Effects of pre-rolling on mechanical properties and fatigue crack growth rate of 2195 Al-Li alloy

While pre-deformation is often conducted before aging treatment to increase the strength and microhardness of 2195 Al-Li alloy, it often increases the fatigue crack growth(FCG) rate and thus reduces the fatigue life of the alloy.To determine the effects and causes of pre-deformation and heat treatment on the mechanical properties and FCG rate of2195 Al-Li alloy, and to provide a suitable calculation model for the FCG rate under different pre-deformation conditions, 2195 Al-Li alloy specimens with different degrees of pre-rolling(0, 3%, 6%, and 9%) were investigated. The experimental results indicate that with the increase of pre-rolling, the density of the T1phase and the uniformity of the S′distribution and the microhardness, tensile strength, and yield strength of the alloy increase and at the same time the FCG rate increases, and thus the fatigue life is reduced. It was also found that the normalized stress intensity factor of elastic modulus(E) can be applied to correlate the FCG rate of pre-rolled 2195 Al-Li alloy with constant C and K parameters.

Strength and structure variation of 2195 Al-Li alloy caused by different deformation processes of hot extrusion and cold-rolling

2195 Al-Li alloy was deformed through extrusion followed by cold-rolling.The textures of the extruded plate and cold-rolled sheet after solutionization were investigated.The longitudinal strength and precipitates after T8 aging were measured and observed,respectively.Compared to those in the sheet,T1(Al2 Cu Li)precipitates in the extruded plate after T8 aging are non-uniform,and their incubation time is shorter.The extruded plate after solutionization is not recrystallized and contains 55.28%deformation textures of Brass and S.In the cold-rolled sheet after solutionization,massive recrystallization occurs and S component disappears.Due to the higher fraction of Brass and S textures with higher Schmid factor and lower equivalent sliding system number,the extruded plate possesses an yield strength not higher or even lower,but a tensile strength higher,than the cold-rolled sheet after solutionization.In addition,during the aging after pre-stretch,these textures promote T1 precipitation on preferred sliding planes of cold-rolled sheet and cause its higher yield strength and tensile strength after T8 aging.

Corrosion Properties of Light-weight and High-strength 2195 Al-Li Alloy

The intergranular corrosion and exfoliation corrosion of 2195 Al-Li alloy treated by multi-step heating-rate controlled aging （MSRC） are studied. The corrosion features of 2195 Al-Li alloys which are respectively treated by high-temperature nucleation MSRC （H-M） and low-temperature nucleation MSRC （L-M） are contrasted. And the corrosion mechanism of 2195 A1-Li alloy is also discussed from the viewpoint of microstructure （types, distribution, etc.） of the strengthening phase. The results show that 2195 A1-Li alloy after H-M is more susceptible to intergranular corrosion and exfoliation corrosion than that of alloy after L-M. The degree of intergranular corrosion increases with the increase of predeformation amount and the surface parallel to the rolling direction is more prone to exfoliation corrosion. The main reason of intergranular corrosion and exfoliation corrosion is the formation of corrosion galvanic couples among T1 phase, θ＇ phase and grain boundary precipitate-free zones （PFZ）.

Microstructure evolution of spray deposited and as-cast 2195 Al-Li alloys during homogenization

