Combined effect of micro-alloying and enhanced pre-aging on the paint-bake response of an AA 6014 alloy

An enhanced pre-aging (EPA) process for 6000 series aluminum alloy automotive sheets has been developed, utilizing a micro-alloying strategy combined with vacancy concentration control. This innovative approach enhances the paint bake (PB) response by up to 70 % without compromising other properties. Microstructural characterizations and analyses of aging precipitation were conducted using positron annihilation spectroscopy, differential scanning calorimetry, transmission electron microscopy, and vacancy kinetics simulations. The measurements reveal that Sn retains a high concentration of supersaturated quenched-in vacancies. During the short high-temperature spike aging (SA) treatment, these vacancies effectively promote nucleation and give rise to a favorable Mg/Si ratio within the clusters/nuclei. In the subsequent conventional pre-aging (PA), simultaneous growth of nuclei and formation of new PA clusters occur. During PB, excess vacancies trapped by Sn/Cu and some clusters are released. This release effectively promotes the transformation of clusters into β" phase precipitates with a higher number density, smaller volume, and more dispersed distribution. This transformation substantially enhances the PB strength increment.