METAL-MATRIX COMPOSITES FABRICATED BY PRESSURE-ASSISTED INFILTRATION OF LOOSE CERAMIC POWDER

A composite fabrication method has been developed in which pressure is applied to infiltrate loose ceramic particles by liquid metal. The pr essure applied has been exerted by two different techniques: centrifug al and squeeze casting. Both techniques are found to be successful in preparing Al-Al2O3 metal-matrix composites. In this method the alumina powder, followed by a rod of aluminium, is inserted into a tube which is heated to a temperature above the melting point of the alloy. In c entrifugal casting the tube is rotated around an axis perpendicular to that of the tube and a centrifugal force is induced which acts on the liquid metal. In squeeze casting a squeezing force is applied to the liquid metal using a similarly heated plunger. The composite is formed by the infiltration of the liquid metal through the powder interstice s under the action of the force applied. The infiltration mechanisms i n both techniques are found to be different. Whilst the formation of t he composite was in the direction of the squeezing force, it was in th e direction opposite to the centrifugal force. However, four cases of infiltration are obtained in both techniques: no infiltration because the pressure applied is lower than the back pressure due to surface te nsion; partial infiltration with remaining metal above the alumina pow der due to low pressing time; full infiltration due to there being suf ficient metal charge and sufficient time; and partial infiltration due to an insufficient metal charge. The infiltration mechanisms in both cases are suggested in view of the application of different processing conditions, different Al2O3 particle sizes and the use of a commercia l aluminium and an Al-Si alloy. The structural features of the composi te, namely the particle distribution, the metal/ceramic interface and the soundness, were studied. Comparison between the structures obtaine d by both techniques is made. In both cases, a high volume fraction of Al2O3 ranging between 50 and 65% was obtained. (C) 1998 Elsevier Scie nce S.A.