We investigate the relationship between inhomogeneously distributed S precipitates and hardness of stress-aged single-crystal Al-Cu-Mg. First, the effect of crystallographic anisotropy is considered and modeled from the results of free-stress aged single-crystal Al-1.2Cu-0.5Mg with (\( 1\bar{1}8 \)), (\( \bar{1}\bar{2}5 \)), (356), and (319) plane orientations. Effect of crystallographic anisotropy depends on the angle between the plane orientation of the single crystal and {012} habit planes of the S precipitates. Second, the effects of the magnitude of the applied stress and direction on the S-laths’ size and distribution are considered. As the applied stress-induced S-laths inhomogeneously distribute during aging, the effect of the single-crystal’s orientation on the distribution of S-laths is modeled. The results show that a single crystal near (111) plane orientation has the lowest stress-orienting effect. Finally, at higher applied stresses, such as 50 MPa, the S precipitates disperse more homogeneously due to the influence of the dislocations. Inhibiting the effect of dislocation depends on the angle between the plane orientation of the single crystal and the {111} dislocation slide planes. A precipitate-strengthening model of the stress-aged Al-Cu-Mg alloys is established based on crystallographic anisotropy, stress-orienting precipitates, and inhibiting the effect of dislocations.

•Al–Cu alloy single crystals were used to study the asymmetric rolling process.•Shear bands consist of S-type grains were observed in asymmetric rolled sheet.•Microstructures of deformation bands were analyzed reference to texture results.•Microstructure of shear bands depends on initial orientations of single crystals.•Texture of the shear bands always rotated towards Copper component.Asymmetric rolling process (ASR) introduces extra shear deformation compared with conventional symmetric rolling (SR), and induces shear bands in microstructures. ASR technique has been applied to manufacture thick aluminum plate with improved mechanical properties due to the fine grains or even ultrafine-grains obtained in shear bands. Single crystal specimens of Al–Cu alloys were prepared in this work to study the microstructure and texture evolution in asymmetric rolled sheet. Microstructures of deformation bands were observed and the microtexture was analyzed through electron backscatter diffraction (EBSD). Compared with the SR process, ASR induced more homogeneous deformation and shear bands were observed in deformation area. Microstructure of shear bands depends on the initial orientations of single crystals. After single crystal with (3, − 1, 9) plane orientation is processed in ASR, shear bands consist of S-type grains were formed, which have uniform size and almost the same twisting degree. But for single crystal with (− 3, 0, 7) plane orientation, shear bands have various twisting degrees and grains distributed inhomogeneously. Texture of the shear bands in ASR processed specimens always rotated towards Copper component for single crystals with different orientations.Download high-res image (523KB)Download full-size image

While it is clear that precipitates near grain boundaries in aluminum alloys nucleate more easily and grow more quickly, the mechanism of how the grain boundaries influence the anisotropic distribution of S precipitates is still unknown. In this study, a bicrystal Al-Cu-Mg alloy specimen was prepared to obtain a single grain boundary. The precipitation distribution and hardness of the Al-Cu-Mg alloy bicrystal were analyzed after stress aging at 30 MPa. Transmission electron microscopy was used to analyze the precipitation behavior of S precipitates, and the hardness was tested along the location on the bicrystal specimen. The hardness of the right area around the grain boundary is lower than that of the left area around the grain boundary after stress aging. The precipitation behavior around the grain boundary is different from that in the matrix. For the left area near the grain boundary with the (1, −1, 8)Al plane orientation, the anisotropic distribution of the S precipitates is inhibited. For the right area near the grain boundary with the (3, 5, 6)Al plane orientation, the anisotropic distribution of the S precipitates is even more pronounced than that in the right matrix. This difference in the influence depends on the crystallographic orientations of the matrix around the two sides of the grain boundary.

Microstructure quantitatively analysis is significant for building the relationship between microstructure and properties, especially the ageing precipitation kinetics in aluminum alloys. In the present paper, a core parameter (aspect ratio) was introduced to describe the morphology change of precipitation. Aspect ratios of T1-plate precipitates in Al-Cu-Li-Zr alloys, S-plate precipitates in Al-Cu-Mg alloys and β-rod precipitates in Al-Mg-Si alloys were quantitatively analyzed. Results indicate that aspect ratios of precipitates increase at the early stage of ageing, then reach the peak and finally decrease slowly. The soft-impingement theory and HHC theory were introduced to model the ageing kinetics. Thermodynamics and kinetics parameters of different precipitates were also calculated in the model. Simulated results of different precipitates agree well with the experimental results.

The stress aging behavior of Al–Cu alloy under various applied stresses, i.e., elastic stress, yield stress and plastic deformation stress, was investigated using single crystals. The resulting microstructures and the yield strength were examined by transmission electron microscopy (TEM) and compression tests, respectively. The results indicate that an elastic stress of 15 MPa is high enough to influence the precipitation distribution of θ′ during aging at 180 °C. The applied stress loading along direction results in increased number density of θ′ on (001)Al habit planes. This result becomes more significant with increasing applied stress and leads to lower yield strength of Al–Cu single crystals during aging. Moreover, the generation of the preferential orientation of θ′ was discussed by the effect of the dislocation induced by applied stress as well as the role of the misfit between the θ′-precipitate and Al matrix. The results are in agreement with the effect of the latter one.

