The Advancement of 7XXX Series Aluminum Alloys for Aircraft Structures: A Review

Altıparmak S.C., Yardley V.A., Shi Z., Lin J.

Additive Manufacturing (AM) processes, also known as 3D printing, enable geometrically complex parts to be produced layer by layer on the basis of three-dimensional (3D) data generated either by scanning physical objects or using design software. Compared to conventional manufacturing processes, AM offers the elimination of production steps, allowing rapid and relatively easy prototyping of physical objects from 3D model designs, and reproduction of existing objects. Over the last two decades, AM processes have become widespread for the manufacturing of complex-shaped components in numerous industrial sectors, one of their main areas of application being in the aerospace industry. This sector makes extensive use of high-strength aluminium alloys because of their high strength-to-weight and stiffness-to-weight ratios and excellent machinability. However, the applicability of AM processes to high-strength aluminium alloys is still limited by the presence of several types of non-negligible issues and defects in additively manufactured (AMed) aluminium components. Over the years, significant research efforts have been directed at minimising or eliminating these defects and thereby expanding the range of applications of AM in high-strength aluminium alloys. This paper reviews the state of the art in AM of high-strength aluminium alloys for aerospace. The focus is on defects and issues in AMed 2xxx and 7xxx series alloys and recent developments in novel hybrid AM processes to minimise or eliminate the defects.

Georgantzia E., Gkantou M., Kamaris G.S.

Over the last few decades aluminium alloys have been increasingly used in the construction sector due to their favourable properties. Thereafter, many research projects have been carried out with the aim to obtain a more comprehensive understanding of their structural performance and develop accurate and reliable design formulae. The scope of this paper is to provide a comprehensive review of research by discussing the reported experimental, numerical and analytical work on structural aluminium alloys. The paper presents an overview of research studies on the mechanical properties of aluminium alloys under monotonic, cyclic and thermal loading conditions. Moreover, a considerable amount of experimental and numerical investigations focussing on the structural performance and design of aluminium columns, beams and beam-columns is reviewed. The performance of connections and composite aluminium-concrete members is also discussed. Comments on the suitability of the international design specifications to structural aluminium alloys are included. Within the review, knowledge gaps are identified and the corresponding research work to fill these gaps is recommended.

Wang K., Wang Y., Yue X., Cai W.

• An experimentally validated finite element based multiphysics model is developed for metal tribocorrosion. • Wear induced residual stress near the edge of the wear track results in nonuniform surface corrosion. • Wear-corrosion synergy leads to time-dependent tribocorrosion rate. • Tribocorrosion maps are constructed to predict material loss as a function of mechanical and corrosion properties. • Surrogate models are used for uncertainty quantification of the finite element model. Past research in tribocorrosion mainly relies on costly and trial-and-error experimental methods to study the materials’ deformation and degradation under extreme conditions. This work developed a finite element based multiphysics model, validated by existing tribocorrosion experiments of two Al alloys, to analyze the synergistic effects of mechanical and corrosion properties on the material degradation mechanisms of tribocorrosion. During consecutive passes of the counter body, significant residual stress was found to develop near the edge of the wear track, leading to highly concentrated corrosion current than elsewhere. Such non-uniform surface corrosion and stress-corrosion coupling led to variations of tribocorrosion rate over time, even though testing conditions were kept constant. Tribocorrosion rate maps were generated to predict material loss as a function of different mechanical and electrochemical properties, indicating a hard, complaint metal with high anodic Tafel slope and low exchange current density is most resistant to tribocorrosion. Finally, two surrogate models, Gaussian process and neural network with dropout, were used for uncertainty quantification of the finite element model.

Ghafouri T., Golshan Bafghi Z., Nouri N., Manavizadeh N.

• Quantum characteristics of side-contacted field-effect diode is studied using the finite-difference method. • Schrödinger equation is solved regarding assessed potentials in the ON and OFF states. • Potential profiles, energy levels, and time-independent/dependent wave functions are studied. • Remarkable potential barriers in the OFF state result in an inability of electron movement from source to drain in low energies. • The transport is feasible in higher states, so that minority carriers contribute to transport mechanism in the highest energies. Numerical approaches play an outstanding role in solution of quantum mechanical problems with due attention to the complexity of analytic solutions for open systems. This paper studies quantum characteristics of the previously proposed side-contacted field-effect diode (S-FED) as an emerging device in the modern system-on-chips (SoCs) using the finite-difference method (FDM). The characteristics obtained by solving the Schrödinger equation and regarding the distinguished potentials in ON and OFF states include energy levels and time-independent/dependent wave functions. The cosine dependency of eigenvalues on longitudinal position conveys level broadening in high states stringing a sequence of probability oscillations in the ON state. Remarkable potential barriers in the OFF state result in an inability of electron movement from source to drain in low energies; nevertheless, by overcoming the total energy to potential barrier, the transport is feasible in higher states, so that minority carriers contribute to transport mechanism in the highest energies.

