Lithium-containing Aluminium Alloys Research Articles

Chromium-Free Conversion Protective Coatings on 1424 Aluminum Alloy

Abstract—It is known that lithium-containing aluminum alloys possess low pitting corrosion resistance. It has been previously shown that an IFKhANAL-3 alkaline chromate-free conversion composition gives the coatings on a series of aluminum alloys, such as AMg3, D-16, and V95, which are comparable to conventional chromate coatings in terms of protective properties. It should also be noted that inhomogeneous CCs that do not adhere to the substrate are formed in standard chromate conversion composition on lithium-containing alloys; therefore, development of the method for the fabrication of protective CC for 1424 alloy is a relevant task. In this work, the effect of various complexing agents on the oxidation and protective properties of the coatings based on 1424 alloy have been studied. The modifying additives under study have decreased the concentration of different oxides in the conversion compositions providing adsorption of corrosion inhibitors and increase in their protective properties. According to the results of accelerated corrosion tests, integrated use of Trilon B and BTA significantly increases the corrosion resistance of the coatings based on 1424 alloy.

Study of the influence of a corrosive medium on the process of damage accumulation and the nature of failure of structural aluminum alloys 1441 and V-1469 under tensile and low-cycle fatigue testing

The kinetics of damage accumulation in the surface layers of lithium-containing structural aluminum alloys 1441 and V-1469 under tensile and fatigue tests under a rigid cycle before and after the impact of corrosive media (a salt mist test chamber and a moderately warm climate of a coastal atmosphere) was studied. The damage parameters of structural aluminum–lithium alloys and the change in the nature of their failure and in the surface relief with consideration of the combined influence of loading and a corrosive medium were evaluated.

Kinetics of Accumulation of Damage in Surface Layers of Lithium-Containing Aluminum Alloys in Fatigue Tests with Rigid Loading Cycle and Corrosive Effect of Environment

High-resolution methods of metal physics research including electron, laser and optical microscopy are used to study the kinetics of the accumulation of slip lines and bands and the corrosion damage in the plastic zone of specimens of aluminum-lithium alloys 1441 and B-1469 in rigid-cycle fatigue tests under the joint action of applied stresses and corrosive environment. The strain parameters (the density of slip bands, the sizes of plastic zones near fracture, the surface roughness in singled-out zones) and the damage parameters (the sizes of pits and the pitting area) are evaluated.

Al–Cu–Li and Al–Mg–Li alloys: Phase composition, texture, and anisotropy of mechanical properties (Review)

The results of studying the phase transformations, the texture formation, and the anisotropy of the mechanical properties in Al–Cu–Li and Al–Mg–Li alloys are generalized. A technique and equations are developed to calculate the amounts of the S1 (Al2MgLi), T1 (Al2CuLi), and δ' (Al3Li) phases. The fraction of the δ' phase in Al–Cu–Li alloys is shown to be significantly higher than in Al–Mg–Li alloys. Therefore, the role of the T1 phase in the hardening of Al–Cu–Li alloys is thought to be overestimated, especially in alloys with more than 1.5% Li. A new model is proposed to describe the hardening of Al–Cu–Li alloys upon aging, and the results obtained with this model agree well with the experimental data. A texture, which is analogous to that in aluminum alloys, is shown to form in sheets semiproducts made of Al–Cu–Li and Al–Mg–Li alloys. The more pronounced anisotropy of the properties of lithium-containing aluminum alloys is caused by a significant fraction of the ordered coherent δ' phase, the deformation mechanism in which differs radically from that in the solid solution.

Visual Detection of Machining Damage on Aerospace Aluminium Alloys during Manufacturing Process

In aerospace industry, chromic acid anodizing (CAA) has been traditionally used as a non-destructive testing (NDT) technique to detect flaws in aluminium alloys. However, with the increasing restriction on the use of chromic acid and the application of lithium-containing aluminium alloys to aircraft structures, the capability of anodizing as a NDT method is challenged. In this work, machining damage was deliberately introduced to an Al-Li-Cu alloy AA2099-T8. Then, the visibility of the machine damage after tartaric-sulphuric acid anodizing (TSAA), which is an environmentally friendly anodizing process, was studied. It is suggested that, with proper lighting condition, it is possible to replace CAA with TSAA for detecting machining damage on lithium-containing aluminium alloys during manufacturing.