The effect of three-stage homogenization on mechanical properties and stress corrosion cracking of Al-Zn-Mg-Zr alloys has been investigated. The results reveal that the interface of η phases provide the favorable sites for Al3Zr dispersoids under the first-stage homogenization at 350 °C, and therefore three-stage homogenization can reduce size and improve density and homogeneity of Al3Zr dispersoids. Uniform, fine and dense Al3Zr dispersoids inhibit recrystallization effectively and improve the uniformity of the grains. Under the same T74 aging temper, the main strengthening mechanisms of the material are η' phases and dispersion strengthening, and as a result the strength of the material increases by 11.02 MPa and 3.11 MPa when compared with one-stage and two-stage homogenization, respectively. Al3Zr dispersoids can pin dislocations and reduce the accumulation of dislocations at grain boundaries significantly, besides, uniform grains contribute a lot to the elongation, leading to a higher elongation and impact toughness of the material. The improved corrosion resistance is due to a decrease in large-angle grain boundaries which are favorable channels for corrosion, and SCC susceptibility (ISSRT) of the material is only 2.6%.

The deformation degree of many integrated stiffened panels expected to adopt the creep age forming process, exceeds the linear elastic range. However, there is no research report on the possible results therefrom. And it is significant for the structural design and process optimization to systematically study the effect of deformation degree on the property and microstructure of the creep aging formed parts and ascertain the suitable deformation range. For this purpose, a series of creep age forming experiments of AA7475 sheet specimens with different curvature radiuses were conducted employing a radius-adjustable forming tool. Results indicated that the strength property varies by a low-to-peak-to-low manner as the increase of the deformation degree, and the radius-thickness ratio of the peak strength maintains a constant. Moreover, the measured range of the property fluctuation amplitude is 3.5–5.8%, which means a certain property difference may exist on different local regions of an integrated component with various curvature radiuses. In order to reduce the possible negative effect, it shall be necessary to pre-consider this factor and design a reasonable deformation range before the practical manufacturing process.

The Al–Cu–Mg–Ag single crystal is prepared to study its precipitation behavior during aging under elevated compression stresses. Transmission electron microscopy (TEM), high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and hardness test are used for this investigation. The results show that a compression stress of 50 MPa leads to more uniform length distribution but slightly lower number density of Ω-phase in the stress-aging sample, as compared to that of the stress-free aging sample. When a higher compressive stress of 200 MPa is applied, the number density of Ω-phase is significantly decreased while a large number density of θ’-phase are precipitated along the dislocation lines introduced by the applied stress. Furthermore, the stress-orientating effect on Ω precipitation can not be observed in Al–Cu–Mg–Ag single crystal during stress aging even under the compressive stress of 200 MPa loading along the [2¯15]Al direction. This result signifies that the applied stress magnitude is not the only critical factor in determining whether the stress-orienting effect of Ω-phase can be generated, as this effect may be also associated with the loading orientation during stress aging.

The effects of stress magnitude and crystallographic orientation on the mechanical properties of stress-aged Al–2Cu alloy single crystal specimens are investigated. The specimens with (1-, 1, 6) plane orientation were stress-aged under 0, 15, 40, and 60 MPa. Specimens with five different plane orientations ((1-, 2-, 6), (3, 1, 3), (1-, 3, 3), (1-, 1, 6), and (1-, 4-, 6)) were stress-aged under stress with the same magnitude of 40 MPa. The results of mechanical properties and TEM microstructure show that the yield stress of stress-aged specimens depends on the crystallographic orientation as well as the stress magnitude. A model based on crystallographic orientation describes the precipitation strengthening of stress-aged Al–2Cu alloy. The calculated yield stresses in the model fit well with the experimental observations. This model results not only provide important insight into solving the anisotropy problem attributed to precipitation strengthening, but also offer a benchmark for choosing the right range of stress in the manufacture of Al–Cu alloys.

•Single-stage quenching tests were conducted based on the end quenching equipment.•Multi-stage quenching tests are reasonably designed from the experimental results.•Coupling control of the cooling rate and the residual stress has been achieved.In order to attain good hardenability in the single-stage quenching process of Al–Zn–Mg–Cu alloys, rapid cooling rate is often desirable. But this would inevitably increase the residual stress. Therefore, the aim of this study is to achieve coupling control of the cooling rate and the residual stress by using the multi-stage quenching process. First, a series of single-stage quenching tests were conducted based on the end quenching equipment. Then in the double-stage quenching tests, a higher cooling rate was obtained comparing to the single-stage quenching. Based on this discovery, three kinds of multi-stage quenching processes were designed based on the experimental results of the single-stage quenching tests. The mechanism of the multi-stage quenching has been analyzed by comparing the cooling curves, the microstructure, the hardening depth, and the maximum residual stress. Furthermore the optimal multi-stage quenching process for 7050 aluminum alloy plate was obtained.