<p>To gain a deeper understanding of how diverse microstructures influence the thermal conductivity of Al-Si alloys, the mechanism governing thermal conductivity variations in ADC12 alloy was elucidated through directional solidification and subsequent heat treatment processes. The contributions of grain boundaries, second phases, and solid-solution atoms to thermal conductivity were qualitatively evaluated. Results show that directional solidification substantially eliminates transverse grain boundaries, and the thermal conductivity reaches 189.59 W·m⁻¹·K⁻¹. After 24 h solid solution treatment, the average grain size decreases from 57.10 µm (as-cast) to 21.65 µm and the aspect ratio of eutectic Si reduces from 8.94 to 4.52. However, the extensive solid solution of Cu in the α-Al matrix induces severe lattice distortion, significantly reducing the thermal conductivity to 135.40 W·m⁻¹·K⁻¹. The achievement of high thermal conductivity is attributed to the substantial elimination of grain boundaries, which serves as a critical mechanism for thermal conductivity enhancement of ADC12 alloy. These findings provide novel strategies and theoretical insights for achieving excellent thermal conductivity in aluminum alloys by customizing their microstructures.</p>