Properties of synthesised oxide-ceramic coatings in electrolyte plasma on aluminium alloys

The effect of alloying of aluminium alloys by copper, magnesium, silicon, zinc and lithium on the phase contents, thickness and microhardness of oxide-ceramic coatings was investigated. All of these coatings were obtained by high-temperature synthesis in plasma of spark discharges in the metal-electrolyte system at an electric voltage of 10 7 V m −H. Coatings consist of α-, γ- and δ- phases of Al 2O 3. In the case of alloying by silicon, Al 2SiO 5 is formed as well. The thickness of the coating reaches a maximum of 500 μm. The coatings of the thickness of 150–220 um and the microhardness of 15–36 GPa (depending on the alloying type and consist electrolyte content) were received when the cathodic and anodic current densities were related as 1.5 during 60 min or 0.9 during 120 min. The coatings obtained on the lithium-containing aluminium alloys have the highest microhardness. The total fraction of copper and magnesium must not be less than 3.5%. The oxide-ceramic coatings, synthesised in electrolyte plasma, are dielectric and have a high resistance to wear and corrosion.

Processing and Response of Aluminum-Lithium Alloy Composites Reinforced with Copper-Coated Silicon Carbide Particulates

Lithium-containing aluminum alloys have shown promise for demanding aerospace applications because of their light weight, high strength, and good damage tolerance characteristics. Additions of ceramic reinforcements to an aluminum-lithium alloy can significantly enhance specific strength, and specific modulus while concurrently offering acceptable performance at elevated temperatures. The processing and fabrication of aluminum-lithium alloy-based composites are hampered by particulate agglomeration or clustering and the existence of poor interfacial relationships between the reinforcing phase and the matrix. The problem of distribution of the reinforcing phase in the metal matrix can be alleviated by mechanical alloying. This article presents the results of a study aimed at addressing and improving the interfacial relationship between the host matrix and the reinforcing phase. Copper-coated silicon carbide particulates are introduced as the particulate reinforcing phase, and the resultant composite mixture is processed by conventional milling followed by hot pressing and hot extrusion. The influence of extrusion ratio and extrusion temperature on microstructure and mechanical properties was established. Post extrusion processing by hot isostatic pressing was also examined. Results reveal the increase in elastic modulus of the aluminum-lithium alloy matrix reinforced with copper-coated SiC to be significantly more than the mechanically alloyed Al-Li/SiC counterpart. This suggests the possible contributions of interfacial strengthening on mechanical response in direct comparison with a uniform distribution of the reinforcing ceramic particulates.

Attempt to predict hydrogen solubility limits in liquid multicomponent aluminum alloys

Using Wagner's model of interaction parameters derived from the data on hydrogen solubility in liquid binary aluminum alloys, an attempt has been made to predict hydrogen solubilities in multicomponent aluminum alloys. Agreement between the predicted and experimentally determined hydrogen solubilities in all of the multicomponent alloys investigated is poor. In the non-lithium containing alloys the poor agreement may be due to the fact that, unlike nitrogen and hydrogen solubilities in ferrous alloys, the hydrogen solubility limits in aluminum alloys are too low to be reliably predicted by Wagner's model of interaction parameter. In the case of the lithium-containing aluminum alloys, combined addition of Li and Cu to liquid aluminum gives rise to a synergistic increase in hydrogen solubility in the multicomponent Al-H-Li-Cu alloy. This is provisionally attributed to the probable interaction effects between Li and Cu which appears to increase lithium affinity to hydrogen. This suggests that, contrary to the assumptions implicit in Wagner's model, the contributions of Li and Cu to the total solubility of hydrogen in aluminum are not independent or free from interactions between the alloying elements. Thus, a correct analytical expression for hydrogen solubility for Al-H-Li-Cu probably requires the solute-solute interaction coefficients. However, it is clear from this study that there is a need for reliable experimental measurement of hydrogen solubility in both binary and multicomponent aluminum alloys.

Recent Developments in Light Alloys

Light alloys, and aluminium alloys in particular, are widely used to model new processes and their association with the aerospace industries often places them at the forefront of emerging technologies. Moreover, knowledge of microstructure/property relationships is providing opportunities to design alloys to meet specific service requirements. These two themes are used as the basis for discussing recent developments in aluminium, magnesium and titanium alloys. Particular attention is given to new ageing practices, lithium-containing aluminium alloys, the role of trace elements in controlling precipitation and to titanium aluminides. Reference is also made to several new materials produced by unconventional methods of processing.

