Commercial Al-Li Alloy Research Articles

New EoL Routes of Al-Li Aircraft Integral LBW and FSW Welded Panels including New Cr-Free Coatings

The end of fife (EoL) of new aircraft panels made of Al-Li alloys in which the stringers and skin were joined, either by laser beam welding (LBW) or by friction stir welding (FSW), was investigated at the lab scale. Different cutting strategies, ranging from cutting only for size reduction to full separation of all materials, including the removal of the welded seam, were defined, with the objective of recycling the maximum amount of panel scrap back into high-quality aircraft Al-Li alloys. Those welded aerostructures were coated with two novel Cr-free coating systems. The effect of the coatings on the recyclability of the panels and the need to eliminate the primer and topcoats were researched. Fading/enrichment of the alloying elements during recycling was determined. The chemical compatibility of the recycled alloys with four commercial Al-Li alloys was examined. The EoL route that maximized closed-loop recycling and the conservation of the valuable alloying elements was identified. Nine out of the ten configurations were found to be compatible with joint recycling. Only the LBW structure with ER4047 filler wire required sorting into scrap fractions and removing the weld seam. Decoating by corundum blasting followed by cutting before remelting is the recommended EoL process.

First-principles insights into complex interplays among nano-phases in an Al-Cu-Li-Zr alloy

Multiple nano-phases and nano-complexes feature the typical microstructures of current commercial Al-Li alloys. We present a combined first-principles and scanning transmission electron microscopy study of various nano-features in an aged Al-Cu-Li-Zr model alloy, in attempt to elucidate the fundamental aspects of their formation and interaction. Our results suggest that highly-diffusive excess Li can segregate to the early-precipitated β’(L12-Al3Zr) nano-phase, to develop a tentative δ’(L12-Al3Li) shell at the early-aging stage. The δ’ shell promotes GP-zone precipitation along the (100) facets but is metastable only, and can eventually dissolve into β’ with time. Consequently, GP-zones can evolve into θ” and θ’ independently under the prolonged aging. Excess Li also weakly segregates to θ’ interfaces to develop an “in-phase” or “anti-phase” δ’ shell. The gained insights help clarify various experimental observations, and their implications for Al-Li alloy design are also discussed.

Composite precipitates in a commercial Al–Li–Cu–Mg–Zr alloy

Composite precipitates with duplex and triplex association of single precipitates commonly found in commercial Al–Li alloys were observed by transmission electron microscopy (TEM) in a re-solution and quenched AA2090 alloy after aging. These novel precipitates were the duplex association of δ′(Al 3Li) and T 1 (Al 2CuLi) as well as the triplex δ′/β′(Al 3Zr)/T 1 and δ′/β′/θ′(Al 2Cu). The reduction of elastic energy associated with composite precipitate formation is suggested as a basic mechanism with plate-like T 1 and θ′ particles serving as preferential sites for nucleation of either δ′ or δ′/β′ spherical particles, which although in much smaller numbers, represent a volume fraction of 10 to 20% of all precipitates.

Microtexture and nanoindentation study of delamination cracking in Al-Cu-Li-X alloys

Commercial Al-Li alloys have strength and weight advantages over non-Al-Li alloys. The fracture behavior of these alloys is unusual and has limited their use. The fracture mode, described as delamination, is intergranular, along the broad grain boundaries parallel to the rolling plane of the plate. Microtexture analyses have shown that delaminations occur along boundaries with greater than 30o misorientation. However, it was observed that relatively few of the high angle boundaries exhibited this behavior. Some grains of the retained deformation texture show high internal misorientation, which is a measure of stored strain energy. Delamination tends to occur between these grains and adjacent, recrystallized grains. Nanoindentation studies indicate a higher hardness for the high internal misorientation grains. These results suggest that the delamination

Early and Late Stages of Demixing of a Commercial Al-Li Alloy

A detailed in situ SAXS investigation is presented on the phase separation process in a commercial aluminum-lithium alloy. Taking advantage of conventional and synchrotron X-ray sources, it has been possible to probe both slow and fast kinetics. Both early and late stages of demixing are probed in a range of different temperatures between 75 and 200°C. The corresponding SAXS data are interpreted in terms of the currently accepted theories. The large temperature range covered in these measurements allows obtaining information on activated processes occurring in these materials.

