Physical properties of graphite/aluminium composites produced by gas pressure infiltration method
Introduction
Porous graphite preforms have already been impregnated by copper, silver, lead and their alloys in order to obtain composites with low thermal expansion combined with relatively high mechanical strength or high electrical conductivity. The disadvantages of these metal materials are their heavy weight and environmental concerns. Therefore light metal—aluminium or magnesium—infiltrated graphites obtain increased attention as potential candidates for lightweight components such as parts of combustion engines or current collectors [1]. In addition to a reduced weight, the low coefficient of thermal expansion (CTE) in combination with excellent tribological behaviour and temperature resistance of carbon materials allow a fuel and oil reduction in comparison to conventional aluminium alloys [2]. As a result the amount of pollution gases in the engine exhausts are drastically reduced [3]. Furthermore, such engines have shown excellent antiseizing properties due to the favourable tribological properties of the graphite.
Graphite piston materials have to meet a number of requirements. In order to sustain the cyclic tensile and pressure loads in the engine, the composites must exhibit a sufficiently high flexural strength of at least 100 MPa and a Weibull modulus m>20. High thermal conductivity, in excess of 50 W m−1 K−1 and a low coefficient of thermal expansion (CTE) ensure the combustion process [4].
One method to produce aluminium/graphite composites is the infiltration of a porous graphite preform with liquid aluminium. The principle of the infiltration of porous material with metal is to force the metal into the preform under external pressure in order to overcome the bad wettability between aluminium and graphite. This paper illustrates characteristic features of graphite/aluminium composites produced by gas pressure infiltration (GPI) method. Physical properties (electrical conductivity, coefficient of thermal expansion) and mechanical properties (flexural strength) are investigated as a function of different metal matrices (Al99.85 or AlSi7Mg) and graphite preforms (with 10 or 13 vol% porosity). Due to the mismatch of coefficients of thermal expansion between graphite and the metal phase, these composite systems may be susceptible to thermal damage. The thermal stability of these composites was therefore investigated by means of thermal cycling. To the authors knowledge no papers exist on porous graphite/aluminium composites produced by GPI method.
Section snippets
Monolithic materials
Two porous graphite preforms have been used, FU2590 (10 vol% porosity) and FU4501 (13 vol% porosity), both manufactured by Schunk Kohlenstofftechnik (Germany), characterised by the processing steps: cold isostatic pressing, carbonising and graphitising. Relevant mechanical and physical properties can be seen in Table 1. Graphite preforms were infiltrated either with 99.85 pure aluminium or an AlSi7Mg alloy. Table 2 shows the chemical composition (wt%) of these two different metal matrices.
Composite production–gas pressure infiltration
The
Material characterisation
Characteristic optical micrographs of graphites infiltrated with the AlSi7Mg alloy are shown in Fig. 2, Fig. 3. The metal phase (bright) has filled the pores homogeneously. However, some residual porosity is visible (black regions in Fig. 2). Closed porosity resulting from graphite production or inhomogeneous infiltration during composite manufacturing may be responsible for residual porosity. Silicon phases in the metal matrix (indicated by arrows in Fig. 3) became visible after etching the
Conclusions
Graphite-based composites have been infiltrated either with pure aluminium or an AlSi7Mg alloy by means of the gas pressure infiltration method. The AlSi7Mg matrix morphology in the graphite composites is strongly affected by the preform and the process parameters. Instead of fine eutectic silicon particles, typical for monolithic AlSi7Mg, coarse silicon phases were formed and inhomogeneously distributed inside the metal phase. An amount of about 10 vol% AlSi7Mg has increased the electrical
Acknowledgments
The authors would like to thank Schunk Kohlenstofftechnik GmbH for delivering the porous graphite preforms.
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