Study of the porosity produced in an aluminum alloy matrix composite due to a T6 heat treatment

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 δ-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 µm) 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.