Effect of anisotropy on Fatigue properties of AA2198 Al-Li plates joined by Friction Stir Welding

Abstract

Al-Li alloys are characterized by a strong anisotropy in mechanical and microstructural properties with respect to the rolling direction. In the present paper sheets, 4 mm thick, of AA2198 Al-Li alloy were joined via Friction Stir Welding (FSW) by employing a rotating speed of 1000 RPM and a welding speed of 80 mm/min, in parallel and orthogonal direction with respect to the rolling one. The joint mechanical properties were evaluated by means of tensile tests at room temperature. In addition, fatigue tests were performed by using a resonant electro-mechanical testing machine under constant amplitude control up to 250 Hz sinusoidal loading. The fatigue tests were conducted in axial control mode with R=?min/?max=0.33, for all the welding and rotating speeds conditions. The damage behavior was studied by applying thermoelastic stress analysis (TSA) to the crack formation and propagation of friction stir welded sheets under cyclic fatigue tests. The fatigue crack propagation experiments were performed on single edge notched specimens. Fatigue tests were carried out up to failure in tension- tension with load ratio R=0.33. The TSA measurement system allowed crack evolution to be observed in real-time during fatigue cycles and stress fields to be derived on the specimens from the temperature variation. The thermoelastic data were used to measure the principal stresses and principal strains on the specimens surface and the crack growth rate during tests. All the results were validated by employing finite element analysis performed with ABAQUS software. SEM observations of the fractured surfaces were done to characterize the weld performances.