Fatigue crack growth mechanisms in similar and dissimilar aluminum friction stir welds: Residual stress, microstructure, and microhardness effects

• Similar and dissimilar friction stir butt welds of aluminum 6061 and 7075. • Contour method more reliably measures residual stress than crack-compliance. • Weld nugget crack growth rates are suppressed by 70–80 % and correlate with K res. • Cracks preferentially tunnel in regions of high residual stress and microhardness. • A method of residual stress data analysis for crack growth prediction is proposed. Spatially varying microstructure, microhardness, and residual stress in friction stir welds cause crack growth rates to fluctuate near weld nuggets and complicate fatigue life predictions. Similar and dissimilar friction stir butt welds of 6061-T651 and 7075-T735 aluminum alloys were characterized by optical microscopy and microhardness traversals, and residual stress mapping by the contour method and fatigue crack growth testing were conducted across and within weld nuggets. Crack growth data were analyzed by the crack-compliance method to calculate residual stresses and consequent stress intensity factors (K res ), and contrasted with the contour method. Constant ΔK crack growth testing across the welds revealed suppression of growth rates relative to the base materials, being lowest in the thermo-mechanically affected zones. Crack growth within the weld nuggets was conducted at decreasing ΔK and resulted in uneven through-thickness crack growth. A method for crack propagation analysis of materials containing heterogeneous residual stresses is proposed using contour method data to calculate K res profiles, indicating that crack growth rates increased in regions of greater K res and microhardness in both crack-weld orientations. The results from these studies can be used to design weld configurations with beneficial residual stresses and enhanced fatigue crack growth resistance for structural applications.