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.

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).

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.

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.

Effects of cooling rates on microporosity in DC casting Al-Li alloy

During the direct chill(DC)casting process,primary cooling from the mold and bottom block,and secondary cooling from the waterjets produce a concave solid shell.The depth of this liquid pocket and mushy zone not only depends on the solidification range of the alloy but also the boundary conditions such as cooling rates.Al-Li alloys solidify in a long solidification range increasing the susceptibility of porosity nucleation in the semi-solid region.In this study,the effects of cooling rate on the porosity formation were quantified for the large ingot casting using X-ray computed tomography(XCT).By characterizing pore size distributions at four different cooling conditions,the correlation between the mechanical properties at both room and high temperatures and the microstructure features was identified.The constitutive equations were constructed.It is found that increasing the cooling rate reduces the grain size,increases the number density of micropores,and minimizes the number of large pores,thereby improving the mechanical performance.Therefore,long mushy zones and deep liquid pockets in Al-Li alloys can be effectively controlled by controlling the boundary conditions of the DC casting solidification process,thereby obtaining castings with excellent mechanical properties.

DYNAMIC RECRYSTALLIZATION AND SUPERPLASTICITY IN Al-Li ALLOY

The behavior of dynamic recrystallization in the superplastic deformation of 8090 and 2091 aluminum-lithium alloys have been investigated.TEM observations indicated that dynamic recrystallization occurs at thetriple junction of grain boundaries.The measurement of grain boundary angle showed that recrystallization indynamic equilibrium exists in the process of superplastic deformation of 8090 Al-Li alloy.It is also indicatedthat,besides the role of refining grains and the grain boundary sliding,dynamic recrystallization playsconcurrently a role of stablizing microstructure.Thus dynamic rccrystallization can be used to induce metalssuperplasticity,which leads to a simplification of pretreatment for superplastic deformation.

MICROSTRUCTURE AND MECHANICAL PROPERTIES OF 8090 Al-Li ALLOY

The change of mechanical properties of the 8090 Al-Li alloy influenced by aging is attributed to the change of δ′-phase particle size and the precipitation of S′-phase.The δ′-phase may easily precipitate and rapidly grow,but the S′-phase can only precipitate with a longer stage of incubation.The precipitation of S′-phase would be promoted by cold working prior to ag- ing.The co-precipitation of δ′- and S′-phase could improve the strength and plastieity of the alloy,for which aging at 190℃.for 20—30 h seems to be optimal.In addition,the precipitate free zone( PFZ)can form at high angle grain boundary and its width is over 200 nm in peak aging condition.But at sub-grain boundary,the formation of PFZ is difficult.

Influence of Ce on the Extrinsic Strengthening Effect of Al-Li Alloy

The influence regular and mechanisms of rare earth element cerium and aged condition on the propertiesand fracture feature of monotonic tension for aluminum-lithium alloy 2090 were researched. The evident plasti-cation effect can be achieved by adding cerium into alloy 2090, and the aged condition has a significant influ-ence on this effect. The plastication effect of cerium at peckaging is strongest, and at underaging and overagingis weader. The mechanisms of strengthening and plastication of cerium for aluminum-lithium alloys were dis-cussed in the theory of the extrinac strengthening and the intrinsic toughening. According to the research results about fracture feature, first the theory of extrinsic strengthening was presented, and then the role of ceri-um on aluminum-lithium alloy 2090 was explored from the unite energy, the anti-phase baundary energy andthe electron bonds.

