The role of Si on the microstructure of Al casting alloys and short fibre composites

Thermal stability and the improvement of the mechanical properties of AI-Si alloys and their composites are important for their application in engine and break components. The study of the three-dimensional microstructure of the materials would be a significant tool for further alloy and composite development by understanding the influence of the 3D architecture of their heterogeneities. In this work the thermal expansion of different AISi1.1, -1.7, -7, -12 alloys and reinforced AISi1.1, 7, 12, 18 alloys with 20%vol. of Al203-short fibres is investigated. A reduction of the thermal expansion is observed by increasing the Si content in both unreinforced and reinforced AI-Si alloys. The addition of random planar alumina fibres reduces the in plane thermal expansion, but the transverse expansion is significantly increased with respect to the matrix. The precipitation kinetics of Si is studied by means of Differential Scanning Calorimetry (DSe). The measured exothermic and endothermic effects produced by the precipitation and dissolution of Si are correlated with an increase and reduction of the coefficient of thermal expansion (CTE), respectively. he results provide a method to use dilatometry for studies of precipitation kinetics of Si in aluminium.