Low cycle fatigue properties of friction stir welded dissimilar 2024-to-7075 aluminum alloy joints

Low cycle fatigue (LCF) tests, uniaxial symmetrical (R = −1) push-pull, were performed on the friction stir welded dissimilar 2024-to-7075 aluminum alloy joints at various strain amplitudes, focusing on the effect of base material (BM) positioning during friction stir welding (FSW). The microstructures and microhardness distribution mapping across the joint cross-section were characterized. It was observed that the BM positioned on the retreating side (RS) was dominant in the upper part of stir zone (SZ), while the middle and lower parts of SZ consisted mainly of the BM on the advancing side (AS). A larger low-hardness zone was present when 2024 aluminum alloy was placed on the AS. Cyclic hardening occurred in both types of dissimilar joints at strain amplitudes higher than 0.4%, below which cyclic stablization sustained during cyclic deformation. BM positioning during FSW affected the fatigue resistance of joints at a higher strain amplitude. Longer fatigue life was obtained when 7075 aluminum alloy was located on the AS. All of the fatigued specimens of joints fractured at the location of softer heat-affected zone on the 2024 side because of the coarsened precipitates, with crack initiation from the specimen surface and crack propagation characterized by typical fatigue striations.