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2 Archives of Foundry Engineering

2 2009

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Oxidation of the AlSi6Cu4 alloy and AlSi6Cu4-graphite particles composite at the elevated temperatures

Konopka Z., Holecek S., Pożar J., Nadolski M., Łągiewka M., Zyska A.

Archives of Foundry Engineering

2009

Vol. 9, iss. 2

61-64

Oxidation process of AlSi6Cu4 alloy as a composite matrix and of AlSi6Cu4 / 8 vol.% graphite particles composite was investigated. Composites were prepared by stirring method for suspension obtaining and by squeeze casting of the suspension. This process was examined by testing specimens annealed during up to 1000 hours at 573 K and 673 K in air atmosphere. The average oxidation state, oxide layer thickness and hardness of examined materials were measured during the annealing time. Obtained results imply the following conclusions: composite oxidizes faster than matrix alloy at both temperatures what is confirmed by higher weight gains and thicker oxide layer. The rate of oxidation of both materials gradually slows down at both temperatures. At initial stages of annealing at 673 K the rate of oxidation of both materials is much higher than that at 573 K. With increasing time of annealing the ratio of oxidation rate at 673 K to the one at 573 K comes down. Hardness of the composite is lower than that of matrix alloy before and during annealing at both temperatures. Drop in hardness at both 573 K and 673 K is the same for matrix and composite, and after about 100 hours the hardness no longer descents.

Corrosion behaviour of the AlSi6Cu4 alloy and cast AlSi6Cu4-graphite particles composite

Konopka Z., Holecek S., Pożar J., Nadolski M., Łągiewka M., Zyska A.

Archives of Foundry Engineering

2009

Vol. 9, iss. 2

65-68

The corrosion behaviour of the AlSi6Cu4 alloy as a composite matrix and of composites with 8% vol. of graphite particles was investigated. The corrosion experiments were performed over a range of elevated temperatures and were carried out in sea water (3.5% NaCl solution). We have focused our attention to the determination of the mode of corrosion attack and to the determination of the rate of corrosion and other corrosion characteristics. Both as-cast and annealed matrix and composite specimens were tested, as well as the 99.9% as-cast aluminium for comparison. Corrosion behaviour of the materials was assessed by the corrosion potential (Ec) and by potentiodynamic (polarization) curves. As expected, composite is less corrosion resistant than the matrix alloy. In addition to pitting, a severe galvanic corrosion occurs as a result of galvanic couple aluminium/graphite formation. Corrosion potentials imply that examined materials would be sufficiently resistant in non or slightly oxidizing solutions without dissolved oxygen. All studied materials corrode very slowly at potentials negative to corrosion potential, while at potentials positive to corrosion potential the corrosion rate goes up by 1 or 2 orders.