The effect of different precipitate microstructures obtained by different heat treatments on fatigue behavior of 7020 aluminum alloy was investigated. The fine Guinier Preston I (GPI) zones in the under-aged alloy can be repeatedly sheared by dislocations produced in cyclic loading, making the fatigue crack initiate difficultly and fatigue crack path propagate tortuously. Fatigue strength and fatigue crack propagation resistance of the alloy with shearable precipitates are much higher than those of the alloy with unshearable precipitates. The peak-aged alloy with continuous grain boundary precipitate (GBP) and narrow precipitate free zone (PFZ) is prone to initiate fatigue cracks and reduce fatigue strength. With the growth of unshearable precipitates, the fatigue strength of the alloy firstly increases and then decreases. Precipitates with moderate size in the over-aged alloy improve the roughness-induced crack closure (RICC) effect. Soft matrix with appropriate width between the precipitates can promote the slip reversibility and relax the crack tip stress. The fatigue strength of the moderately over-aged alloy reaches to 122.1 MPa at 10⁷ cycles of loading, and the fatigue crack growth rate (FCGR) is 35.6% slower than that of the peak-aged alloy at ΔK of 10 MPa·m^1/2.

The influences of aging treatments on microstructures and the corrosion properties of an Al–Cu–Li alloy were investigated through an immersion test in intergranular corrosion (IGC) solutions, a potentiodynamic polarization test, and electrochemical impedance spectra (EIS), combined with scanning and transmission electron microscopy. The results demonstrated that the Al–Cu–Li alloy displayed outstanding comprehensive mechanical properties and IGC resistance after treating with pre-strain deformation and a double aging process (PDA). Both the PDA and pre-strain followed by creep aging (PCA) treatments significantly increased the number densities of T₁ and θ’ precipitates in the grain interior. The increase in precipitates in the grain interior greatly reduced the Cu-rich precipitates on the grain boundaries and inhibited the formation of a precipitate-free zone (PFZ). The electrochemical characteristics of the Al–Cu–Li alloy were influenced by the precipitates in the grain interior and grain boundaries. The studied alloy gained high IGC resistance due to the refinement of its microstructure, and the main corrosion mode was intra-granular pitting corrosion; thus, the corrosion diffusion rate was slowed down.

The microstructures and mechanical properties of friction stir welded (FSW) Al–Zn–Mg alloy plate under different travel speeds were investigated. Both the average grain sizes (AGSs) of the shoulder affected zone (SAZ), nugget zone (NZ), and the widths of thermo-mechanically affected zone (TMAZ) decreased with the increase of travel speed. Moreover, the AGSs of NZ are always about 60% of that of SAZ at different travel speeds. The fractions of high-angle grain boundaries (HAGBs) in the FSW joints reduce with the distance away from the stir zone (SZ). Furthermore, the initial η’ strengthening precipitates in NZ and TMAZ dissolve and GP zones form during subsequent natural aging, so that the hardness is similar in the two zones. The precipitate evolution in the heat-affected zone (HAZ) at hardness minima are affected by travel speeds, which induce the hardness minima and ultimate tensile strength (UTS) of FSW joints and increase with the increase of travel speed, and a fracture tends to occur at hardness minima location of HAZ during tensile testing.

In this paper, the microstructure of friction stir welded between the Al–Li–Cu alloy and the Al–Zn–Mg–Cu alloy was studied using optical microscope(OM), electronic backscattered diffraction (EBSD), SEM, and TEM. The hardness profile revealed that the range of heat affect zone for the Al–Zn–Mg–Cu alloy was slightly wider than the Al–Li–Cu alloy when they suffered from the thermal transient. Additionally, the characterization of precipitates inside the different zones was obviously different, which corresponded to the microhardness distribution profile. At the periphery of the kissing line, the Al–Li–Cu and the Al–Zn–Mg–Cu alloys mutually diffused during the weld process. The magnesium element in the Al–Zn–Mg–Cu alloy diffused into the Al–Li–Cu alloy. But the copper and zinc had no change because of the low diffusion coefficient in aluminum. The heat affected zone of the Al–Zn–Mg–Cu alloy showed a higher corrosion susceptibility immersed into the corrosion environment, and the Al–Li–Cu alloy did not show severe corrosion.