Si modification is a common practice in the production of aluminum–silicon components. The presence of various elements in recycled alloys can affect the modifying role of Sr. In this research, the effect of different Sr/Cd ratios on the grain refinement of aluminum dendrites and the modification of eutectic Si was evaluated in two widely used alloys, AC4C and ADC12. Thermal analysis using a two-thermocouple method was applied to predict solidification kinetics, including the nucleation temperature, growth temperature, recalescence, coherency point, and solid fraction. Compared to the Sr-modified alloy, different Sr/Cd ratios showed no effect on the nucleation and growth temperatures, coherency temperature, temperature and time interval to coherency, or the solid fraction at the coherency point. Also, the eutectic growth temperature of AC4C and alloy ADC12 remained constant within the modification temperature range, at 564 °C and 566 °C, respectively. Microstructural observations using optical and SEM confirmed the thermal analysis results. At the same time, different Sr/Cd ratios did not have a significant effect on the secondary dendrite arm spacing, Si morphology, Si interspacing, Si length, thickness, and aspect ratio. The hardness of the alloys remained constant at approximately 73 VHN for the AC4C alloy and 75.5 VHN for the ADC12 alloy. It can be concluded that the presence of Cd did not show a detrimental effect on the modification role of Sr. On the other hand, a comparison of the two alloys showed that the solid fraction at the coherency point decreases by 50 pct, and the Si solid fraction increases with the increase of Si. Furthermore, the increase in Si led to finer aluminum dendrites (89 to 93 to 48 to 52 μm), but the Si inter spacing did not show a significant change (4.5 to 5.6 μm).
