Artificial aging time influencing the crack propagation behavior of the aluminum alloy 6060 processed by equal channel angular pressing

Abstract

The combination of a severe plastic deformation (SPD) by equal-channel angular pressing (ECAP) with a subsequent heat treatment significantly influences the precipitation characteristics of age-hardenable aluminum alloys. The aging kinetics is accelerated and the morphology of the precipitates is affected by the induced strain. In the present study, the influence of the artificial aging time and the resulting precipitation morphology on the fatigue threshold was investigated for the aluminum alloy 6060 with conventional grain size (CG) and after ECAP processing. Four artificial aging conditions are compared for the as-extruded and ECAP processed material, respectively. The resulting microstructure is examined by transmission (TEM) and scanning transmission electron microscopy (STEM) and related to the processing condition and the aging time. The mechanical properties are characterized by tensile and hardness testing and crack propagation tests are conducted in order to determine the threshold ΔKth. The ECAP processed conditions exhibit a lower resistance against crack propagation regardless of the aging time, when compared to the as-extruded conditions. Therefore, the grain size is determined as the major influencing factor on the fatigue threshold. In contrast, the effect of the aging time and the precipitation morphology is only minor. When compared to incoherent precipitates, coherent ones promote planar slip resulting in an increased threshold against crack propagation for both CG and finer grained, bimodal microstructures. However, the size of the coherent precipitates and the resulting lattice distortion is decisive to achieve higher fatigue thresholds.