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Infiltration of Fibrous Preforms by a Pure Metal: Part I. Theory
A. MORTENSEN, L.J. MASUR, J.A. CORNIE, and M.C. FLEMINGS General expressions are derived to describe fluid flow and heat transfer during infiltration of fibrous preforms by a pure metal. Analytical solutions to the problem are given for the case of unidirectional infiltration into a uniform preform of aligned fibers under constant applied pressure. Calculations are carried out for infiltration kinetics (including total infiltrated length) and temperature distribution, using as an example alumina fiber/aluminum composites. Limiting cases leads to very simple expressions. Initial fiber temperatures both above and below the metal melting point are considered. In the case of fibers at a temperature significantly below the metal melting point, it is concluded that the factor most strongly influencing infiltration is the solidification of metal in the interfiber region. In the calculations, it is assumed that this solidification is in the form of a uniform solid metal sheath around the fibers. Metal superheat, when present, serves to progressively remelt the solidified sheath from the upstream end of the preform. Fiber volume fraction and initial temperature are predicted to have a major effect on infiltration kinetics, while metal superheat exerts a relatively minor influence. When no external heat extraction is present and a constant pressure is applied to the metal, flow through the preform continues indefinitely. For the case of external heat extraction, flow ceases when sufficient solidification occurs to block flow.
existence of a critical preheat temperature based upon physical constants of the metal and particles, above which the particles must be heated in order for infiltration to occur. Their calculations are based upon an overall thermal balance between the particles and the metal. Hosking and Netz t3~ have studied the infiltration of an aluminum alloy into a packed bed of particles ranging in size from 3 to 9 mm. They present a statistically derived expression that is based upon their experimental results. On the flow of liquid metals through assemblies of fibers, Fukunaga has made experimental measurements and theoretical calculations of the permeability of a fibrous preform, which are summarized in a recent publication, t4] Fukunaga and Goda propose the formation of a layer of solidified metal on the fibers and verify its existence through the use of mechanical tests on leachedout fibers. (5] They then derive a Kozeny-type expression for the infiltration coefficient which agrees with experimental measurements. [6] Fukunaga and Goda also propose a critical infiltration velocity as the mechanism for cessation of flow. Girot e t al. t7'81 have performed a numerical analysis of the infiltration of liquid alloys into fibrous preforms. They propose that flow ceases when the metal cools to its liquidus and, therefore, do not account for the release of latent heat of solidification by the metal in their calculations. Clyne et a l . t9.t~ have investigated the critical pressure necessary for melt infiltration and the effect of infiltration pressure on fiber preform deformation. They also explain the evolution of the composite microstructure, including an interpretation of the role of metal superheat. Recently, Martins e t al. [z2] have adapted a model by Brittin (]3] for infiltration of a straight capillary to the infiltration of a metal matrix composite. This treatment is geometrically simplified but has the advantage of including inertial and end-drag effects that might arise in some instances.
I.
INTRODUCTION
T H E liquid metal infiltration process for fabricating fiberreinforced metals consists of the injection and subsequent solidification of liquid metal within the interstitial spaces of a fiber preform. Fluid flow aspects of the infiltration process can be treated readily following the extensive literature on flow of fluids through porous media, an important topic in soil science and chemical engineering. The problem of interest here has additional heat flow and solidification aspects, since the fibers are often held at a temperature significantly below the melting point of the matrix to minimize chemical reactions. In an adiabatic infiltration problem, heat will be exchanged between the metal and the fibers. In a nonadiabatic problem, heat flow will additionally take place between the composite and its surroundings, as is usually the case in casting ses. These heat transfer phenomena are of importance, because they influence both infiltration kinetics and the resulting metal microstructure in the composite. Because of its considerable engineering relevance to metal matrix composite fabrication, the infiltration process has already been studied by several authors, from both theoretical and experimental standpoints. Nagata and Matsuda(J.2} have investigated the infiltration of particles ranging in size from 37 to 3400/~m. They propose the