Xing Y., Song L., He X., Qiu C.

In this article a generalized finite difference method (GFDM), which is a meshless method based on Taylor series expansions and weighted moving least squares, is proposed to solve the elliptic interface problem. This method turns the original elliptic interface problem to be two coupled elliptic non-interface subproblems. The solutions are found by solving coupled elliptic subproblems with sparse coefficient matrix, which significantly improves the efficiency for the interface problem, especially for the complex geometric interface. Furthermore, based on the key idea of GFDM which can approximate the derivatives of unknown variables by linear summation of nearby nodal values, we further develop the GFDM to deal with the elliptic problem with the jump interface condition which is related to the derivative of solution on the interface boundary. Four numerical examples are provided to illustrate the features of the proposed method, including the acceptable accuracy and the efficiency.

Li G., Cui S.

The evolution of crystal damage in rolled AA5182-O aluminum alloy sheet was studied by experiment and finite element simulation in this paper. Grain size characteristic on rolling anisotropy was determined by electron backscatter diffraction (EBSD) test and section line method, and the average grain size of R00°and R90° with the rolling direction was achieved. An improved grain modeling method was proposed by adjusting base rectangle and regularity coefficient based on Voronoi method. It established six finite element models of uniaxial tension grains, which considers anisotropic grain size and regularity coefficient. The Gurson–Tvergaard–Needleman (GTN) model was applied to analyze damage evolution of elements in grains, and whose damage parameters were achieved by the tensile test and the response surface method. The mechanical properties of grain boundaries were characterized by cohesive zone model. The Abaqus software was adopted to conduct the uniaxial tension finite element analysis to investigate influence of anisotropic grain size, number, and regularity coefficient on plasticity damage evolution of this material. The results show that when regularity coefficient of is 0.01 and 007, the fracture elongation of 54 grains model which is considering grain sizes of R00° and R90° is the closest to experimental results.

FAN R., ATTARILAR S., SHAMSBORHAN M., EBRAHIMI M., GÖDE C., ÖZKAVAK H.V.

The use of a constrained groove pressing (CGP) method to plastically deform AA6063 aluminum alloy led to the improved surface properties. It was found that hardness magnitude is dramatically improved and its uniformity is considerably decreased after the first pass, while subsequent passes result in better hardness behavior for the processed material. Also, the elongated grains formed in the first pass of the CGP are gradually converted to the equiaxed counterparts by adding pass numbers. Eventually, higher corrosion resistance of the sample by imposing the CGP process is related to the quick formation of passivation film and the change in the morphology of the second phase and precipitates which hinder their electrochemical reactions and decrease the potential localized attack sites.

Kim S., Moon S.K.

Parts with complex geometry have been divided into multiple parts due to manufacturing constraints of conventional manufacturing. However, since additive manufacturing (AM) is able to fabricate 3D objects in a layer-by-layer manner, design for AM has been researched to explore AM design benefits and alleviate manufacturing constraints of AM. To explore more AM design benefits, part consolidation has been researched for consolidating multiple parts into fewer number of parts at the manufacturing stage of product lifecycle. However, these studies have been less considered product recovery and maintenance at end-of-life stage. Consolidated parts for the manufacturing stage would not be beneficial at end-of-life stage and lead to unnecessary waste of materials during maintenance. Therefore, in this research, a design method is proposed to consolidate parts for considering maintenance and product recovery at the end-of-life stage by extending a modular identification method. Single part complexity index (SCCI) is introduced to measure part and interface complexities simultaneously. Parts with high SCCI values are grouped into modules that are candidates for part consolidation. Then the product disassembly complexity (PDC) can be used to measure disassembly complexity of a product before and after part consolidation. A case study is performed to demonstrate the usefulness of the proposed design method. The proposed method contributes to guiding how to consolidate parts for enhancing product recovery.

Wang Y., Cao L., Wu X., Tong X., Liao B., Huang G., Wang Z.

