Accelerating precipitation hardening by natural aging in a 6082 Al-Mg-Si alloy

It is well known that long time natural aging (NA) after quenching from solution treatment will significantly reduce the precipitation hardening kinetics and peak hardness of most 6xxx aluminum alloys during later artificial aging (AA). Here we demonstrate an effective strategy to accelerate precipitation hardening, taking advantage of NA. It is found that by a short time pre-aging (PA) at AA temperature, NA for up to 1 year can reduce the time to peak strength in a 6082 alloy during later AA. A simultaneous increase in yield strength and uniform elongation at peak-aged condition can be achieved as a result of finer and denser age-hardening precipitates than those formed by direct AA treatment. Quantitative characterization of the precipitate microstructure by annular dark field scanning transmission electron microscopy (ADF-STEM) and atom probe tomography (APT) reveals that PA generates a small fraction of fine β″ needle precipitates composed of 6–9 β″-eyes and a substantially high density of GP-zones composed of 3–5 β″-eyes, which are stable at room temperature and can grow easily into β″ upon AA. During NA after PA, more GP-zones with at least 2 β″-eyes form while the larger GP-zones inherited from PA grow further, both of which can act as the precursors of β″ precipitates during later AA.