Strengthening mechanisms of pre-stretching strain on friction stir welded 2024-to-7075 dissimilar aluminum alloys joints: Yield sequence effect

Pre-stretched strains, 2%, 4%, and 6%, were conducted on the friction-stir-welded dissimilar 2024-to-7075 aluminum alloy joints for strengthening purpose. Tensile tests and microhardness measurements were performed to evaluate the strengthening effect. Transmission electron microscopy and high resolution transmission electron microscopy examinations were employed to characterize precipitates and dislocation structures of the as-welded and pre-stretched joints to elucidate the intrinsic strengthening mechanisms. Results showed that the yield and tensile strengths of the pre-stretched 6% joint respectively increased 147.9 MPa and 31.1 MPa when compared with the as-welded joint. The strain hardening rates of 2024 lower hardness zone (LHZ)-II and 7075 LHZ were higher at the pre-stretched strain rate of 1 mm/min than those of 2 mm/min based on the microhardness area evolution. The yield sequence of the as-welded joint was 2024 LHZ-II, 2024 LHZ-I, 7075 LHZ, stir zone (SZ), and 2024 base material. The appearance of dislocation tangles, dislocation walls, dislocation cells, dislocation pile-ups in the 2024 LHZ-II and 7075 SZ combined with the severe interactions between dislocation structures and precipitates were the dominant strengthening factors. It was very interesting that dislocations provided a diffusion path for Mg atoms and Cu atoms in 2024 LHZ-II during the pre-stretching process.