Thermal and thermomechanical effects on defect evolution in an Al–Li superplastic alloy

A commercial Al–Li alloy, in which the superplastic microstructure is developed by “strain-assisted continuous recrystallization” in early stages of flow, was studied using positron annihilation lifetime spectroscopy. Results revealed that exposing the material to a temperature of 525 °C (optimal temperature of superplastic deformation for this alloy) led to agglomeration of single vacancies into vacancy clusters of size of approximately four vacancies. Evidence for superplastic strain-induced cavitation was not found at 450 °C up to an elongation of 432% and at 525 °C up to 341%, the initial strain rate of deformation in both cases being 1.0 ×10−3 s−1. These results have practical significance.

Mixed mode fatigue crack growth in the commercial AlLi alloy, 8090

Crystallographic fatigue crack propagation along planar slip bands has been recognized to occur over a particularly wide range of conditions in Al-Li alloys due to the presence of coherent, shearable [delta][prime] (Al[sub 3]Li) precipitates, in conjunction with the strong underlying crystallographic texture often developed in these materials during secondary processing. Such Stage 1 propagation typically produces a highly tortuous crack path during conventional mode 1 fatigue testing. While the net crack growth direction remains parallel to the nominal mode 1 direction, crack tortuosity has been generally recognized as beneficial to fatigue resistance, as both local mixed mode crack-tip loading conditions and roughness induced closure may significantly reduce the driving force for crack extension (i.e. crack shielding'' occurs). Recent investigations of commercial Al-Li plate in the damage tolerant condition have however shown that co-planar Stage 1 crack propagation along preferred [111]-plane orientations may give rise to sustained macroscopic crack deviation from the nominal mode 1 growth direction. Such crack growth raises a number of important questions with regard to conventional, nominally mode 1 based structure lifting and fatigue testing methods when applied to these materials. Work has therefore been carried out to characterized fatigue crack growth during sustained co-planar Stage 1more » propagation in a commercial Al-Li alloy, with particular reference to mixed mode loading conditions.« less

Dynamic strain ageing and crack propagation in the 2091 AlLi alloy

Dynamic strain aging (DSA) phenomena, which stem from the dynamic interaction of mobile dislocations and diffusing solute atoms, are observed in many dilute alloys at intermediate temperatures. In uniaxial deformation, DSA may lead to a negative strain rate sensitivity (SRS) of the flow stress and to the occurrence of jerky flow, alias the Portevin-Le Chaetelier (PLC) effect. As little is known to date about the influence of DSA and the PLC effect on fracture-related properties, the aim of the present study is to investigate ductile fracture in a commercial Al-Li alloy, the 2091 alloy. In the standard treatment conditions used all through this work, this alloy exhibits serrated yielding in uniaxial deformation, associated with a depletion of the SRS. The present study is, therefore, focused on properties related to one single heat treatment, for which fracture is purely intragranular. Experimental results are reported and discussed on the influence of strain rate and temperature on both uniaxial deformation and the tearing behavior of the 2091 alloy. Evidence is found for an anomalous behavior of the tearing resistance in the temperature domain where DSA occurs. Further, heterogeneous deformation is shown to take place in such conditions in the plastic zone ahead ofmore » the crack tip region.« less

Effect of heat treatment environment on Li depletion and on mechanical properties in Al-Li alloy sheets

The loss of lithium during solution heat treatment of various commercial Al-Li alloy sheets as a function of the heat treatment environment is investigated. The thickness of the soft layer formed due to solutionizing at 530‡C for 30 min in laboratory air, argon and salt bath environments is determined by microhardness measurements. Li loss from the surfaces is interpreted with the help of normalized microhardness profiles. Results showed that prior Li-depleted layer does not hinder further loss of Li from the surfaces and the heat treatment environment has relatively little effect on the loss of Li. The loss of tensile strength due to the Li depletion in the peak-aged condition is found to be more pronounced in 1 mm thick sheet than in 1.5 mm or 2 mm thick sheets. Possible reasons for loss of strength in 8090 series alloys after the heat treatment are discussed. Explanations for the observed ductile fracture features in Li-depleted layers is offered.

