It is difficult to characterize the variation of corrosion resistance of the narrow areas in friction stir welding (FSW) joints due to the large temperature gradient. In this paper, the welding thermal simulation was performed to simulate the heat affected zone (HAZ) and thermo-mechanical affected zone (TMAZ) of the FSW 7075-T6 aluminum alloy, and the corrosion resistance and microstructure of the simulated samples were studied. Results show that the corrosion potential changes greatly under different thermal simulation temperatures. The pitting corrosion of the HAZ simulated samples presents two pitting potentials, but for the TMAZ simulated samples, two pitting potentials will gradually evolve to one pitting potential with the increase of the maximum temperature. The electrochemical impedance spectroscopy results show that the corrosion mechanism of the HAZ and TMAZ is completely inconsistent, which is related to the differences in precipitate and grain characteristics.
This paper studies the influence of the regression re-aging (RRA) process on the mechanical properties and corrosion resistance of 7075. In the RRA heat treatment, the material properties are markedly affected by the variation of parameters in the regression stage. When the regression temperature and the regression duration increase, the mechanical properties of the material decrease. There was no intergranular corrosion in the samples treated by RRA, while obvious intergranular corrosion occurred in the T6 sample. Compared with the T6 sample, the RRA-190-0.5 sample with optimized parameters has a higher density of the precipitated phase and narrower precipitate-free zone, and the precipitated phase along the grain boundary is discontinuous, isolated, and short. Compared with the single-stage peak aging treatment, the RRA treatment can considerably improve the corrosion resistance and stress corrosion resistance but slightly reduce the strength. The best heat treatment process condition is 120°C/24 h + 190°C/0.5 h + 120°C/24 h.
In this paper, the effects of compressive pre-deformation and successive pre-artificial aging on the compressive creep aging behavior and microstructure evolution of the Al-Cu-Li alloy have been studied. Severe hot deformation mainly occurs near the grain boundaries during the compressive creep initially, which steadily extends to the grain interior. After that, the T1 phases will obtain a low radius–thickness ratio. The secondary T1 phases in pre-deformed samples usually only nucleate on dislocation loops or Shockley incomplete dislocations induced by movable dislocations during creep, which are especially prevalent in low plastic pre-deformation. For all pre-deformed and pre-aged samples, two precipitation situations exist. When pre-deformation is low (3% and 6%), solute atoms (Cu and Li) can be consumed prematurely during pre-aging at 200 °C, with dispersed coherent Li-rich clusters in the matrix. Then, the pre-aged samples with low pre-deformation no longer have the ability to form secondary T1 phases in large quantities during subsequent creep. When dislocation entangles seriously to some extent, a large quantity of stacking faults, together with a “Suzuki atmosphere” containing Cu and Li, can provide the nucleation sites for the secondary T1 phase, even when pre-aged at 200 °C. The sample, pre-deformed by 9% and pre-aged at 200 °C, displays excellent dimensional stability during compressive creep because of the mutual reinforcement of entangled dislocations and pre-formed secondary T1 phases. In order to decrease the total creep strain, increasing the pre-deformation level is more effective than pre-aging.
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