Hardness of composite materials with high fibre contents
Krzysztof Naplocha, Zdzisław Samsonowicz Politechnika Wrocławska, Instytut Technologii Maszyn Automatyzacji, ul. Łukasiewicza 3/5, 50-375 Wrocław
Annals 1 No. 2, 2001 pages 199-202
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abstract The reinforcement of aluminum alloys is being investigated as a means of improving their hardness and other properties. Published literature on hardness of aluminum-based composites is however rather limited and available data tend to concentrate upon one kind of material from wide family of composites. It is hard to state which factor significantly affects on hardness. We may find only general parameters, common for all composites such as: fibre contents, properties of fibres and matrix, arrangement of fibres, quality of interface between fibre and matrix. In this study fibre contents changed from 10 to 54 vol. % was particularly treated. Two series of composite material were fabricated. First contained 10÷20 vol. % of fibres was obtained by using squeeze casting method. Production process consisted of placing of heated porous preforms (700°C) in the heated die, pouring of overheated (780°C) molten alloy AK9 alloy into the die and pressing it in order to infiltrate the porous preforms with the molten alloy. The infiltration pressure for the particular samples was increased from 30 to 150 MPa and kept as long at the moment of alloy solidification was reached. In order to eliminate porosity in the composite material, a special design of the cast was applied, which allows for an escape of the air inside preform during infiltration. Figure 2 shows effect of infiltration pressure on hardness. There was noticed increase of hardness with increase of fibre contents especially for composite materials fabricated by using the lowest value of infiltration pressure and increase of infiltration pressure caused very negligible increase of hardness of samples strengthened with 20% of ceramic fibres. The highest hardness of 140 HB was that of the composites containing 20% vol. of the fibre produced at high infiltration pressures. Hardness of unreinforced alloy was 80% lower than that. Manufacturing process of second series allows introducing to aluminum alloy high quantity of fibres, unusually for these types of composites. As a result of this outstanding increase of hardness HB was observed. Composite reinforced with 54 vol. % of fibre posses 255 hardness (Fig. 2). It is three times higher then hardness of matrix. The microscopic observations confirmed obtaining a proper link between the fibres and the matrix, unfortunately we also observed micro cracks on the fracture of same samples (Fig. 3). It proves present considerable stress inside the composite and force to search a solution of this problem. Presented results are first steps of work focused on composite material reinforced with high fibre contents.