In this paper,a comparative study on the spray deposited and as-cast 2195 alloy was carried out to reveal their microstructure evolutions and differences during the homogenization process.The dissolution of the secondary particles and the diffusion of solute were studied based on microstructure characterization and kinetics analysis.The precipitation behavior of Al3Zr dispersoids and its influence on recrystallization were investigated by using TEM and EBSD characterization.It was found that the large-size particles at triangular grain boundaries dissolve slower than the intragranular phases and other grain boundary phases.The required homogenization time depends on the dissolution processes of the large-size phases at grain boundaries.The size of grain boundary phases in the spray deposited alloy is much smaller than that in the as-cast alloy,so the homogenization time required for the spray deposited alloy is significantly shorter.Two-stage and ramp heating homogenization processes can promote the precipitation of Al3Zr dispersoids in the two alloys.In the spray deposited alloy,the dispersoids tend to precipitate at the positions of the T1 plates dissolved,which causes a non-uniform distribution and decreases the recrystallization resistance of the alloy.However,the distribution of the dispersoids in the as-cast alloy is more uniform after the homogenization,which brings a stronger inhibition on the recrystallization.According to the microstructural characterization and kinetics analysis results,it can be concluded that the homogenization with a slow ramp heating is suitable for the two 2195 alloys,and a shorter holding time can be used for spray deposited alloy,e.g.12 h at 500℃,while the holding time for the as-cast alloy is no less than 35 h at 500℃.

Characterization of localized corrosion pathways in 2195-T8 Al-Li alloys exposed to acidic solution

The corrosion properties of aluminum-lithium(Al-Li) alloys, which are potential materials used to construct for tanks of liquid rockets or missiles, are essential for safe propellant storage and transport. In order to manifest the corrosion resistance of the 2195 Al-Li alloy in practical propellant tanks filled with N2O4, the alloy was soaked in 30% nitric acid solution, an accelerating corrosion environment, to test its corrosion behavior. Scanning electron microscopy(SEM) and transmission electron microscopy(TEM)were used to characterize microstructure and corrosion morphology of the alloy. Focused ion beam(FIB),combined with SEM, was used to demonstrate localized corrosion features and the propagation of corrosion pathways beneath the alloy surface. It was found that the corrosion network was formed with most intergranular corrosion and sparse intragranular corrosion. Additionally, the distribution and number of intermetallic particles influenced the localized corrosion degree and the direction of corrosion pathways. Aggregated particles made corrosion pathways continuously and caused more severe corrosion. The results from this work were valid and useful to corrosion prevention and protection for storage safety on propellant tanks in N_(2)O_(4).

Role of alloying and heat treatment on microstructure and mechanical properties of cast Al-Li alloys:A review

Due to the prominent advantages of low density,high elastic modulus,high specific strength and specific stiffness,cast Al-Li alloys are suitable metallic materials for manufacturing complex large-sized components and are ideal structural materials for aerospace,defense and military industries.On the basis of the microstructural characteristics of cast Al-Li alloys,exploring the role of alloying and micro-alloying can stabilize their dominant position and further expand their application scope.In this review,the development progress of cast Al-Li alloys was summarized comprehensively.According to the latest research highlights,the influence of alloying and heat treatment on the microstructure and mechanical properties was systematically analyzed.The potential methods to improve the alloy performance were concluded.In response to the practical engineering requirements of cast Al-Li alloys,the scientific challenges and future research directions were discussed and prospected.

Predict the evolution of mechanical property of Al-Li alloys in a marine environment

The ocean is one of the essential fields of national defense in the future,and more and more attention is paid to the lightweight research of Marine equipment and materials.This study it is to develop a Machine learning(ML)-based prediction method to study the evolution of the mechanical properties of Al-Li alloys in the marine environment.We obtained the mechanical properties of Al-Li alloy samples under uniaxial tensile deformation at different exposure times through Marine exposure experiments.We obtained the strain evolution by digital image correlation(DIC).The strain field images are voxelized using 2D-Convolutional Neural Networks(CNN)autoencoders as input data for Long Short-Term Memory(LSTM)neural networks.Then,the output data of LSTM neural networks combined with corrosion features were input into the Back Propagation(BP)neural network to predict the mechanical properties of Al-Li alloys.The main conclusions are as follows:1.The variation law of mechanical properties of2297-T8 in the Marine atmosphere is revealed.With the increase in outdoor exposure test time,the tensile elastic model of 2297-T8 changes slowly,within 10%,and the tensile yield stress changes significantly,with a maximum attenuation of 23.6%.2.The prediction model can predict the strain evolution and mechanical response simultaneously with an error of less than 5%.3.This study shows that a CNN/LSTM system based on machine learning can be built to capture the corrosion characteristics of Marine exposure experiments.The results show that the relationship between corrosion characteristics and mechanical response can be predicted without considering the microstructure evolution of metal materials.

