Influence of Mg and Ti on both eutectic solidification and modifying efficiency in Sr-modified Al-7Si cast alloys

Magnesium and titanium are the main alloying elements always present in a commercial A356 alloy, and strontium is commonly added to achieve a good degree of modification of the eutectic structure. Whilst most of the studies have been focused on the role of strontium on commercial A356 alloy, little attention has been paid to understand the possible interaction of magnesium and titanium with the modifying efficiency of strontium. In the light of these aspects, the present study is aimed at investigating the effects of magnesium and titanium on the microstructural evolution of Sr-modified Al-7Si, Al-7Si-0.4Mg and Al-7Si-0.4Mg-0.12Ti alloys. To assess the role of Sr on the eutectic solidification path, the analysis of the cooling curves combined with a quantitative microstructural evaluation of the eutectic silicon particles has been carried out. Thermal analysis results highlight how the alloys that contain magnesium show a significant decrease, higher than 5 degrees C, of the thermal parameters of the eutectic solidification, with respect to the Al-7Si alloy. On the other hand, titanium seems to have only a slight effect on the same solidification characteristics. Metallographic investigations and the related statistical analysis of some geometrical parameters of the silicon particles indicate that the presence of magnesium and titanium induces a variation of both dimension and morphology of the particles. As a result, these experimental findings suggest that the effect of alloying elements, such as titanium and magnesium, on the thermal parameters obtained from cooling curves has to be taken into consideration when the thermal analysis is used to assess the strontium modification efficiency.