Milling Parameters Optimization of Al-Li Alloy Thin-Wall Workpieces Using Response Surface Methodology and Particle Swarm Optimization

To improve the milling surface quality of the Al-Li alloy thin-wall workpieces and reduce the cutting energy consumption.Experimental research on the milling processing of AA2195 Al-Li alloy thin-wall workpieces based on Response Surface Methodology was carried out.The single factor and interaction of milling parameters on surface roughness and specific cutting energy were analyzed,and the multi-objective optimization model was constructed.The Multiobjective Particle Swarm Optimization algorithm introducing the Chaos Local Search algorithm and the adaptive inertial weight was applied to determine the optimal combination of milling parameters.It was observed that surface roughness was mainly influenced by feed per tooth,and specific cutting energy was negatively correlated with feed per tooth,radial cutting depth and axial cutting depth,while cutting speed has a non-significant influence on specific cutting energy.The optimal combination of milling parameters with different priorities was obtained.The experimental results showed that the maximum relative error of measured and predicted values was 8.05%,and the model had high reliability,which ensured the low surface roughness and cutting energy consumption.It was of great guiding significance for the success of Al-Li alloy thin-wall milling with a high precision and energy efficiency.

ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY DURING CORROSION PROCESS OF 8090 Al-Li ALLOY IN EXCO SOLUTION

The corrosion behavior and electrochemical impedance spectroscopy (EIS) features of 8090 Al-Li alloys in EXCO solution were investigated, and the EIS was simulated using an equivalent circuit. At the beginning of immersion in EXCO solution, the EIS is comprised by a depressed capacitive arc at high-mediate frequency and an inductive arc at low frequency, and the inductive component decreases and disappears with im- mersion time. Once exfoliation or severe pitting corrosion is produced, two capacitive arcs appear in the EIS. These two capacitive arcs are originated from the two parts of the corroded alloy surface, the original flat alloy surface and the new inter-face exposed to the aggressive EXCO solution due to the exfoliation or pitting corrosion. Some corrosion development features of 8090 Al-Li alloys in EXCO solution can be obtained through simulated EIS information.

ELECTRO-SUPERPLASTICITY IN 2091 Al-Li ALLOY

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INTERFACE MORPHOLOGIES AND SOLUTE SEGREGATION OF Al-Li ALLOY 8090 DURING UNIDIRECTIONAL SOLIDIFICATION

The solid-liquid interface morphology and solute segregation behaviour of AI-Li alloy 8090 during unidirectional solidification were studied by the liquid metal quenehing method under varied processing conditions.When solidification rate,R<O.13 or>O.75 mm/min (temper- ature gradient,G_L=130℃/cm),the structure revealed of planar or dendritic interface respectively.With the increase of R,the interface morphology becomes cellular from planar gradually,within a narrow range.And the greater the R,the,finer the dendrite.Segregation of element Cu and impurity elements Fe and Si are quite severe,the interface morphology markedly influences on solute segregation.During solidification at coarse dendrite interface, their segregation ratios are rather great and solidified structure is coarse.

Thermal Stress Analysis of Welded Joint in 1420 Al-Li Alloy Induced by Thermal Cycling

A model of double grains under plane stress state has been established. According to the double grain model, thermal stress induced by thermal cycling in welding fusion zone is numerically simulated by finite element method, and the microstructures before and after thermal cycling are observed. The effect of thermal stress on weld microstructure is discussed. Experimental and analysis results show that the difference between the coefficients of thermal expansion and elastic modulus for grains along different crystal directio n can produce alternate thermal misfit stress and strain near boundaries under thermal cycling. At the temperature of upper and lower limit, thermal stress nearby grain boundary reaches maxima. Thermal stress induced changes in microstructure, which expressed by the sending dislocations from boundaries to matrix, piling up against the boundaries and the increasing of dislocation density.

THE CORROSION RESISTANCE OF AN Al-Li ALLOY IN 3.5 wt.-%NaCl

The influences of heal treatment and pH value on corrosion potential and cyclic polarization curve or an Al-Li alloy in 3.5 wt.-%NaCl were investigated.Microstructures of the alloy were combined to analyze and explain the development of cyclic polarization curves and the variation of corrosion potential within 24 h.The results revealed that the preferential dissolution of the precipitates in the alloy increased the susceptibility of the alloy to localized corrosion.Prolonging artificial ageing treatment caused heavy precipitation of T1 phase in matrix and both precipitation of phases containing copper and widening of PFZ along the grain boundary,and thus worsened the corrosion resistance.