Corrosion behavior of 2195 and 1420 Al-Li alloys in neutral 3.5% NaCl solution under tensile stress

The corrosion behaviors of 1420 and 2195 Al-Li alloys under 308 and 490 MPa tensile stress respectively in neutral 3.5% NaCl solution were investigated using electrochemical impedance spectroscopy(EIS) and scanning electron microscope(SEM). It is found that the unstressed 1420 alloy is featured with large and discrete pits, while general corrosion and localized corrosion including intergranular corrosion and pitting corrosion occur on the unstressed 2195 alloy. As stress is applied to 1420 alloy, the pit becomes denser and its size is decreased. While, for the stressed 2195 alloy, intergranular corrosion is greatly aggravated and severe general corrosion is developed from connected pits. The EIS analysis shows that more severe general corrosion and localized corrosion occur on the stressed 2195 Al-Li alloy than on 1420 Al-Li alloy. It is suggested that tensile stress has greater effect on the corrosion of 2195 Al-Li alloy than on 1420 Al-Li alloy.

Grain structure and microtexture evolution during superplastic deformation of 5A90 Al-Li alloy

The microstructural evolution of banded 5A90 A1-Li alloy during superplastic deformation at 475℃ with an initial strain rate of 8× 10^-4 S^-1 was studied using EBSD technique. The results showed that, before deformation, the grain shape appeared to be banded, the most grain boundaries belonged to low-angle boundaries, and the initial sheet had a dominate of { 110}（112） brass texture. During deformation, there were grain growth, grain shape change, misorientation increasing and textural weakening. The fraction of high-angle boundaries increased rapidly once the flow stress reached the peak value. Corresponding deformation mechanism for various stages of deformation was suggested. Dislocation activity was the dominant mechanism in the first stage, then dynamic recrystallization occurred, and grain rotation was expected as an accommodation for grain boundary sliding （GBS）. At large strains, GBS was the main mechanism.

Effect of thermo-mechanical treatment process on microstructure and mechanical properties of 2A97 Al-Li alloy

2A97 Al-Li alloy was processed by thermo-mechanical treatment at different pre-stretch deformations of 0, 3% and 6%. The microstrucatre observation results reveal that some δ＇ and T1 precipitates are found in a（Al） matrix of 2A97 alloy processed by the heat treatment with no pre-stretch deformation. When the pre-stretch deformation is 3% and 6%, respectively, amounts of tiny T1 and a few of S＇ precipitates precipitates are observed in the microstructures of 2A97 alloy. The tensile test results show that the tensile properties of 2A97 alloys are improved via thermo-mechanical treatment. When the pre-stretch deformation is from 0, 3% to 6%, the ultimate tensile strength values of the 2A97 alloys increase gradually from 447.7, 516.5 to 534.3 MPa, and the elongations decrease from 17.6%, 12.8% to 10.2%, respectively. Moreover, with increasing pre-stretch deformation amount from 0 to 6%, the in-plane anisotropy value of 2A97 alloys becomes more obvious.

Fatigue crack initiation and early propagation behavior of 2A97 Al-Li alloy

The fatigue crack initiation and early propagation behavior of 2A97 Al-Li alloy was studied. The smooth specimens were fatigued at room temperature under constant maximum stress control when stress ratio （R） is 0.1 and frequency （f） is 40 Hz. Microstructure observations were examined by optical microscopy, transmission electron microscopy, scanning electron microscopy and electron back scattered diffusion, in order to investigate the relationship between microstructure and fatigue crack initiation and early propagation behavior of 2A97 alloy. The results show that the fatigue cracks are predominantly initiated at inclusions and coarsen secondary phases on the surface of 2A97 alloy. The fatigue crack early propagation behavior of 2A97 alloy is predominantly influenced by the interactions between grain structure and dislocations or persistent slip bands （PSBs）. When the misorientation of two neighbouring grains is close to the orientations of the favorable slip plane within these two grains, high-angle grain boundary severely hinders the PSBs passing through, and thus leads to crack bifurcation and deflection.