Long-time annealing at ambient temperature of a mildly treated commercial AI-Li alloy

Abstract Mildly treated samples of a commercial Al-Li alloy have been studied, after ageing at ambient temperature for different times over a period of nearly a year, by small-angle X-ray scattering. It was found that the structure of the alloy changed over time and that precipitate particles increased in dimension. A two-step particle growth rate has been observed and attributed to two different mechanisms. First, nucleation events are taking place whereas the second slower step is attributable to coarsening. During the latter stage the Lifshitz-Slyozov law of R 3 against t was followed.

Ａｌ‐Ｌｉ合金の鋳造割れ性

Effects of Li content and grain-refiner addition on the hot-tearing characteristics of Al-Li binary word alloys (0-3mass%) and the commercial Al-Li alloys (2090, 2091, 8090 and 2219+1.5mass%Li) were investigated by using hindered type hot-tearing test (I-beam test). High strength alloys, 2024 and 7075 were also investigated to compare with the commercial Al-Li alloys. In Al-Li binary alloys, the tearing tendency was decreased with increasing Li content, but was not improved by the grain-refiner addition, and the efficiency of grain refinement was less than in the pure aluminum. In the commercial Al-Li alloys, the tearing tendency was same as 2024 and 7075 alloys in the case of no additioning grain-refiner, but was not so improved by the grain-refiner addition, while in 2024 and 7075 alloys remarkably improved, and the efficiency of grain refinement was less than in 2024 and 7075 alloys.

ＤＣ鋳造におけるＡｌ‐Ｌｉ合金の鋳造割れ

The prevention of hot-tearing in the commercial Al-Li alloys ingots (150mm×350mm) produced by DC casting was studied. Crack started at the caved part of ingot surface which was contaminated by oxide. The molten metal surface was ventilated by Ar gas to inhibit the generation and contamination of oxide. The stress by solidification shrinkage was reduced by decreasing casting speed, because the faster casting speed caused larger shrinkage. But the pouring temperature and coolant flow rate had no influence. No crack was detected in 2024 alloy. Shrinkage of Al-Li alloys was larger than 2024 alloy, and that was the one of the reasons why Al-Li alloy easily cracked. The faster casting speed causes the deeper sump.

Analysis of small-angle scattering patterns from a commercial Al-Li alloy by means of a model incorporating a repulsive step potential

Abstract Small-angle X-ray scattering measurements are reported for a commercial Al-8·49%Li-51% Cu (atomic composition) alloy solution treated at 520°C and thermally aged for several times at several temperatures. Data have been analysed by means of a model of ellipsoidal precipitate particles previously proposed by some of us and by a modification of this model where, in the interparticle interference term, allowance is made for interactions between the precipitate particles at longer range than previously. This was achieved by the introduction, in addition to the hard-sphere interaction potential, of a potential step. Our fits indicate that the precipitate particles interact through a repulsive potential of the order of 0.6k B T. Particle dimensions obtained from the fit indicate that the Lifshitz-Slyozov coarsening mechanism holds. However the activation energy for volume diffusivity obtained for the present alloy is 15% smaller than those previously reported for other commercial alloys and for pure bi...

Kinetics of particle growth and particle composition of a commercial Al-Li alloy by means of small-angle scattering

Abstract Extending previous work done on a similar alloy, small-angle X-ray scattering measurements are reported for a commercial Al-Cu-Li alloy (2–68% Cu and 1–88% Li by weight). Data have been analysed by means of an ellipsoidal model of precipitate particles using a method previously proposed by some of us which, incorporating interparticle interference effects, reproduces the fuli scattering intensities of the samples studied. Particie dimensions obtained from the fit indicate that the Lifshitz-Slyozov coarsening mechanism holds for ageing at higher temperatures and longer times. The activation energy for volume diffusivity obtained is similar to that of pure binary alloys. The scattering-curve fitting parameters include the volume fractions of the δ-phase which decrease as expected with increasing temperature. The scattering contrast between δ and the matrix is observed to increase with temperature. On the basis of these observations, it would appear that the lithium concentrations in both the matrix...

