The hardness, toughness, wear resistance, and fatigue behavior of materials can be improved through a deep cryogenic treatment (DCT). During this treatment, low temperatures (−100 °C to −196 °C) are maintained and then increased to room or higher. In this work, an indirect-extrusion plate of 7A99 ultra-high strength aluminum alloy was subjected to a T6 (peak aging) treatment and a T6-DCT treatment. The influence of the T6-DCT treatment on the mechanical properties, grain morphologies, precipitates, and atom–cluster distribution was investigated via tensile testing, electron backscatter diffraction, transmission electron microscopy, and three-dimensional atom probe analysis. The tensile strength (maximum: 705 deep cryogenic treatment), yield strength (maximum: 678 MPa), and elongation (maximum: 14%) of the T6-DCT-treated alloy were higher than those of the T6-treated alloy. Moreover, the T6-DCT treatment resulted in (i) grain size refinement and increased uniformity of the microstructure (homogeneous distribution of η’-MgZn₂- and η-phase precipitates), and (ii) reduced segregation degree of Zn, Mg, and Cu atoms in the matrix (fraction of small atom clusters (sizes: 10–20 nm, 20–50 nm) increased, fraction of large clusters (size: >1,000 nm) decreased). Therefore, DCT can refine the precipitates and increase the uniformity of the precipitate distribution, thereby improving the strength and plasticity of the alloy.

A non-isothermal ageing process was proposed for an Al-Zn-Mg-Cu alloy aiming to accommodate the slow heating/cooling procedure during the ageing of large components. The evolution of microstructure and microchemistry was analyzed by using transmission electron microscopy, high-angle annular dark field imaging, and energy dispersive spectrometry. The age-hardening of the alloy was examined to evaluate the strengthening behavior during the non-isothermal process. The corrosion behavior was investigated via observing the specimens immersed in EXCO solution (solution for Exfoliation Corrosion Susceptibility test in 2xxx and 7xxx series aluminum alloys, referring ASTM G34-01). Secondary precipitation was observed during the cooling stage, leading to increased precipitate number density. The distribution of grain boundary precipitates transits from discontinuous to continuous at the cooling stage, due to the secondary precipitation’s linking-up effect. The solutes’ enrichment on grain boundary precipitates and the depletion in precipitate-free zones develops during the heating procedure, but remains invariable during the cooling procedure. The corrosion in NIA (Non-isothermal Ageing) treated specimens initiates from pitting and then transits to intergranular corrosion and exfoliation corrosion. The transition from pitting to intergranular corrosion is very slow for specimens heated to 190 °C, but accelerates slightly as the cooling procedure proceeds. The transition to exfoliation corrosion is observed to be quite slow in all specimens in non-isothermal aged to over-aged condition, suggesting a corrosion resistance comparable to that of RRA condition.

Large-scale, high-strength aluminum alloy forgings are essential components in the aerospace industry, with benefits including increasing strength and decreasing weight. Accurate shape-property control is the secret to forging quality. This study uses the alloy 7050 to experimentally evaluate the parametric influence of cold compression on residual stress and mechanical characteristics. The evolutions of mechanical properties, microstructure and residual stress are theoretically studied using various cold compression strains from 1% to 5% on an equivalent part, of which the results are further applied on a complicated rib-structured die forging. It is demonstrated that increasing the compression strain reduces the tensile strength of the material, but has little impact on conductivity and fracture toughness. According to the TEM results, compression also encourages the precipitation and growth of precipitated phases, particularly in positions with high dislocation densities after aging. Cold compression significantly reduces residual stress; nevertheless, as compression strain increases, residual stress first decreases and then increases. With the use of rib-structured forging, it is observed that the compression strain for 7050 aluminum alloy ranges from 2% to 4%, and the combined pressing method of the rib and web improves the uniformity of residual stress.

The influence of iron (Fe)-containing constituent particles on the behavior of fatigue crack initiation and propagation of Al-Cu-Li-Mg-Zr alloys has been studied using fatigue crack growth (FCG) tests and in-situ fatigue testing and detailed metallographic examination based on scanning electron microscopy. Experimental results show that the alloy with a low level of Fe content (2A97-T3 sheet) exhibited a lower density, accompanying equivalent tensile strength and FCG rate compared to the damage-tolerant 2524-T3 sheet. It was found that the fatigue b growth of both alloys is dominated by transgranular mode, accompanied by intergranular expansion, and the high level of Fe content alloy presents more characteristics of intergranular. Coarse constituent particles were detrimental to the resistance against FCG. It is postulated here that the micro-cracks formed around the coarse Fe-containing particles are merged with the primary crack to produce a bridging effect, accelerating the growth of fatigue cracks in the alloy with a high level of Fe content.

The effects of 1%~5% cold compression on the mechanical properties, microstructure and residual stress of 7050 aluminum alloy were investigated, and the results were applied to the complex die forging with stiffener structure. The results show that with the increase of cold compression, the tensile strength of forging decreases gradually, but the conductivity and fracture toughness of the forging have little change. TEM microstructure shows that with larger cold compression (5%), the dislocation density was relatively higher, which promoted the precipitation and growth of precipitated phases at positions with higher dislocation density during aging process. The ultrasonic tests of residual stress show that with the increase of cold compression, the residual stress in the forging decreases first, and then increases when cold compression reaches 5%. The results of the die forging of 7050 aluminum alloy with bar structure show that the residual stress of the die forging is obviously eliminated after 2-4% cold compression, and the deformation after machining is obviously less than of the uncompressed forgings. The combination of bar and web is more beneficial to the uniform distribution of residual stress of the die forging with bar structure.