We report structural, mechanical, and thermal properties of diecast ADC12 aluminum alloys characterized using synchrotron X-ray diffraction (XRD), scanning electron microscopy, energy dispersive X-ray (EDX) analysis, thermal conductivity (λ), Vickers hardness (Hv), and stress-strain measurements. We also studied the effect of post-annealing performed in a vacuum atmosphere on the mechanical properties of diecast ADC12 alloys. EDX and XRD results revealed that Al2Cu and AlCu3 grains are formed, well dispersed in Al base and highly crystalline. Ultimate tensile strength (UTS) of 307.9 ± 9.1 MPa and elongation of 2.98 ± 0.62% were estimated. λ was 129.3 ± 0.27 W/mK and Hv was approximately 130. Both values were significantly higher than the reported values. At annealing temperatures ranging from 25 to 200°C, UTS and Hv values remained constant, while as the annealing temperature increased to 500°C, these values gradually decreased. This is because stabilization of the microstructure improves toughness and ductility.