<p>In this study, the Cu content was varied at 0.0 and 0.5 wt%, and the Zr content at 0.0 and 0.2 wt%, while the solution-treatment time was adjusted from 6 to 72 h to analyze the phase transformations and evaluate the fatigue properties of Al–Si–Mg alloys based on Cu and Zr contents and heat-treatment conditions. The morphology and area fraction of the precipitates under different heat-treatment conditions were measured. As a result, a significant number of platelet (Al,Si)₃(Ti,Zr) phases were observed, which were formed by the substitution of some elements in Al₃Zr. The results confirmed that as the solution-treatment time increased, the Zr precipitates partially dissolved into the matrix with some being reprecipitated after dissolution. In addition, the metastable L1₂ (Al,Si)₃(Ti,Zr) phase transformed into the stable D0₂₃ (Al,Si)₃(Ti,Zr) phase due to the addition of Cu. Tensile and fatigue tests were conducted to evaluate the impact of these changes on the mechanical properties of the alloys. The results showed that, in the case of Zr-added alloys, the elongation of the material at high temperatures significantly increased with longer solution-treatment times, and the presence of Zr particles induced substantial deflection, thereby improving the fatigue characteristics.</p>