Dynamic analysis of unidirectional pressure infiltration of porous preforms by pure metals
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INTRODUCTION
OVER the past 2 decades, advanced composite materials have progressed from a laboratory curiosity to a production reality. In principle, composites can be constructed of any combination of two or more materials. Metallic composites or metal-matrix composites (MMCs) reinforced with fibers are currently of significant interest. They offer the opportunity to tailor a material with a combination of properties unavailable in any single material, e.g., combining the very high tensile strength and modulus of elasticity of various types of fibers with the low density of a metal such as aluminum, titanium, or magnesium to obtain a composite material with a higher strength-to-density or modulus-to-density ratio than any single known alloy. Among the several MMC fabrication processes available, the liquid infiltration process referred to as squeeze casting is receiving increasing attention because of its economic feasibility. Liquid metal-matrix infiltration or pressure infiltration, as shown schematically in Figure 1(a), uses pressurized inert gas to force a liquid metal into a preheated porous preform of reinforcement material. Unlike other MMC fabrication methods, pressure infiltration is conducted within the controlled environment of a pressure vessel. This makes it possible to cast complex structures in thin-walled low strength molds. High infiltration pressures can be applied, keeping very low differential pressures between the inside and the outside of the mold, thus reducing the mold required wall thickness and minimizing costs. DHIMAN K. BISWAS, formerly Graduate Student, Department of Chemical Engineering, Cleveland State University, is Software Engineer, Comsys, Information Technology Services, Raleigh, NC 27612. JORGE E. GATICA, Associate Professor, and SURENDRA N. TEWARI, Professor, are with the Department of Chemical Engineering, Cleveland State University, Cleveland, OH 44115-2425. Manuscript submitted March 26, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
For the pressure infiltration process, the pressure gradient, infiltration velocity, temperatures of the preform and superheated melt, and physicochemical properties of the preform are critical variables determining the microstructure of the final composite. Before solidification occurs, the preform permeability is constant and the infiltration velocity is only a weak function of the infiltration length (cf. Figure 1(b)). For pure metals, as solidification starts, a twophase (liquid 1 solid) region emerges with a time-space varying solid fraction, and the infiltration dynamics become strongly dependent on the infiltration length and solid fraction (cf. Figure 1(c)). This two-phase zone is confined between two sharp fronts: a remelting front at the point where the superheated melt enters the two-phase zone and an infiltration front. These two fronts have independent dynamics resulting in a two-phase zone that expands with the infiltration time. Because of their considerable engineering relevance to MMC fabrication, infiltration processes
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