Damage Evolution in AA2099 and AA6061 Aluminum Alloys Under Quasi-static and Dynamic Mechanical Loading

Abstract

In this research, the behavior of AA2099 and AA6061 aluminum alloys under quasi-static and dynamic mechanical loading conditions was examined. The effects of temper condition on the dynamic response of both alloys were investigated as well as the microstructural evolution associated with the formation of adiabatic shear bands in these alloys. Cylindrical specimens of the alloys were solutionized at 540 ºC for 2 h, water quenched, cold worked (T8) and then followed by natural aging, one-step artificial aging or two-step artificial aging to produce different temper conditions. Quasi-static compression, microhardness, direct impact and dynamic torsion tests were thereafter conducted on the aged specimens. Microhardness and quasi-static compression test results showed that strain hardening prior to precipitation hardening increased deformation resistance in AA2099 alloy, but it made no difference in the deformation resistance of precipitation-hardened AA6061 alloy. The two-step aged AA2099 alloy showed higher deformation resistance compared to the one-step aged or naturally aged alloy. Although AA2099 alloy showed superior mechanical properties under quasi-static loading, the reverse was observed under dynamic mechanical loading at high strain rates. Both direct impact and dynamic torsion test results showed that AA2099 and AA6061 failed by adiabatic shear band formation at high strain rates. Under dynamic shock loading conditions, strain hardening prior to precipitation hardening reduced the deformation resistance of AA2099 alloy but increased that of AA6061. The higher strength of the naturally aged AA2099 compared to the one-step aged alloy and the higher strength observed in one-step aged than in the two-step aged alloy indicated that precipitation of second phase particles promoted adiabatic shear banding in AA2099 alloy. The results of optical and scanning electron microscopy showed that depending on the temper condition and strain rates, both deformed and transformed shear bands formed in the two alloys under dynamic shock loading. While dissolution of second-phase particles occurred inside the shear bands formed in AA2099, very fine equi-axed grains of average size of 620 nm were observed inside the transformed bands of AA6061 alloy. Intense localized thermal softening led to the dissolution of the reinforcing second-phase particles, causing reduction in the deformation resistance and promoting shear strain localization in AA2099 alloy. Investigation of the transformed band in AA6061-T6 alloy using synchrotron light radiation at the Canadian Light Source showed the presence of less silicon inside the transformed band than in the region adjacent to the shear band.