The microstructure, hardness and corrosion resistance of aluminum alloy 7085 after different retrogression and re-aging (RRA) treatments were investigated. The results showed that the properties of alloy 7085 are sensitive to the temperature and dwell time of the retrogression treatment. Desirable mechanical property and corrosion resistance can be obtained for the alloy pre-aged at 120 °C for 24 h, followed by a retrogression at 160 °C for 1.5 h and re-aging at 120 °C for 24 h. After the optimized RRA treatment, the hardness of alloy 7085 is improved by 10.2% as compared with the peak-aged one, which is mainly due to the distribution of dispersed rod-like η′ precipitates in the matrix; while the improved corrosion resistance is mainly attributed to the discrete and coarse η phase with higher Cu content, and narrow precipitate-free zone about 45–50 nm in width along the grain boundaries.

Sharma K., Srinivas G.

In one way or the other, all of the world’s major scientific inventions have been dependent on the materials available at that time. Edison was able to invent light bulb only because Tungsten, a material capable to sustain high temperature, was available to him. Wright brothers were able to make their airplane fly because their engine was made out of aluminium and not steel, which kept their aircraft light weight. Materials play an important part in defining the function of the structure, they are used for making. In the 21st century entire world is moving towards automation and artificial intelligence so it becomes necessary that the structures, that are made, be intelligent and smart so as to adapt to their surrounding thus increasing efficiency and reducing the complexity in designing. There is a need to develop smart structures for aerospace application which can suffice the demands of this expanding industry. Smart materials like Shape Memory Alloys (SMA), piezoelectric materials, Carbon Fiber Reinforced Polymer (CFRP), Shape Memory Polymer (SMP) etc. are the materials that make up the backbone for latest aerospace application. Developing materials that can be used for morphing application is of strategic and economic importance for both civil and military application. For morphing application, the element of the material should be one that can possess variable stiffness and allow the shape change to be a reversible process. This paper reviews different types of smart materials that are used in the field of aerospace and explores their application. This paper will help future students and researchers gain a concise knowledge of smart materials used in the world of aerospace and will pave a way for future work that needs to be done in this field.

Xie P., Chen S., Chen K., Jiao H., Huang L., Zhang Z., Yang Z.

The influence of heat treatment on stress corrosion cracking resistance of a low-Cu containing Al-Zn-Mg-Cu aluminum alloy has been investigated. The results show that the stress corrosion cracking resistance is significantly improved by step-quench and aging heat treatment whereas retrogression and re-aging heat treatment and two-stage over aging heat treatment could hardly enhance the stress corrosion cracking resistance compared with peak aging. The improved stress corrosion cracking resistance of the low-Cu containing Al-Zn-Mg-Cu aluminum alloy subjected to step-quench and aging heat treatment is mainly attributed to grain boundary precipitates with high Cu content, large size and discontinuous distribution.

Xu X., Mao Q., Jiang Z., Vitus T., Zhang T., Jia W., Zhu C., Wang H.

The aluminum alloy extrusion materials were treated by multi-stage solution aging, and the microstructure and mechanical properties were characterized and tested. The results show that with the increase in solid solution temperature and time, the strength shows a trend of increasing first and then decreasing, and reaches a maximum value in G3. Under the conditions of 450 °C × 2 h + 460 °C × 2 h + 470 °C × 2 h in solid solution process and 121 °C × 5 h + 133 °C × 16 h in the aging process, the aluminum alloy has excellent comprehensive properties, with a strength of 828.0 MPa and elongation of 8.1%.

ZHAO K., LIU J., YU M., LI S.

The thorough-thickness inhomogeneity of precipitate distribution and pitting corrosion behavior of 95 mm-thick 2297 Al−Li alloy rolled plate was investigated using scanning electron microscopy, transmission electron microscopy and electrochemistry method. Precipitate distribution and pit size were statistically analyzed to obtain quantitative information and corresponding correlation. The population density and the size fraction of precipitate on different sections in the thick plate are ranked from high to low in the following order: quarter-section (QS) > surface section (SS) > mid-section (MS). After 300 min potentiostatic polarization, the number and the total volume of pits are ranked from high to low as QS>SS>MS, indicating a higher pitting susceptibility of the plate in QS with more precipitates. The through-thickness inhomogeneity of pitting corrosion in 2297 Al−Li alloy thick plate is mainly ascribed to inhomogeneous precipitate distribution.

Krasnikov V.S., Mayer A.E.

