Effect of Welding, Aging, and Stress Ratio on Fatigue of GMAW and FSW AA 6061‐T651 Joints

ABSTRACT This study investigates the fatigue behavior of AA6061‐T651 aluminum alloy joints produced by gas metal arc welding (GMAW) and friction stir welding (FSW), in as‐welded (AW) and postweld aged (PWA) conditions. Postweld aging was performed at 200°C for 8 h, and fatigue tests were conducted at stress ratios of R = 0.1 and 0.5. The PWA–FSW joints exhibited the highest fatigue strength, reaching about 90 MPa at R = 0.1, representing nearly 30% improvement over untreated FSW joints and 40%–45% over GMAW joints. Increasing the stress ratio reduced fatigue life due to diminished crack‐closure effects and multiple crack initiation sites. Microstructural analysis using OM, OIM, and TEM revealed finer grains and stable precipitate distributions in PWA–FSW joints, directly enhancing fatigue resistance. Fractographic examination confirmed distinct crack initiation, propagation, and final fracture regions influenced by stress ratio. The experimental S–N results agreed well with Eurocode 9 design curves for structural design validation.