Fatigue Crack Growth Behaviour in Friction Stir Welded Aluminium-Lithium Alloy Subjected to Biaxial Loads

In this article, biaxial load fatigue crack growth tests are reported. Specimens were made of an advanced aluminium-lithium alloy AA2198-T8 joined by the friction stir welding process, capable of producing advanced integral metallic structures that can offer significant cost and weight savings over the current joining methods. Two material rolling directions are considered in relation to the welding and crack growth direction. Welding-induced initial distortion was measured before the experiment for better result interpretation. Test specimens are representative of two different weld orientations, that is longitudinal weld parallel to the material rolling direction and circumferential weld perpendicular to the material rolling direction for investigating the inherent material anisotropy of aluminium-lithium alloys. In all tests, the fatigue crack was initiated in the thermo-mechanical process zone of the weld and propagated parallel to the weld joint line. It is shown that the rolling direction of the selected aluminium alloy strongly affects the crack growth path. The specimens welded orthogonally to the rolling direction exhibit a shorter fatigue crack growth life than the specimens welded parallel to the rolling direction.