Archeology-inspired enhancement strategy for aluminum alloys

Metal casting naturally produces a fine-grained chill layer at the mold interface, although this surface layer is commonly removed during machining. Inspired by archeological observations of ancient Cu-Sn bronzes, we investigated the effects of preserving this naturally formed metallic "skin" in modern A356 aluminum alloys. In bronzes, retaining the chill layer reduced corrosion current densities in neutral, alkaline, and acidic environments. In A356 alloys, preserving the chill layer increased ultimate tensile strength by up to 10%, improved elongation from 4.1% to 5.8%, enhanced fatigue endurance by 14%, and increased surface hardness by 20%. These improvements are associated with a naturally formed hierarchical grain structure, in which a fine-grained surface and a coarser interior promote Hall-Petch strengthening and hetero-deformation-induced hardening. These results highlight the functional significance of the chill layer and suggest its preservation as a design consideration for improving cast aluminum alloys without alloy redesign or additional surface processing.