Hot deformation behavior and microstructure evolution of 1460 Al-Li alloy

The hot deformation behavior and microstructure evolution of 1460 Al-Li alloy were investigated by isothermal compression test conducted at various strain rates（10-3-10 s-1） and temperatures（573-773 K）. The flow stress curves were corrected by considering the friction at the platen/specimen interface and the temperature change due to the deformation heating. The effects of strain, strain rate and temperature on the deformation behavior were characterized by the Zener-Hollomon parameter in a hyperbolic-sine equation, and the constitutive equations were established according to the peak flow stress associated with dynamic recovery, dynamic recrystallization and the dissolution of T1 phases. In the entire strain rate and temperature range, the prediction capabilities of the developed constitutive equation are proved to be feasible and effective with a linear correlation coefficient and an average absolute relative error coefficient of 0.9909 and 6.72%, respectively.

EFFECT OF HEAT TREATMENTS ON TENSILE PROPERTIES AND MICROSTRUCTURE OF 2195 ALLOY

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Through-thickness inhomogeneity of precipitate distribution and pitting corrosion behavior of Al-Li alloy thick plate

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.

Microstructure evolution of aluminum-lithium alloy 2195 undergoing commercial production

Microstructures of three kinds of typical product states for commercially fabricated alloy 2195 were observed.It is found that the hot-rolled plate is characterized by a fibrous structure containing fine,polygonized substructures;and the cold-rolled sheet was characterized by a "pan-caked" grain structure containing high density dislocation cells.The product under near peak-aging temper is proved to contain a large amount of dispersive,plate-shaped T1(Al2CuLi) precipitates,together with a small fraction of θ'(Al2Cu) plates,exhibiting a desirable combination of mechanical properties.Analyses using scanning electron microscopy reveal that many coarse,irregular-shaped Al7Cu2Fe constituent particles exist in all product states,which indicates that intermediate heat treatments have little influence on this iron-caused,detrimental phase.The formation and evolution of microstructures for different product states of alloy 2195 were discussed in view point of the commercial production condition.

Microstructures evolution and mechanical properties disparity in 2070 Al-Li alloy with minor Sc addition

The microstructures and mechanical properties of Al-3.35Cu-1.2Li-0.4Mg-0.4Zn-0.3Mn-0.1Zr （mass fraction, %） alloywith Sc addition or free Sc were investigated through tensile test, SEM, EPMA and TEM. The addition of 0.082% （mass fraction） Scelement leads to the formation of Cu-rich and Sc-contained nano-sized Al3（ScZr） particles and W phase particles. The Al3（ScZr）particles can inhibit recrystallization to a certain extent and impede recrystallized grain growth during solution treatment. It is foundthat W phase cannot dissolve in supersaturated solid solution during the solution heat treatment, and the Cu content in thesolutionized matrix is decreased, which causes a decrease in the fraction of Cu-contained strengthening precipitates with T1 （Al2CuLi）and θ＇ （Al2Cu） under T8 aging condition. Due to the formation of the W phases, the small Sc addition causes a little reduction in thestrength.

Flow curve correction and processing map of 2050 Al-Li alloy

Hot compression tests of 2050 Al-Li alloy were performed in the deformation temperature range of 340-500°C and strain rate range of 0.001-10 s-1 to investigate the hot deformation behavior of the alloy.The effects of friction and temperature difference on flow stress were analyzed and the flow curves were corrected.Based on the dynamic material model,processing map at a strain of 0.5 was established.The grain structure of the compressed samples was observed using optical microscopy.The results show that friction and temperature variation during the hot compression have significant influences on flow stress.The optimum processing domains are in the temperature range from 370 to 430°C with the strain rate range from 0.01 to 0.001 s-1,and in the temperature range from 440 to 500°C with the strain rate range from 0.3 to 0.01 s-1;the flow instable region is located at high strain rates(3-10 s-1)in the entire temperature range.Dynamic recovery(DRV)and dynamic recrystallization(DRX)are the main deformation mechanisms of the 2050 alloy in the stable domains,whereas the alloy exhibits flow localization in the instable region.