We investigate the interaction of edge dislocation with θ′ phase in aluminum matrix using atomistic simulations. The thickness of θ′ phase is chosen to be constant of 2.2 nm and the diameter is varied in range from 3 to 10 nm. It is shown that first interactions of dislocation with θ′ phase occur according to the mechanism of Orowan loop formation around the obstacle. In this case, the cross-slip processes, the formation of dislocation jogs in adjacent slip plane and the emission of vacancies upon the return of a segment to the initial slip plane are possible. During these interactions, the material of θ′ phase is subjected to high shear stresses up to 3 GPa in a layer of 2 nm thickness. Such a high stress leads to a θ′ phase cutting on the third or fourth overcoming of the obstacle by dislocation. A study is carried out of the dependence of the average stress in the system on the size of inclusion and the distance between inclusions. It is shown that an increase in the diameter of inclusion causes an increase in the average stresses in the system in proportion close to the square root of the diameter of the inclusion. Increasing the distance between inclusions causes the inversely proportional reduction of the average stress. The conducted investigation of the strain rate sensitivity showed that in the case of high shear rates, the average stresses in the system continuously increase that does not allow applying the time averaging procedure to them. The described effect is also registered in the case of pure aluminum. The existence of two regions on the temperature dependence of the average stresses in the system on the strain rate in the case of θ′ phase, previously described by Yanilkin et al. (2014) for the Guinier-Prestone zones, is confirmed. In the case of high strain rates, heating of the system leads to a decrease in the dislocation velocity, while at low strain rates the dislocation velocity increases with increasing temperature at a fixed shear rate. An interesting result obtained with long-term molecular-dynamics simulations when the tracing time is up to 2 ns, there is a tendency to reduce the average stress in the system through time. This result can be explained by destruction of the structure and form of θ’ phase. The law of motion of a dislocation in the approximation of the constancy of θ′ phase properties is proposed to describe the response of the system to shear deformation. The model contains a dislocation mass, phonon friction, and takes into account the effect of the inclusion of θ’ phase through an increase in the elastic energy of a dislocation during the formation of the Orowan loop around an obstacle.

Ashkenazi D.

The history of aluminum is rather short since it was discovered only in the nineteenth century, yet it has become an important part of everyday life. This article reviews the history of aluminum through technological breakthroughs as well as from cultural and social perspectives, beginning with its discovery, through the nineteenth and twentieth centuries until today; and aims to suggest possible future trends and applications for aluminum alloys. Aluminum has a high strength-to-weight ratio combined with excellent thermal conductivity and good corrosion resistance. Therefore, aluminum is an attractive material for many applications, including transportation, electrical and packaging industries, architecture, and food industries. It is also a recyclable metal, which provides both environmental and economic advantages. The commercial use of aluminum started at the end of the nineteenth century and continues to grow today with the development of new advanced aluminum alloys. Consequently, from a cultural perspective, aluminum is considered a symbol of modernity. Technological breakthroughs generate economic growth and social benefits. Present applications of aluminum include new choices, such as 3D printing, composite materials, nano-rods, biomedicine devices and aerospace uses. Based on the excellent properties of aluminum, its low price, combined with its significant scrap value and a growing recycling market, as well as its accelerating global production, it is expected that the aluminum industries will considerably grow through the twenty-first century and aluminum will continue to be a major part of our everyday culture. Therefore, based on the increasing growth of aluminum production and consumption, additional research and development effort is needed in the following years to minimize the negative environmental side effects associated with the technological developments related to aluminum production and at the same time creating further technological innovation.

Karahan B., Tekin K.C., Malayoglu U.

Beyss O., Breuer U., Zander D.

Tan Y., Zhou Y.

Lin Y., Wang L.

Zheng Z., Li L., Xie C., Ruan X., Yang J.

ABSTRACTThis study constructs a kernel average misorientation (KAM) evolution model based on the crystal plasticity finite element (CPFE) model, simulating the cyclic deformation of circular‐like single‐edge notched (CL‐SEN) specimens under variable strain amplitudes and mean strains. The relationship between KAM and cyclic plasticity (plastic strain amplitude and mean plastic strain) is analyzed. The results show that the CL‐SEN does not affect the linear relationship between KAM and plastic strain amplitude. For the first time, a linear relationship between KAM and mean plastic strain is proposed, with the influence of strain amplitude on KAM being significantly greater than that of mean strain. Additionally, the distribution patterns of KAM and plastic strain are consistent, further confirming their close correlation. The findings enhance understanding of fatigue damage mechanisms and provide guidance for improving the fatigue performance of materials.

Arcieri N., Manfredi D., Actis Grande M.

Kumar R., Mondal S.

