Keywords: friction stir welding, residual stress, contour method, two parameters crack growth model, FEM-DBEM approach.

Friction stir welding (FSW) is an innovative joining technique developed and patented by The Welding Institute of Cambridge in 1991. Conceptually, the FSW process is quite simple: a non-consumable rotating tool, constituted by a shoulder and a pin, is inserted between the adjoining edges of the parts to be welded and moved along the desired weld line. The tool movements lead to a local increase of the work piece temperature, with heat generation, caused by frictional effects and plastic deformation; as a consequence the induced softening allows the processing material to flow around the pin, according to complex patterns, resulting in a solid state weld. In recent years, the friction stir welding process has been applied in several sectors, such as aeronautical, automotive, and nautical among others; moreover, much effort has been spent by several research groups worldwide to provide a deeper understanding of process effects on microstructure, mechanical properties, and residual stress. In particular, an accurate knowledge of the process induced residual stress is highly desired for the effective implementation of the technique in safety-critical components and to predict the fatigue behavior of friction stir welded assemblies.

This paper deals with a numerical investigation on the influence of residual stresses, induced by the friction stir welding process, on fatigue crack growth in AA2024-T3 butt joints. The computational approach is based on the coupled usage of the finite element method (FEM) and the dual boundary element method (DBEM). In particular, linear elastic FE simulations have been performed to evaluate the process induced residual stresses. The computed residual stress field is then superimposed to the stress field produced by a remote fatigue traction load and a crack propagation is simulated by the DBEM. A two-parameter crack growth law is used for the crack propagation rate assessment. The DBEM code BEASY and the FE code ANSYS are coupled in the aforementioned numerical approach using in house developed routines.

An asymmetric stress distribution, characterized by tensile stress in correspondence of the weld line, balanced by compressive stresses into the contiguous material, has been found, with a tensile stress peak has been localized in the advancing side and below the top surface. Moreover, the residual stress effect on crack growth rates and on crack shape, depends on the following material parameters: the stress-strain curve, the thresholds of the driving forces, and the constant amplitude crack growth curves.

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