Electron-beam weld microstructures and properties of aluminium-lithium alloy 8090

Lithium-containing aluminium alloys are of considerable current interest in the aerospace and aircraft industries because lithium additions to aluminium improve the modulus and decrease the density compared to conventional aluminium alloys. Few commercial aluminium-lithium alloys have emerged for use in the aerospace industry. One such candidate is 8090, a precipitation-hardenable Al-Li-Cu-Mg alloy. The influence of electron-beam welding on the microstructure and mechanical properties of alloy 8090 material has been evaluated through microscopical observations and mechanical tests. Microscopic observations of the electronbeam welds revealed an absence of microporosity and hot cracking, but revealed presence of microporosity in the transverse section of the weld. Mechanical tests revealed the electronbeam weld to have lower strength, elongation and joint efficiency. A change in microscopic fracture mode was observed for the welded material when compared to the unwelded counterpart. An attempt is made to rationalize the behaviour in terms of competing mechanistic effects involving the grain structure of the material, the role of matrix deformation characteristics, grain-boundary chemistry and grain-boundary failure.

Effect of carbonate and lithium ions on the corrosion performance of pure aluminium

Abstract The effect of the presence of lithium ions in carbonate solutions on the corrosion behaviour of aluminium was studied taking into consideration the particular importance of the use of lithium-containing aluminium alloys in aeronautics. dc electrochemical polarization measurements were performed on pure aluminium (99.999%) in mixed solutions of Na 2 CO 3 and Li 2 CO 3 , the results showing that the presence of Li + in solution improves the passivation of aluminium, but only above a critical concentration of carbonates. The presence of chlorides in those solutions above a certain level is deleterious for the passivation films, since it leads to localized attack. This process seems to be related to the easy formation of lithium choloride containing species, which promote local film dissolution.

The presence and consequences of precipitatefree zones in an aluminium-copper-lithium alloy

The addition of lithium to aluminium alloys has the potential for providing a class of high-strength alloys with exceptional properties suitable for aerospace applications. Potential candidates are precipitation hardenable and belong to the Al-Li-Cu family. The intrinsic microstructural features have a pronounced influence on the mechanical response of these alloys. In this work, the mechanisms responsible for the formation of precipitate-free zones along grain boundaries in precipitation-strengthened lithium-containing aluminium alloys were examined. The influence of grain morphology and the nature and type of precipitate coverage at the grain boundary in controlling the formation of these zones was analysed. The presence and influence of these zones along the grain boundaries on mechanical properties was studied for an Al-4.5Cu-1.21 Li alloy. It was found that while strength is comparable with existing high-strength alloys, the ductility decreases due to the presence of precipitate-free zones. The degradation in ductility is attributed to the particular mode of plastic deformation of this alloy, and to the restriction of plastic deformation in narrow planar zones along the grain boundaries. Fracture occurs when a critical local strain is reached in these zones. The overall consequences of precipitate-free zones along grain boundaries on mechanical properties are discussed in the light of competing effects involving the nature of matrix-strengthening precipitates, grain-boundary particles and deformation characteristics.

Violent oxidation of lithium-containing aluminum alloys in liquid oxygen

A strong exothermic and quite well known thermite reaction involving aluminum, oxygen and transition metals (Fe, Cr, Ni, etc.) has apparently been initiated during impact testing of Alcoa aluminum alloy #2090 in liquid oxygen at NASA-MSFC. In some instances, this reaction, essentially an oxidation process, has been so intense that the Inconel 718 cup containing the aluminum alloy disk and associated impacter has melted raising certain safety concerns in the use of this alloy. Reaction products as well as the test specimen surfaces have been studied with surface science techniques like XPS/ESCA, SIMS and AES. Typically, in order to initiate the thermite reaction a temperature of approximately 1000°C is necessary. The mechanism responsible for this oxidation is of great interest. The analysis of the reaction products together with a theoretical analysis, including digital modeling has been pursued. There is strong evidence that the large relaxation energy of the aluminum oxide coating, formed during the aluminum alloy cleaning process, is causing a highly localized energy release during fracture or lattice deformation which is enhancing the oxidation process to a runaway condition. The presence of alkali atoms (Li) enhances the likelihood and intensity of the oxidation reaction. The details of the surface studies will be discussed.

Weldability of aluminium-lithium alloy 2090 using laser welding

Lithium-containing aluminium alloys are of considerable current interest in the aerospace and aircraft industries because lithium additions to aluminium improve the modulus and decrease the density compared to conventional aluminium alloys. Many such alloys are under development for aircraft applications, which usually involves mechanical fastening. While aluminium-lithium alloys are fusion weldable with gas metal arc, gas tungsten arc and electron beam processes, they suffer from problems of weld porosity, heat-tearing cracking, poor penetration and low joint efficiency. In this paper, the weldability of aluminium-lithium alloys is briefly reviewed. The weldability of commercial aluminium-lithium alloy 2090 in the peak-aged condition was studied using laser welding. The quality of the welds was evaluated through mechanical tests (hardness and tensile tests) and microscopical observations. Mechanical property data and microscopical observations of the welds on prior surface-prepared (milled) material revealed a low degree of the weld surface degradation and an absence of porosity. This coupled with the attractive joint efficiencies suggest the superiority of the laser welding to conventional arc welding of this alloy. The performance of laser-welded butt joints is rationalized.

