Numerical prediction of microporosity formation during the solidification of a pure metal within a porous preform

During the elaboration of metal-matrix composites (MMCs) by infiltration, the appearance of microporosities is more frequent and more difficult to control than in traditional foundry because the fibrous preform slows down the circulation of the liquid metal. With the aim of modelling solidification shrinkage and its prediction, we will present in this paper an algorithm which takes into account : (i) the volume drop due to the solidification shrinkage, (ii) the nucleation of microporosities and (iii) the apparent compressibility of the liquid. It is demonstrated that the apparent compressibility is due to the presence of gas trapped in the liquid during the impregnation of the preform. Secondly, we will present an experimentation with solidification of molten metal infiltrated into a preform. The solidification of the metal in the preform is presented according to various experimental configurations in order to show the influence of the solidification parameters. Lastly, numerical and experimental results will be compared for conditions with and without riser which validates the numerical model. (C) 2002 Elsevier Science Ltd. All rights reserved.