Small-angle scattering from a commercial Al-Li alloy

Abstract Small-angle X-ray scattering measurements are reported for a commercial A1-2 wt% Li alloy solution treated at 520°C and thermally treated for several times at several temperatures. Data have been analysed by means of a method previously reported which, as it incorporates interparticle interference effects, reproduces the scattering intensities of the samples studied. During the thermal treatment the precipitate volume fraction remains constant, while the number density decreases with increasing time at the various temperatures. The scattering observations from the earliest time treatments are consistent with a precipitate-coarsening mechanism in which phase separation is complete and the average particle volume grows linearly with time. The precipitate and matrix concentrations are determined from the analysis of the scattering curves and indicate considerable non-stoichiometry in the precipitate. The interparticle spacing and particle size are scaled in a fixed ratio to one another in the course...

Mechanical properties of Al–Li alloys Part 1 Fracture toughness and microstructure

Abstract Mechanisms influencing the ambient temperature mechanical properties of commercial Al–Li alloys 2090, 8090, 8091, and 2091 are examined as a function of plate orientation, with specific emphasis on the role of microstructure. In Part 1, results on the uniaxial tensile and plane strain fracture toughness properties are presented and the behaviour is discussed in terms of the role of the matrix and grain boundary precipitates, associated precipitate free zones (PFZs), and the occurrence of short-transverse delamination. It is seen that in general peak aged microstructures show an excellent combination of strength and toughness (L–T, T–L), equal to or exceeding that shown by traditional 2000 and 7000 series high strength aluminium alloys. The superior toughness of peak aged compared with naturally aged microstructures seems to be associated with widespread matrix precipitation of platelike precipitates (T1 in Al–Li–Cu alloys and S in Al–Li–Cu–Mg alloys), β′-dispersoids and second phase particles whi...

Mechanical properties of Al–Li alloys Part 2 Fatigue crack propagation

AbstractMechanisms influencing the ambient temperature mechanical properties of commercial Al–Li alloys 2090, 2091, 8090, and 8091 are examined, with specific emphasis on the role of microstructure. In Part 2, results on fatigue crack propagation behaviour are presented for both ‘long’ (≥ 5 mm) and ‘microstructurally small’ (~1–1000 μm) cracks and compared with behaviour in traditional high strength aluminium alloys. In general, it is found that the growth rates of long fatigue cracks in Al–Li alloys are up to two to three orders of magnitude lower than in traditional 7000 and 2000 series alloys, when compared at an equivalent stress intensity range ∆K. By contrast, corresponding growth rates of microstructurally small fatigue cracks were up to two to three orders of magnitude higher than the long crack results. Such observations are attributed to the prominent role of crack tip shielding in Al–Li alloys resulting from the tortuous and deflected nature of the crack paths which results in a reduced crack t...

The influence of lithium on the ageing of A 7000 series alloy

The present commercial Al-Li alloys are based on the Al-Cu or Al-Cu-Mg alloy systems. In this paper, the influence of lithium on Al-Zn-Mg-Cu alloys is considered. It is shown that lithium inhibits the formation of GP zones and meastable MgZn2, which results in a decrease in the natural ageing behaviour. As the lithium concentration increases, coarse MgZn2 is formed at lithium concentrations less than 1 wt%, which decreases the extent of artificial age hardening, as well as natural ageing. At lithium concentrations above 2 wt%, ordered Al3Li is formed, but instead of MgZn2 precipitating, a coarse T phase is produced. As a result dual phase precipitation hardening is not obtained in lithium containing 7000 series alloys strengthened by MgZn2. Strengthening can be obtained if the Zn, Mg and Cu concentrations give S phase strengthening, rather than MgZn2.