<p>Solidification conditions greatly affect the final quality of components. Therefore, investigating solidification characteristics is very important. This study examines the influence of the modification process and cooling rates on hyper-eutectic Al–17 pct Si alloys using thermal analysis. Accordingly, the cooling curves of these alloys were plotted at cooling rates of 0.14, 0.26, 0.84, and 2.04 °C/s. The effect of the cooling rate and the addition of phosphorus as the modifier on the solidification characteristics of these alloys was examined. The findings from the cooling curves and microstructural evaluations indicate that changes in cooling rate and the addition of phosphorus significantly influence the nucleation and growth of primary silicon crystals, as well as the solidification range, solid fraction, and characteristics of the eutectic reaction. Increasing the cooling rate raises the nucleation temperature of primary silicon, enhances nucleation undercooling, elevates the nucleation temperature of eutectic phases, increases the final solidification temperature, and expands the solidification range. Moreover, modification with phosphorus leads to an increase in the nucleation temperature of primary silicon crystals and eutectic phases, extends the solidification temperature range and time, while also resulting in a decrease in the final solidification temperature compared to the unmodified state.</p>