Directionally solidified Al-Si-Mg alloys

This work analyzes the influence of cooling rate and chemical composition on the microstructural evolution and mechanical properties of Al–Si–Mg alloys obtained by directional solidification. Compositions of 3 wt% Si and 8 wt% Si were studied, incorporating Mg additions of 2wt. % and 5wt. % for the Al–3 wt% Si alloy and 4wt. % for the Al–8 wt% Si alloy. The objective was to evaluate the effect of these variables on macro- and microstructural parameters, such as grain size, columnar-to-equiaxial transition (CET), secondary dendritic spacing (λ₂), eutectic interlamellar spacing (λe), and silicon particle size.

The results show that increasing the cooling rate produces a marked refinement of the macro- and microstructure, evidenced by the decrease in λ₂ and λ_e, as well as by the reduction in grain size. This structural refinement results in a significant increase in Vickers microhardness (HV). Furthermore, the increased silicon content promotes an increase in the eutectic fraction, contributing to the material's hardening.

On the other hand, the addition of Mg promotes the formation of Mg₂Si precipitates, generating an additional increase in hardness through combined mechanisms of solid solution hardening, precipitation, and microstructural refinement. A correlation was also established between microhardness and variables such as λ₂ and λ_e through a multiple regression model, demonstrating the interaction between thermal and microstructural factors.

Finally, it was observed that ternary alloys exhibit lower λ₂ values ​​and higher HV values ​​compared to binary alloys, even at lower cooling rates.