Elsevier

Carbon

Volume 41, Issue 5, 2003, Pages 1017-1024
Carbon

Physical properties of graphite/aluminium composites produced by gas pressure infiltration method

https://doi.org/10.1016/S0008-6223(02)00448-7Get rights and content

Abstract

Graphite/aluminium composites have been produced by means of gas pressure infiltration method. Two porous graphite preforms with a porosity of 10 and 13 vol%, respectively, have been infiltrated using either a commercially 99.85 pure aluminium or an AlSi7Mg alloy. Thermal expansion coefficient, electrical conductivity and flexural strength have been determined as a function of graphite preforms and metal matrices. To investigate the susceptibility of this composite system to thermal damage, specimens were thermally cycled between 60 and 300 °C up to 1020 cycles. Infiltrated graphites exhibited a significantly higher electrical conductivity (0.34–0.51 m/Ω mm2) compared to porous graphite preforms depending on graphite type and metal matrix. Thermal cycling did not influence electrical conductivity. The coefficients of thermal expansion of the composites were at least three times lower than for monolithic aluminium. Thermal cycling has reduced these values even more, most likely due to stress relaxation processes. The infiltration of porous graphite preforms with AlSi7Mg alloy or Al99.85 has increased the flexural strength of the composites resulting in values up to 105 MPa. The decrease in mechanical strength due to thermal cycling was about 10%.

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.

References (23)

  • C. Carcia-Cordovila et al.

    Pressure infiltration of packed ceramic particulates by liquid metals

    Acta Mater

    (1999)
  • Cited by (77)

    • Improving the Young's modulus of Mg via alloying and compositing – A short review

      2022, Journal of Magnesium and Alloys
      Citation Excerpt :

      Moreover, carbon fibers possess good high-temperature chemical stability and do not react with molten Mg, which enables the fabrication of Cf/Mg composites through the melt processing techniques. Recently, continuous carbon fiber reinforced metal matrix composites (CF-MMCs) are usually fabricated by gas pressure infiltration [32,77], low pressure infiltration [67,76] and pressure infiltration techniques [78,79]. Specifically, preforms of carbon fibers are prepared by entangling or winding the long fibers unidirectionally to a designed shape by a computer numerical control (CNC) winding machine.

    View all citing articles on Scopus
    View full text