Combined effect of pre-aging and Ag/Cu addition on the natural aging and bake hardening in Al-Mg-Si alloys

The combined effect of pre-aging and Ag/Cu addition on natural aging and bake hardening in Al-Mg-Si alloys were studied by Vickers microhardness measurement, tensile test, atom probe tomography and transmission electron microscopy. The results showed that Ag and/or Cu addition could further enhance the positive effect of pre-aging on bake hardening for Al-Mg-Si alloys. Although the pre-aged alloys with individual Ag/Cu addition had higher paint bake hardness, the hardness in T4P temper was relatively high due to effectively forming of Cluster(2) during pre-aging. While the pre-aged Ag-Cu-added alloy had low T4P temper hardness and higher bake hardening response. The strong interaction energies between Ag/Cu atoms and Mg atoms were responsible for the facilitated clustering and precipitation processes. After paint bake treatment, most of Ag atoms accumulated preferentially at β″/α-Al matrix interface, while Cu atoms entered into the interior of β″resulting in disordered structure within the precipitate.