Synergistic Effect of Cu Addition and Pre-Straining on the Natural Aging and Artificial Age-Hardening Behavior of AA6111 Alloy

This study systematically investigates the synergistic effects of Cu addition (0-0.7 wt.%) and 2% pre-straining on the artificial aging, natural aging (NA), and bake-hardening response (BHR) of AA6111 alloy. The results reveal that Cu significantly enhances age-hardening capacity and accelerates artificial aging kinetics. The 0.7Cu alloy achieved a 14% higher peak hardness (106.9 HV) than the Cu-free alloy (93.8 HV) while reducing peak aging time by 50% (from 10 h to 5 h). Pre-straining further promoted hardening rates, shortening peak aging times to 2 h for the 0.7Cu alloy. Natural aging (NA) severely suppressed BHR in non-pre-strained alloys, reducing paint baking (PB) increments by 75-77.5% after 14 days. However, the introduction of pre-straining not only reduces the negative effects of NA but also improves the BHR. TEM analysis demonstrated that Cu addition accelerated the precipitation of fine GP zones and β″ phases while pre-straining introduced dislocations that acted as heterogeneous nucleation sites for Q' phases, refining precipitates and suppressing NA cluster formation. The synergistic combination of 0.7Cu and pre-straining achieved a superior BHR yield strength increment of 68.1 MPa with retained ductility, highlighting its potential for automotive applications requiring balanced formability and post-forming strength.