Study of the porosity produced in an aluminum alloy matrix composite due to a T6 heat treatment
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6/14/04
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Study of the Porosity Produced in an Aluminum Alloy Matrix Composite Due to a T6 Heat Treatment A. GARCÍA-ROMERO, A.M. IRISARRI, M. ANGLADA, and C. GARCÍA-CORDOVILLA In the present article, the reactions and phase transformations occurring between the fibers and the matrix during the T6 thermal treatment of an industrial composite are analyzed. The composite (taken from prototype diesel piston heads) consisted of an aluminum alloy conforming to AFNOR AS12UNG (Al-12 pct Si-1 pct Mg-1 pct Cu-1 pct Ni) that was reinforced with SAFFIL -alumina short fibers. Different heat-treating temperatures, holding times, and heating rates were considered. As a result of all these treatments, small to medium size (between 1 to 10 m) holes were observed at the matrixfiber interfaces, although the amount and size varied depending on the heat-treating conditions. This porosity was attributed to the volume contraction associated with the reaction between the silica contained in the preform (colloidal silica is the binder for the alumina fibres) and the magnesium present in the aluminum alloy to produce MgO and Si during the solution heating treatment. This local volume contraction is higher than 28 pct.
I. INTRODUCTION
DISCONTINUOUSLY reinforced aluminum (DRA) composites are increasingly being considered for structural applications in the aerospace and automotive industries because they show exceptionally good wear resistance, specific strength, and specific stiffness. Moreover, they possess higher hardness and better high-temperature performance when compared to the corresponding unreinforced alloys.[1] Their industrial use, however, requires a better understanding of their in-service behavior and the development of acceptable industrial production routes, both technical and economical. Locally reinforced components are very attractive at an industrial scale because the characteristics offered by the composite are employed only in the places where they are actually required, without suffering the drawbacks of lower toughness, higher heterogeneity, and higher cost that are usually found when the reinforcement fibers are present in the entire component. The use of -alumina short fibers as local reinforcement in automotive pistons was one of the first applications industrially envisaged for these materials. As a result, various studies have already been published regarding the production and the characterization of -alumina short fiber-reinforced aluminum alloys. In several studies, it has been reported that during the infiltration of the preform, the magnesium contained in the molten alloy reacts with the reinforcement fibers. This reaction can be produced with the silica binder of the preform being reduced to silicon (that enters into the alloy composition), while the magnesium is oxidized to MgO,[2,3,4] or with the alumina reinforcement fibers themselves forming spinel MgAl2O4.[5,6] Both reaction products, MgO and the spinel, usually form on the fibers. As a A. GARCÍA-ROMERO and A.M. IRISARRI, Doctor
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