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Microalloying is an effective method to improve the mechanical properties of 6063 Al–Mg–Si alloy. Here, in this work, the effects of trace Cu (0.14 wt.%) and Cr (0.10 wt.%.) additions on the microstructure and mechanical properties of 6063 Al–Mg–Si alloy were studied. The results show that the additions of two elements can cause significant age-hardening effects, and the yield and tensile strengths of the alloys are significantly increased after T6 heat treatment. Further, it is found that the Cu and Cr additions show different strengthening mechanisms in the alloy. Cu addition leads to a finer and denser distribution of the main strengthening phases in the peak aging of the alloy, as well as a large number density of the precipitated phases, resulting in an increase in the dislocation density of the alloy during strain and hindering the dislocation movement. However, Cr addition acts as an inhibitor of grain growth and results in grain refinement, which also increases the hindrance of dislocation movement during the plastic deformation of the alloy, resulting in an increase in the dislocation density, thereby strengthening the alloy. [ABSTRACT FROM AUTHOR]