Effect of Rare Earth Cerium on Yield Strength Anisotropy of Al-Li Alloy Sheet and Its Theoretical Prediction

The variation of yield strength along rolling direction, transverse direction and 45° to transverse direction of 2090 Al-Li alloy and 2090+Ce alloy sheet containing rare earth cerium was comparatively investigated. The difference of deformation texture in these two alloy sheets was analyzed by means of X-ray orientation distribution function (ODF). The results show that cerium has the effects of enhancing the Brass and S rolling texture components and reducing the recrystallized texture components of Cube and Goss. This is the reason that the anisotropic degree of yield strength in 2090+Ce sheet is higher than that of 2090 alloy sheet. The prediction of yield strength along various orientations in two alloy sheets was done based on Taylor/Bishop-Hill model, and the strengthening effect of grain boundary was evaluated using Hall-Petch relationship. A modified plastic inclusion model was proposed using the concept of grain-orientation factor and T1 phase orientation factor by fitting with tensile test results.

Effect of large strain cross rolling on microstructure and properties of Al-Li alloy plates with high magnesium content

Transmission electron microscopy(TEM),electron backscattered diffraction imaging(EBSD)and X-ray diffractometry were used to analyze the microstructure and texture characteristics of Al-9.8Mg-1.5Li-0.4Mn alloy cross-rolled and extruded plates,and the tensile properties and deep drawing performance were measured.The results show that the occurrence of dynamic recrystallization was promoted,the grains were refined and the preferred orientation of the recrystallized grains was improved by large strain cross rolling.Compared with CBA and CCB rolling methods,CBB rolling method significantly reduced the orientation density of the typical Brass texture{110}?112?in the extruded plates.The orientation densities of Copper texture{112}?111?and Brass texture{110}?112?on theβorientation line in the CBB rolled plates were the lowest,and there were no typical texture features in the plates.Meanwhile,better deep drawing could be gained in the CBB rolled plates,and the mechanical properties of the 0°,45°and 90°directions were basically the same.The tensile strength,yield strength and elongation at room temperature for the CBB rolled plates were 617 MPa,523 MPa and over 20.1%,respectively.The deviation of the mechanical properties at different directions was less than 3%.

Preparation and Corrosion Resistance of Anodic Oxidation Coatings on 2198 and 5A90 Al-Li Alloys

2198 and 5A90 Al-Li alloys were anodized with a constant DC potential in 18%H_2SO_4solution(Solu.A) and the mixture solution of 18%H_2SO_4+5%C_2H_2O_4(Solu.B) at room temperature. 12 and 11 V was optimized as the applied oxidation potential for 2198 and 5A90 alloys, respectively. Cross-sectional morphology, surface morphology and elements distribution of anodic oxidation coatings were observed by scanning electron microscope equipped with energy dispersive X-ray analysis(SEM/EDX). Corrosion resistance was tested by potentiodynamic polarization plot in 3.5%NaCl solution. The results showed that the thicknesses of coatings obtained at the selected potential in Solu.A and Solu.B were about 50 μm/110 μm for 2198 alloy and 80 μm/110 μm for 5A90 alloy. In both solutions, anodic oxidation coatings of 2198 alloy were primarily composed of Al oxides; those of 5A90 alloy were mainly consisted of Al oxides and a small amount of Mg oxides. The results of potentiodynamic polarization showed that anodic oxidation coatings of 2198 and 5A90 Al-Li alloys had better corrosion resistances than that of untreated alloys.