Flow effects on the fractal dimension of dendrites in an AlSiMg cast alloy

Abstract

The microstructure of directionally solidified AlSi7Mg0.6 alloys (A357) mainly consists of primary phase Al-dendrites in an AlSi eutectic matrix with some Mg2Si particles dilute in the interdendritic region. The dendritic network is conventionally characterized by measuring the primary dendrite spacing and the specific surface area on sections perpendicular to the growth direction, and the secondary arm spacing on longitudinal sections. These global measures of the dendrite network do not, however, adequately describe the branched appearance of secondary and tertiary arms. We therefore took up the idea of, for instance, Bilgram and co-workers to further characterize the microstructure by determining the fractal dimension of the primary phase dendrites. We used samples of A357 directionally solidified with a 3 K/mm temperature gradient and solidification velocities of 20 to 1200 µm/s. Samples were solidified with and without rotating magnetic fields (RMF), which induced a fluid flow of around 10 mm/s as a maximum velocity. Different methods were used to determine the fractal dimension on cross sections taken through the samples. A clear dependence of fractal dimension on solidification velocity is seen, while the effects of fluid flow are less apparent. We discuss our experimental observations with reference to the scarce literature data.