The influence of graphite particles on thermal expansion of metal composites AlSi6Cu4/graphite
Marian Mitko Politechnika Częstochowska, Katedra Odlewnictwa, al. Armii Krajowej 19, 42-200 Częstochowa
Annals 4 No. 10, 2004 pages 175-178
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abstract Metal composites reinforced with synthetic graphite particles have been obtained by squeeze casting of AlSi6Cu4 (AK64) cast alloy mixed with graphite particles. Composites containing 4, 8, and 12% (by volume) of graphite have been obtained. Also pure AlSi6Cu4 alloy (i.e. without graphite addition) has been squeeze cast for the purpose of comparison. Exemplary structures of such obtained materials have been shown in Figure 1. Determining of the coefficient of thermal expansion has been performed with use of the LS-4 optical dilatometer. Measuring cycle has covered heating of specimens from the temperature of 25 to 415°C and subsequent cooling back to 25°C, both with the rate of 180°C/h. Results of calculating average values of the coefficient of thermal expansion (α25-T) have been shown in Figure 2 for the data obtained on heating, and in Figure 3 for the data gathered during the cooling period. Dimensional changes of specimens after dilatometric examination have been presented in Table 1. A rapid growth in the coefficient of thermal expansion - from 20÷22 • 10−6 K−1 to 26÷28 • 10−6 K−1 - occurs during heating from 200 to 300°C both for examined composites and for AlSi6Cu4 alloy. Further heating to 415°C results in decreasing of the coefficient of thermal expansion to 25÷27 • 10−6 K−1. During the cooling stage dropping of the coefficient of thermal expansion has more even character. The dimensional growth after the heating and cooling cycle is much larger for composite specimens than for the specimens of pure matrix alloy. Thermal expansion of composites as compared with with the value for matrix alloy is the smaller the greater is the graphite particles percentage in composite material. These differences exceed 10% at the graphite percentage of 8÷12%, and reach 5% at the graphite content of 4%. This suggests the existence of strong bonding at the particle-matrix interface. The increase of graphite particles content is advantageous for the serviceability of AlSi6Cu4/graphite composites not only because of diminishing the frictional wear and the coefficient of friction, but also due to decreasing of thermal expansion of composite material. Improving the dimensional stability as well as decreasing working stresses can be achieved by employing a proper thermal treatment of elements made of such composites, which should result in prolonging their lifetime. Key words: aluminium alloys, composites, graphite particles, thermal expansion