Since aluminum alloys have the low melting point and high thermal conductivity, friction stir weld (FSW)has been successfully used in the aerospace industry as an alternative to traditional welding methods. In the service of FSWed structures, the residual stress and external load would result in a secondary deformation and residual stress redistribution. Therefore, it is necessary to investigate the effect of residual stress on the FSWed fatigue responses. This paper studied the fatigue crack growth behavior of 2024-T3 and 7075-T6 homogeneous and dissimilar FSWed joints. The finite element model was built to calculate the fatigue crack growth rate of FSWed specimen, and the results were compared with the experiment results. It was demonstrated that residual stress significantly affected on the fatigue crack growth rate. Tensile residual stress promoted the crack growth, and it offset the decrease of fatigue crack growth rate by grain refinement. The numerical results also indicated that the longitudinal residual stress showed the greatest effect on the crack deflection. Under the same welding speed, the fatigue crack growth rate of 2024 increased with the increasing rotational speed. For the same rotational speed, the fatigue crack propagation rate of 7075 decreased with the increasing welding speed.

In this work, the effect of residual stress on mixed-mode crack propagation behavior in friction stir welded (FSW) 7075-T6 panel under biaxial loading was investigated. The cruciform sample was designed and manufactured by FSW. Residual stress profiles across the welded sample were measured by the X-ray diffraction technique. Crack propagation behaviors were simulated with five different biaxial loading ratios. Stress intensity factors (KΙ and KΙΙ) were evaluated by finite element method (FEM) and used to study the effects of residual stress on crack behaviors. It was observed that residual stress has a considerable effect on the mixed-mode crack growth. In most of the cases, the crack deflection is mainly affected by residual stress at the beginning of crack propagation. The variation of crack propagation path is strongly linked with the residual stress as well as the biaxial loading ratio. In addition, KΙ and KΙΙ are susceptible to residual stress under biaxial loading conditions. Residual stresses contribute to a higher proportion of KΙΙ compared to that of KΙ. KΙ and KΙΙ in the retreating side are more affected by the residual stress.

The bi-axial 7075-T6 Al alloy samples were designed and manufactured by Friction Stir Welding (FSW). The residual stress profiles were measured. Crack propagation behavior were simulated with five different biaxial loading ratios. ABAQUS software was used to calculate the stress intensity factors (SIFs) and to study the effects of residual stress on crack behaviors. It is found that residual stress can affect the effect of biaxial loading ratio on crack growing. Mode I and mode II SIFs are susceptible to residual stress when λ= 0.5~2. Cracks at the retreating side are much more affected by the residual stress. At given biaxial loading ratio, the distribution characteristics of SIF components have similarity on the advancing side and the retreating side.