• The combined addition of Al−Ti−La master alloy and Nd significantly refines the α-Al dendrites and eutectic Si in Al−7Si alloy, reducing secondary dendrite arm spacing from 18.3 to 11.9 μm and eutectic Si length from 8.6 to 5.0 μm.
• T6 heat treatment (535 °C/3h + 165 °C/3h) further spheroidizes and reduces the size of eutectic Si, leading to substantial improvements in microhardness (24.2%), ultimate tensile strength (11.6%), and elongation (194.0%) compared to as-cast state.
• The modifiers lower the nucleation temperature of eutectic Si, suppressing its growth, and the formation of Ti2(Al,Si)20(La,Nd) phase near eutectic Si particles inhibits their coarsening.
• The combination of modification and heat treatment offers a promising route to enhance the mechanical properties of hypoeutectic Al−Si alloys for automotive and aerospace applications.