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This paper investigates hybrid aluminum castings produced by the overcasting method, targeting lightweight solutions for the engineering industry. The novelty of this work lies in the systematic evaluation of how foaming pressure parameters and subsequent core surface treatments influence the final mechanical properties of these complex components. The principle of this innovative technique is to overcast a porous cellular core, which is created by foaming a molten aluminum alloy. Porous cores were fabricated using various foaming pressures, and their influence on mechanical performance was assessed through uniaxial compression and impact tests. The results demonstrate that controlled foaming pressures, specifically starting pressures of 0.1–0.2 MPa and stabilizing pressures of 0.05 MPa and 0.101 MPa, yield porous cores with the most consistent and highest compressive strength. Crucially, the integrity of the final hybrid casting is highly dependent on the surface treatment of the core prior to overcasting. X-ray tomography revealed that treating the core with 10% H3PO4 acid effectively prevents molten metal penetration, resulting in a compressive strength three times higher than that of untreated or improperly treated castings. Furthermore, impact testing showed that the porous cores exhibit an average impact toughness 3.4 times higher than solid specimens of the same dimensions, highlighting their superior energy absorption capabilities.