Recent developments in the weldability of lithium-containing aluminium alloys

The weldability of aluminium-lithium alloys is reviewed with emphasis on alloys that are being commercialized: 2090, 8090 and Weldalite™ 049. These alloys are weldable, although hotcracking susceptibility has been reported for 2090 and 8090 when welded with certain conventional filler alloys. Mechanical property data from weldments made by several techniques are emphasized. Weld-zone porosity problems, discussed at length in a 1985 review in this journal, have largely been controlled by proper weld pretreatment and gas shielding. Interest in the weldability of aluminium-lithium alloys has taken on increased significance because of their potential to reduce the weight of space launch systems, whose fabrication most often involves welding.

Micromechanisms governing fatigue behaviour of lithium-containing aluminium alloys: Srivatsan, T. S. and Coyne, Jr, E. J. Mater. Sci. Technol. June 1989 5, (6), 548–555

The production of austempered ductile iron (ADI) with uniform microstructure and properties is constrained by the austempering process vis-à-vis the quenching medium. This is as a result of the stringent operating parameters with costly facilities. This limitation has restricted the application of ADI, despite its inherent mechanical and chemical properties. An emerging technology channelled towardsovercoming this limitation is by austempering with forced air cooling equipment, which is accessible, available and cost-efficient. This work characterizes the behaviour of the forced air cool ADI in simulated mine water due to the strategic importance of the mining industry in the global economy. The study establishes the influence of sample section thickness (5, 15, and 20 mm) oncorrosion performance. Electrochemical experiments were performed on the forced air cooledADI at atmospheric pressure and room temperature with methods such as open circuit potential (OCP) and anodic polarisation. The post-corrosioncharacterizations were performed using field emission scanning electron microscopy (FESEM) equipped with energy dispersive spectroscopy (EDS). The research highlighted that small section thickness has a more favourable performance compared with larger section. Consideration is also accorded to the capability of the ADI in the studied environment.

The effect of grain-refining additions to lithium-containing aluminium alloys

Influence des additions de zirconium, manganese et chrome sur la microstructure des alliages Al-Li. Determination des caracteristiques mecaniques, de corrosion et des modes de rupture des alliages apres addition des elements. Interpretation des resultats basee sur la microstructure

Mechanisms of fatigue crack propagation in metals, ceramics and composites: Role of crack tip shielding

Crack tip shielding phenomena, whereby the “effective crack-driving force” actually experienced at the crack tip is locally reduced, are examined with reference to fatigue crack propagation behavior in metals, composites and ceramics. Sources of shielding are briefly described in terms of mechanisms relying on the production of elastically constrained zones which envelop the crack (zone shielding), on the generation of wedging, bridging or sliding forces between the crack surfaces (contact shielding) and on crack path deflection and meandering. Examples are taken from the fatigue behavior of high strength lithium-containing aluminum alloys, aluminum alloy-aramid fiber-epoxy laminate composites, and zirconia ceramics. It is shown that, whereas crack tip shielding can provide a potent means of enhancing “resistance” to crack growth, such extrinsic toughening mechanisms can result in the apparently anomalous behavior of “small cracks” and to the susceptibility of brittle materials to fatigue failure.

Corrosion resistance of Al–Li alloys

Experiments designed to investigate the corrosion resistance of several lithium-containing aluminium alloys are described. Intergranular corrosion was investigated using the NAWL test, exfoliation corrosion using the EXCO test, and stress corrosion using the C-ring test. It was found that intergranular corrosion varied with extrusion parameters and was more severe in copper-containing alloys. Alloys containing less than 0·4% Cu were not susceptible to stress corrosion. In alloys that failed, susceptibility increased as the aging treatment was extended from the underaged to the peak aged temper and also with increasing copper content. The addition of copper to ternary Al–Mg–Li alloys also increased the exfoliation corrosion attack.MST/494

Corrosion resistance of Al–Li alloys

Experiments designed to investigate the corrosion resistance of several lithium-containing aluminium alloys are described. Intergranular corrosion was investigated using the NAWL test, exfoliation ...