Aluminum/carbon composites have garnered significant attention due to their promising thermal and mechanical properties. However, traditional manufacturing processes typically involve high-pressure techniques to achieve part densification, rendering them costly and less practical for large-scale applications. In this study, aluminum-AlSi12/diamond composites were produced using a pressureless, liquid-phase sintering technique. By carefully optimizing the sintering parameters, the densification of the composites reached 86%. Notably, this was further improved to 94% when the diamond particles were coated with copper, demonstrating the benefits of surface modification in enhancing densification. This innovative approach highlights the potential of free sintering as an effective method to achieve high-density composites without resorting to expensive high-pressure methods.