Residual stress and microstructure effects on fatigue crack growth in AA2050 friction stir welds

In this work, results of a study conducted on fatigue crack propagation in friction stir welded AA2050 and the effects of FSW induced residual stresses, as well as changes in the microstructure, are presented. Longitudinal residual stress profiles across butt welded 2050 plates were determined using the cut compliance technique and fatigue crack growth testing was conducted on compact tension specimens machined from the friction stir welds. Tests were performed with the crack propagating nominally perpendicular to the weld and with a constant, applied, cyclic, stress intensity factor. Two different material tempers were investigated and in both cases residual stresses were found to have a major effect on the fatigue crack propagation. It was shown that compressive residual stresses are present in the vicinity of the weld, leading to crack closure and a decrease in the fatigue crack growth rate as the crack approaches the weld. Once in the weld nugget, the crack propagation rate increases. This increase is believed to be linked in part to tensile residual stresses but also to a microstructural effect present when the crack grows through the recrystallized nugget. It was also observed that other closure mechanisms, such as oxide-induced closure may affect the fatigue behavior of the 2050 FSW's. Two methods to predict fatigue crack growth rates in the weld zones were attempted: using the residual stress profiles determined by cut compliance and using the effective stress intensity factor, Delta K-eff, obtained during fatigue crack growth testing. (C) 2007 Elsevier Ltd. All rights reserved.