Effect of Si content on microstructure, mechanical, and thermal/electrical conductivities of Al-xSi-0.3Mn-0.3Mg-0.14Fe alloy prepared by super-slow-speed die-casting

• Increasing Si content from 6.5% to 8.5% promotes eutectic segregation band formation in super-slow-speed die-cast Al-Si-Mn-Mg alloys. • Optimal mechanical properties (quality index) are achieved at different Si contents depending on heat treatment: as-cast (7.5% Si, QI=376.1 MPa), T5 (6.5% Si, QI=373.4 MPa), and T6 (8.5% Si, QI=432.2 MPa). • T6 heat treatment significantly enhances both thermal and electrical conductivities by spheroidizing eutectic silicon and precipitating Mg and Si solutes, reducing electron scattering. • Thermal and electrical conductivities decrease slightly with increasing Si content, indicating a strong correlation with Si content in the alloy.