Residual stresses in friction stir welded parts of complex geometry

Residual stresses play a key role on the mechanics underlying the fatigue crack growth propagation of welded joints. Indeed, compressive residual stresses may induce a beneficial enhancement of the fatigue life under loading condition whereas tensile residual stresses may act to increase the stress distribution at crack tip, resulting in a life-threatening condition of the welded structure. In-process distortion and final geometry of welded joints are also affected by residual stresses. In this paper, the longitudinal residual stress distributions in friction stir welding (FSW) joints were investigated for butt and skin-stringer geometries, including lap and T configurations. To measure residual stresses, the cut-compliance and the inverse weight-function methodologies were adapted for skin-stringer FSW geometries via finite element analysis. AA2024-T4 and AA7075-T6 aluminum alloys were used to weld dissimilar skin-stringer joints whereas butt joints were made of AA2024. The effect of most relevant process parameters as well as the cooling during welding process was also investigated for a better understanding of welding residual stresses. Our findings suggest that FSW of complex skin-stringer geometries produces higher residual stresses than those of butt joints, and that the cooling water flux further reduces residual stresses. Changes of process parameters did not affect markedly residual stress distribution.