Since Al7075 aluminum matrix composites (AMCs) have desirable material qualities that meet consumer demands, such as high hardness, high stiffness, improved yield strength, strength-to-weight ratio, high thermal conductivity, low coefficient of thermal expansion, superior wear, and corrosion resistance, their use has steadily increased over time. These are also lightweight and have better properties as compared to alloys. Due to these characteristics, researchers have recently become very interested in various possible uses in space, automotive, aircraft, submarine, and other structural applications. The present work reviews all the research on inorganic reinforcement-based stir cast Al7075 composites and focuses on their microstructural, mechanical, tribological, and machining properties. Single inorganic reinforcement, which improves material properties, is used when synthesizing stir-cast Al7075 metal matrix composites (MMCs). Generally, the reinforcements based on inorganic in their stir-cast Al7075 MMCs are highlighted with their feasible applications. The effects of different reinforcements with different sizes and weight percentages (wt%) of oxides, nitrides, carbides, borides, and other inorganic particles are analyzed. Further stir cast Al7075 composite’s properties like microstructural, mechanical, tribological, and machining process parameters, and their outcomes studied by various researchers are thoroughly discussed.

Bharat N., Jain R., Bose P.S., Kumar V.

Aluminum alloys of the 7xxx family have found widespread usage in a broad variety of technical applications in recent years. Aluminum matrix composites are being used in greater quantities across a variety of industries, including aerospace and military, as well as the automotive industry. The strength-to-weight ratio, strength, stiffness, and hardness of components made of aluminum-based composites are higher than those of monolithic materials, and they also exhibit better tribological behavior. Composite materials could include these characteristics. The properties of aluminum matrix composites for bearing components heavily depend on the best choice of the reinforcing material and the production process. As a result, alloys made of Al 7xxx are the most essential materials for structural use. In this current study, the development trend and impact of reinforcements on Al 7xxx alloys for engineering applications are presented, and the existing issues are described in a concise manner.

Zheng Z., Ma P., Chen L., Liu C.

Al-Mg-Li alloy is an ideal lightweight structural material for aerospace applications due to its low density, high specific strength, and excellent low-temperature performance. This study examines the mechanical properties and microstructural evolution of Al-Mg-Li alloy subjected to cryogenic and room temperature cold rolling, which induces large plastic deformation. Compared with room temperature rolling, cryogenic rolling significantly reduces surface cavity formation, thereby enhancing the alloy’s rolling surface quality. After cryogenic rolling by 80% and subsequent natural aging, the yield strength of artificially aged Al-Mg-Li alloy reaches 560 MPa, delivering a 60% increase compared to the traditional T6 state with a slight reduction in elongation from 6.5% to 4.6%. The specific strength achieves 2.23 × 105 N·m/kg, outperforming conventional Al-Cu-Li and 7xxx-series Al alloys. The depth of intergranular corrosion decreases from 100 µm to 10 µm, demonstrating excellent corrosion resistance enabled by the new method. Transmission electron microscopy reveals that finely distributed δ′ (Al3Li) is the primary strengthening phase, with high-density dislocations further enhancing strength. However, coarsening of δ′ (from ~2.9 nm to >6 nm) induced by ensuing artificial aging results in coplanar slip and reduced elongation. Lowering the post-aging temperature inhibits δ′ coarsening, thereby improving both strength and elongation. Our results provide valuable insights into optimizing the properties of Al-Mg-Li alloys for advanced lightweight applications.

Shao Y., Peng W., Zhao Y., Oleksandr M., Fu B.

Liu W., Chen N., Liu J., Chi Y., Luo Z., Cai D., Ding C.

Ekladious A., Wang J., Chowdhury N., Baker A., Chiu W.K.

Ekladious A., Wang J., Chowdhury N., Chiu W.K.

Zhang L., Xing S., Zhai H., Hou H., Wang Z., Liu S.

In this paper, regression and re-aging treatment (RRA) was performed on the 7050 alloy, and the evolution in the microstructure of the 7050 alloy was observed by metallography and SEM. The effect of regression and re-aging treatment on the hardness of the alloy was investigated; the maximum hardness was 84.576 HRB at 180 °C/30 min. It was found that with the increase in regression treatment time, the size of the precipitates on the grain boundaries gradually increased, while the number of the precipitates inside the grain decreased accordingly. In the course of the experiment, the η’ (MgZn2) transforms into the η (MgZn2). As a result, the hardness of the alloy showed a decreasing trend. Meanwhile, the precipitates on the grain boundaries gradually increased in size, while the number of precipitates within the grain boundaries correspondingly decreased. These results reveal the influence of the re-aging treatment on the microstructure and mechanical properties of 7050 aluminum alloy, providing an important experimental basis for further optimizing the performance of the alloy.

Zhang X., Bai L., Wang D., Shi H., Liu J., Li Z., Bao X., Ma Y.