Abstract: Microstructural analysis was performed on the as-cast microstructure of the ADC12 aluminum alloy castings. Solution treatment and aging treatments with different processes were conducted to the alloy samples. The microstructure and hardness were analyzed. The results show that after solution aging of the ADC12 aluminum alloy, the microstructure contains Al-Si compound phases within the α-Al matrix. Eutectic silicon exhibits a spherical, fine granular morphology with aging temperature rises. During the aging treatment at 165℃-185℃, peak aging occurs when the alloy exhibits its highest hardness, which is attributed to the precipitation of θ′, β′, M′, and S′ phases that are coherent with the matrix. The occurrence of overaging leads to a decrease in alloy hardness, as these strengthening phases transform into stable θ phase (CuAl₂), β phase (Mg₂Si), M phase (MgZn₂), and S phase (Al₂CuMg), thereby losing coherent interfaces with the matrix. The ADC12 aluminum alloy exhibits the highest hardness after aging at 165℃, reaching HRB 77.7. The heat treatment process of solution treatment at 535℃ for 3 h followed by aging at 165℃ for 6 h is optimal in this experiment.