PRECIPITATION IN BINARY Al-Li ALLOYS

Studies were carried out on the precipitation in two sets of binary Al-Li alloys by means of hardness tests,DSC analyses and TEM observations.When the alloys were aged for constant time,a peak appeared on the plot of temperature dependence upon their hardness in the range of low temperatures,and the endothermic dissolution peak was quite distinct on the DSC curve also at low temperatures.These are believed that an Li-rich GP zone formed in the al- loys aged at low temperatures,but no more GP zone if aging at elevated temperatures.The hardness of the alloys raises rapidly with coarsening of δ′-phase grains.The two endothermic peaks of δ′ dissolution on the DSC curve relate to the structural adjustment at twice of the δ′ precipitation.The equilibrium δ-phase,being of higher thermal stability,nucleates and grows directly from the supersaturated solid solution with the aid of the gram boundary.

MICROSTRUCTURAL VARIATION AND FRACTURE MODES IN Al-Li ALLOYS

The correlation between the diverse fracture modes and the microstructural variation with recrystallization in the Al-Li alloys under heat treatment was observed by means of triplanar optical and scanning electron microscopy.The growth,propagation and feature of their frac- tures were described from the viewpoint of microstructure,energy and geometry.

MECHANICAL PROPERTIES OF Al-Li ALLOY 8090 AT CRYOGENIC TEMPERATURES

Cryogenic strength,ductility and toughness of AI-Li alloy 8090 were found to be superior to those at ambient temperature,and up to optima at about 20 K.Different aging regimes may only influence on its cryogenic yield strength and elongation.

EFFECTS OF IMPURITIES AND RARE EARTH ADDITION ON THE TOUGHENING LEVELS OF Al-Li BASED ALLOYS 2090 AND 1420

The effects of the impurities Fe, Si, Na, K and a rare earth addition, Ce, on the intrinsic and extrinsic toughening levels have been investigated for the Al-Li based alloys 2090 and 1420. 29K reduction in the toughening level for the alloy 2090 with impurities 0.42% Fe+Si or 0.0132% Na+K i4 identified to be caused by the impurities.An improvement on the fracture toughness can be made by adding 0.05%-0.25% Ce to the alloy 2090. The reason behind this is that Ce microalloying can not only enhance both the intrinsic toughening level and the extrinsic toughening level but also suppress the embrittling behavior of the impurities. However, 0.06%-0.15% Ce microalloying fails to bring about any beneficial effect to the toughening level for the alloy 1420.

IN-SITU OBSERVATION OF S'PHASE IN Al-Li BASE ALLOY USING TEM SERIAL ROTATION TECHNIQUE

An in situ observation of the s'phase morphology and its orientation with the matrix in an Al-Li base alloy was carried out by means of double-tilt rotating around[220]a in a transmission electron microscope(TEM).The results show that the s'phase precipitates in the form of bundles.The units of s'phase are lath-shaped,grow along the<100>,orientation,and have habit planes of{210}*.Many units of the s'phase grow in the same orientation and get together to form a plate-shaped bundle of s'phase laths which lie on the{110}a planes.

FATIGUE CRACK PROPAGATION IN Al-Li ALLOY 8090

Fatigue crack growth rates of Al-Li alloy 8090 in air were found to be strongly dependent up- on the aging conditions.The naturally aged and underaged specimens showed the highest re- sistance to the fatigue crack propagation.The fatigue crack growth resistance of the overaged specimen is the lowest and that of the peakaged specimen in between.As compared to air, 3.5% NaCl solution does not change the effect of aging conditions on the fatigue crack growth,but causes a decrease of the resistance to the fatigue crack growth under the same ag- ing condition.Both short cracks from notch and physically short cracks showed much higher rates of fatigue crack propagation in comparison with long cracks under the same aging condi- tion and stress intensity level.The growth behavior of the short crack depends on its type.The growth rate of short crack from notch decreases first to a minimum and then increases with in- creasing △K.However,the physically short crack grows at a progressively increasing rate. The effects of aging conditions are explained in terms of the slip planarity of dislocations and the cyclic slip reversibility.The observed short crack behavior is considered to be dependent on the crack closure and the local plasticity near the crack tip.