High Cycle Fatigue Behavior of AA 6351 and AA 7050 Aluminum Alloys

Fatigue is the main cause of mechanical failure in aircraft structures, in which aluminum alloys are employed in approximately 70% of their structural components. Among the microstructural characteristics of aluminum alloys, the hardening precipitates provided by the ageing heat treatment have an important influence in their mechanical properties. In this context, current studies have shown that the two-step ageing heat treatment (T6I4) improves the mechanical properties of 6xxx and 7xxx aluminum alloys. This investigation presents a study of high cycle fatigue behavior of aluminum alloys AA 6351 (T6 and T6I4) and AA 7050 (T7451 and T6I4) as well the influence of microstructural characteristics and two-step ageing heat treatment in the fatigue properties of these alloys. Fatigue tests were performed on smooth and notched specimens. For the adopted two-step ageing conditions, AA 6351 experienced a drop in fatigue resistance of smooth specimens, while AA 7050 remained at the same level of the as-received condition. The two-step ageing was also observed to decrease the notch sensitivity of the studied alloys.