Effects of the secondary aging heat treatment T6I4 on fracture toughness and fatigue crack growth resistance of AA7050 alloy

icaf2023 Tracking Number 78

The tensile response and the fatigue behavior of aircraft age-hardenable aluminum alloys are strongly influenced by factors like the size, spacing and volume fraction of strengthening precipitates. Recent papers have pointed out that the secondary aging T6I4, by producing a higher density of fine strengthening precipitates as compared to the conventional T7451 temper, can improve the ability of AA 7050 alloy to accommodate plastic strain during loading and promote a more uniform strain distribution. In this sense, the present work had as main goal to evaluate the effects of T6I4 aging on the fracture toughness and fatigue crack growth behavior of AA 7050 plate samples. The alloy was previously hot rolled at approximately 400°C to 75 mm thick plate and received in the commercial T7451 temper condition. Its chemical composition was determined by optical emission spectrometry. The T6I4 condition was obtained from the as-received material by means of a solution treatment (486C / 4h) followed by two-step ageing (145C / 30 min and 65C / 24h). The fracture toughness of the material in both T7451 and T6I4 conditions was determined using the Chevron notch methodology according to ASTM E1304-97(2020) standard. The fatigue crack growth tests were conducted under constant amplitude (R = 0.1 and 0.5) and variable amplitude loadings. In the former case, C(T) notched specimens in accordance to ASTM E647-15e1 standard were employed and the crack length was measured by the compliance method. In the latter case, the Mini FALSTAFF spectrum loading of dog-bone test specimens was adopted and the crack length was measured by quantitative fractography according to the method developed by DSTG (Australia). The obtained results showed an expressive increase of the materials’ KQvM values for the T6I4 condition, as well as a significant decrease on the crack growth rate, especially during the early stages of fatigue at lower constant amplitude ratio, possibly due to a more pronounced crack closure. The specimens treated to T6I4 condition and tested under spectrum loading also presented fatigue lives greater than twice the fatigue life presented by the T7451 material condition.