Impact of pre-aging on the tensile and bending properties of AA 6061

Abstract

The impact of low temperature pre-aging on the artificial age-hardening of AA 6061 (Al–1.1Mg–0.63Si) has been investigated using differential scanning calorimetry (DSC) as well as by measuring the electrical resistivity, and confirmed by microstructure observation using scanning electron microscopy and transmission electron microscopy (TEM). Tensile tests were performed to determine the mechanical properties of the samples. During the pre-aging treatment GP zones I formed at 0–30°C. These GP zones I retarded the precipitation of the intermediate β′ phase during artificial aging and led to a decrease in the strength of the alloy. During the pre-aging treatment β′′ precipitates formed at 40–70°C. Increasing the pre-aging temperature significantly increases the strength after artificial aging. On the other hand, increasing the pre-aging temperature decreases the size of the precipitation-free zones (PFZ) adjacent to the grain boundaries. It was found that the tensile strength was greater if the PFZs were smaller than if they were larger. Grain boundary precipitates (GBP) did not significantly contribute to the hardening or strengthening. The specimens subjected to a lower pre-aging temperature followed by the same artificial aging step had better bendability than did the specimens subjected to a higher pre-aging temperature.