The effect of Sn additions on the semi-solid microstructure of an Al–7Si–0.3Mg alloy

The influence of Sn additions on both the binary Al–Si equilibrium phase diagram and the microstructural evolution of an Al7Si0.3Mg alloy (A356) having a solid/liquid ratio close to 0.5 were studied. Application of the THERMOCALC method showed that in the range 0.2–10.0 wt.%, Sn decreases all the L→S transformation temperatures. The microstructural evolution was studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and by quantitative optical microscopy. Results showed that Sn additions decreased the rate of attainment of the equilibrium liquid fraction besides reducing the kinetics of particle spheroidisation and growth. This behaviour was attributed to a decrease of solid–liquid surface energy brought about by the presence of Sn as small globules within the Si or Al phases and at the Al–Si interface. This interpretation was supported by experimental measurements of particle contiguity and by dihedral angle calculations. The less accentuated effect observed on sample 2.0Sn, was explained by a different spatial distribution of the Sn globules.