert conditions that are typically required for atomization and deposition minimize surface oxidation and other deleterious surface reactions. In addition, spray-atomization and spraydeposition processes may potentially be used for nearnet shape manufacturing of difficult-to-form materials, such as intermetallics and discontinuously reinforced metal-matrix composites. Inspection of the large volume of available scientific literature unequivocally shows that the microstructure of spray-deposited materials is intimately linked to the solidification environment experienced by the droplets prior to, during, and following impact, first with the deposition surface and, subsequently, with each other. For example, the presence of a spheroidal grain morphology, which is typically associated with spray-atomized and spray-deposited mat
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