<p>The effects of 0.3 wt.% Cu addition on the age-hardening behavior and intergranular corrosion (IGC) resistance of an Al-Mg-Si alloy were systematically investigated. Hardness evolution, tensile properties, differential scanning calorimetry, transmission electron microscopy (TEM), immersion corrosion tests, and electrochemical measurements were employed to clarify the role of Cu during aging. The results show that Cu significantly accelerates precipitation kinetics, leading to a higher initial hardening rate and a markedly increased peak-aged strength. TEM observations reveal that Cu promotes the formation of a high number density of fine intragranular precipitates, mainly β″ phase accompanied by minor Cu-containing precipitates, which enhances the strengthening effect. However, Cu addition also aggravates intergranular corrosion. The Cu-containing alloy exhibits deeper IGC penetration and higher corrosion current density, indicating reduced corrosion resistance. This deterioration is attributed to Cu segregation and the formation of more continuous grain boundary precipitates, which increase electrochemical heterogeneity and facilitate anodic dissolution along grain boundaries. These results demonstrate that while Cu microalloying effectively improves age-hardening response, it simultaneously compromises intergranular corrosion resistance, highlighting the need for a balanced alloy and heat-treatment design.</p>