Abstract: Friction stir welding was used to butt-weld 2.8 mm thick 7050-T7451 high-strength aluminum alloy extruded profiles and machined plates. The influence of rotational speed on weld formation was studied, and the fatigue strength and S-N curves of joints in the vertical (LT) and parallel (L) directions relative to the weld, under optimized process parameters, were compared. The results show that at a rotational speed of 200 r/min, tunnel-type defects occur in the stir zone(SZ); however, defect-free joints are obtained at 600 r/min and 1 000 r/min. The upper part of the SZ primarily consists of base material(BM) from the retreating side(RS), while the middle and lower parts form an onion-ring structure through the alternate lamination of the two materials. Across different rotational speeds, the hardness distribution across the joint cross-section exhibits a “W” shape, with the lowest hardness located in the heat-affected zone(HAZ) on the machining side, which is wider than the HAZ in the extruded profile. Regarding fatigue performance, the LT-direction joint exhibits a fatigue strength of 228.3 MPa, exceeding the L-direction joint’s strength of 213 MPa. In terms of fatigue fracture characteristics, increasing stress levels correlate with a decrease in the crack propagation zone area, an increase in the instantaneous fracture zone area, and a reduction in fatigue life. LT-direction fractures are rougher and located in the machining-side HAZ, featuring large dimples (some elongated) and large-sized second-phase particles. In contrast, L-direction fatigue fractures are relatively smooth, displaying numerous small dimples containing small second-phase particles.

Key words: friction stir welding, 7050-T7451 high-strength aluminum alloy, weld formation, fatigue performance