## 4 Conclusions  
This study quantitatively clarified how tailoring the Cu:Mg mass ratio reshapes the solidification path, precipitation sequence, and service behavior of hypoeutectic Al-Si cast alloys designed for an elevated-temperature wear service. After the heat treatment, the Cu:rich alloy (Cu:Mg = 3:1) preserved a thermally stable \(\alpha\)-Al(FeMn)Si network and maintained supersaturated Cu in a solid solution, whereas the Mg-rich alloy (1:3) developed coarse, lamellar Mg\({}_{2}\)Si together with brittle \(\beta\)-AlFeSi residues. These contrasting microstructures produced divergent properties. At 25 \({}^{\circ}\)C the Cu-rich alloy reached an ultimate tensile strength (UTS) of 280 MPa, a yield strength (YS) of 240 MPa, an elongation (EL) of 4.2%, and a hardness of 108 HV, outperforming its Mg-rich counterpart (210 MPa, 144 MPa, 2.2%, 88 HV). The strength retention at 150 \({}^{\circ}\)C and 250 \({}^{\circ}\)C followed the same trend: 165 MPa versus 124 MPa at 250 \({}^{\circ}\)C. Tribological tests conducted under a constant normal load of 10 N (5 cm s\({}^{-1}\), 25-250 \({}^{\circ}\)C) showed that friction coefficients remained within 0.38-0.43 for the Cu:rich alloy versus 0.30-0.42 for the Mg-rich alloy, while specific wear rates were consistently lower for the Cu-rich material--6.15 \(\times\) 10\({}^{-4}\) mm\({}^{3}\) N\({}^{-1}\) m\({}^{-1}\) at 250 \({}^{\circ}\)C compared with 8.24 \(\times\) 10\({}^{-4}\) mm\({}^{3}\) N\({}^{-1}\) m\({}^{-1}\). Importantly, at the fixed load the wear rate displayed an inverse relation to both the bulk hardness and UTS (\(\Delta\)WR \(\approx\) -7 \(\times\) 10\({}^{-5}\) mm\({}^{3}\) N\({}^{-1}\) m\({}^{-1}\) for every 10 HV or 40 MPa increase), whereas greater elongation correlated with a shorter run-in period but did not significantly affect the steady-state wear. The SEM and 3D-profilometry confirmed that the Cu-rich matrix forms a stable, oxide-rich tribofilm and suppresses delamination, whereas the eutectic-free paths and \(\beta\)-AlFeSi fragmentation dominate the Mg-rich alloy. Collectively, the data demonstrate that a Cu-dominant chemistry favors solid solution strengthening, fine network phase reinforcement, and thermally robust tribofilms, providing a clear performance margin over a Mg-dominant alloy governed by coarse Mg\({}_{2}\)Si precipitation. Overall, this work identifies the Cu:Mg ratio as a key lever for balancing solid solution versus precipitation hardening and elucidates its quantitative impact on the coupled metrics of strength, ductility, and wear resistance. The findings offer a compositionally guided microstructural design strategy for developing heat-resistant Al-Si alloys with predictable wear behavior under elevated-temperature and dynamic loading conditions. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app15148047/s1](https://www.mdpi.com/article/10.3390/app15148047/s1)]([https://www.mdpi.com/article/10.3390/app15148047/s1](https://www.mdpi.com/article/10.3390/app15148047/s1)), Figure S1. JMatPro results: (a) Al-Si isopleth showing the hypoeutectic window, with the red-boxed region enlarged as an inset, and phase-fraction-temperature profiles for (b) Alloy A and (c) Alloy B. Figure S2. Peak deconvolution of the overlapping \(\alpha\)-Al (111) and Al\({}_{2}\)Cu (211) reflections in the 38.0-39.0\({}^{\circ}\) 20 window for (b) as-cast Alloy A (AC_A) and (c) T6-treated Alloy A (HT_A). Raw data are shown as black line, the total pseudo-Voigt fit as a red line, the \(\alpha\)-Al (111) component as a blue line, and the Al\({}_{2}\)Cu (211) component as a green line. Table S1. Chemical composition of hypoeutectic Al-Si-(Cu)-Mg alloys with varying Cu:Mg ratios (wt. %). Conceptualization Methodology, J.B.; Software, J.B.; Validation, J.B. and E.L.; Formal Analysis, J.B.; Investigation, J.B.; Resources, J.B., Y.K. and E.L.; Data Curation, J.B.; Writing--Original Draft Preparation, J.B.; Writing--Review and Editing, J.B.; Visualization, J.B.; Supervision, Y.K.