The influence of the reinforcement type on thermal expansion of metal matrix composites
Małgorzata Łągiewka, Zbigniew Konopka, Andrzej Zyska, Maciej Nadolski
Quarterly No. 4, 2009 pages 380-383
DOI:
keywords: composites, thermal expansion, coefficient of thermal expansion, aluminium alloys
abstract The examination results concerning thermal expansion of AlMg9 alloy matrix composites have been presented in the work. Composites reinforced with chopped carbon fibre or graphite particles have been produced for experimental purpose. The length and the diameter of the applied carbon fibre has been equal to 4 mm and 7 μm, respectively, while graphite particles has been of 100÷160 μm average size. The volume percentage of the reinforcement has been selected to be 5, 10, or 20%. The composite castings have been produced by suction of beforehand prepared composite suspension into silica pipes of 9 mm diameter. The examination of thermal expansion has been held by means of an automatic dilatometer. The examined cylindrical specimens have the length of about 35 mm and 7 mm diameter. Specimens have been heated up to 400°C at the rate of 1K/s and then cooled to the ambient temperature at the same rate to determine their dimensional changes. The recorded changes of length against temperature have been presented in the form of dilatometric curves, which in turn have enabled to determine dimensional changes after heating and cooling cycle. The greatest shortening of specimens - 0.046 mm - has been observed for pure matrix alloy, the least one has been recorded for composite with 20% of graphite, which has revealed no dimensional changes at all. Then the instantaneous coefficients of thermal expansion have been calculated according to Formula 1. The results of examinations show that the coefficient of thermal expansion is greater for composites than for matrix alloy. The differences are of minor significance for composites containing graphite particles, but for composites with carbon fibre the coefficient of thermal expansion reaches much greater values. The reason of this phenomenon can be the microporosity of composite castings, which is difficult to avoid during production of composite suspension. Another reason can be also the lack of adhesive bond between the matrix and the reinforcement in the examined composites. The performed tests allow for undoubted conclusion that introducing particulate graphite reinforcement into the matrix alloy results in the dimensional stability of castings, while introducing chopped carbon fibre, though improves the dimensional stability, nevertheless to the significantly less degree than graphite particles.