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Status and prospects for aluminium-lithium alloys in aircraft structures

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Abstract

This paper examines the status and prospects for widespread application of AlLi alloys in aircraft structures. Particular attention is paid to the damage tolerance engineering properties of fatigue crack growth and fracture. At present the application of AlLi alloys is very limited. This is because they are not direct replacements for conventional alloys and the basic material costs are usually too high. Widespread application of AlLi alloys may well require a new generation of alloys, especially for primary structures. This will take at least five years. In the meantime, experience will be gained by selective application of the current generation of AlLi alloys in secondary structures.

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... Moreover, Li enhances the elastic modulus of Al, nearly 6% per unit weight % of Li addition (Sankaran & Grant 1980; Peel et al 1984; Westwood 1990). These apart, Al–Li alloys have been found to exhibit superior mechanical properties as compared to the conventional Al alloys in terms of higher specific strength, enhanced resistance to high cycle fatigue, fatigue crack growth and monotonic as well as cyclic fracture at cryogenic temperatures (Welpmann et al 1984; Jata & Starke 1986; Peel 1989; Quist & Narayanan 1989; Starke & Quist 1989; Lavernia et al 1990; Venkateswara Rao & Ritchie 1992; Wanhill 1994). Further, Al–Li alloys are 210 ...
... technologically more attractive as compared to the newer structural materials such as Ti alloys and composites, which are prohibitively expensive (Westwood 1990; Wanhill 1994). However, these advantages of Al–Li alloys have not still made them a viable alternative due to their low ductility (Lin et al 1982; Sanders & Starke 1982, 1989 Gregson & Flower 1985; Vasudevan & Doherty 1987; Webster 1987), inferior low cycle fatigue resistance (Sanders & Starke 1982; Venkateswara Rao & Ritchie 1992; Eswara Prasad et al 1996, 1997 Eswara Prasad & Rama Rao 2000) and inadequate fracture toughness not only in the in-plane but also, more significantly, in the through-thickness directions (Jata & Starke 1986; Gregson & Flower 1985; Suresh et al 1987; Venkateswara Rao & Ritchie 1989; Lynch 1991; Eswara Prasad et al 1993, 1994). ...
... technologically more attractive as compared to the newer structural materials such as Ti alloys and composites, which are prohibitively expensive (Westwood 1990; Wanhill 1994). However, these advantages of Al–Li alloys have not still made them a viable alternative due to their low ductility (Lin et al 1982; Sanders & Starke 1982, 1989 Gregson & Flower 1985; Vasudevan & Doherty 1987; Webster 1987), inferior low cycle fatigue resistance (Sanders & Starke 1982; Venkateswara Rao & Ritchie 1992; Eswara Prasad et al 1996, 1997 Eswara Prasad & Rama Rao 2000) and inadequate fracture toughness not only in the in-plane but also, more significantly, in the through-thickness directions (Jata & Starke 1986; Gregson & Flower 1985; Suresh et al 1987; Venkateswara Rao & Ritchie 1989; Lynch 1991; Eswara Prasad et al 1993, 1994). These alloys also suffer from a high degree of crystallographic texture and grain fibering, and consequently anisotropy in the mechanical properties (Peel et al 1988; Starke & Quist 1989; Eswara Prasad 1993; Eswara Prasad et al 1993; Jata et al 1998). ...
Article
Aluminium-lithium alloys hold promise of providing a breakthrough response to the crying need for lightweight alloys for use as structurals in aerospace applications. Considerable worldwide research has gone into developing a range of these alloys over the last three decades. As a result, substantial understanding has been developed of the microstructure-based micromechanisms of strengthening, of fatigue and fracture as well as of anisotropy in mechanical properties. However, these alloys have not yet greatly displaced the conventionally used denser Al alloys on account of their poorer ductility, fracture toughness and low cycle fatigue resistance. This review aims to summarise the work pertaining to study of structure and mechanical properties with a view to indicate the directions that have been and can be pursued to overcome property limitations.
... ightweight alloys such as aluminum (Al) alloys are crucial to the transportation sector, especially the aerospace industry. Aluminum alloys have a long history in the industry because of their low weight and considerable strength [1][2][3][4]. Therefore, the history of aircraft material cannot be told without aluminum occupying a prime position. ...
... The first major deployment of the alloy in aircraft components was in the 1950s in the form of Al-Li 2020, even though its development started in 1920. Later generations of the alloy which have found application in aircraft components include Al-Li 2090, 2091, 8090 and 8091 [1][2][3][4]. The high elastic modulus of Al-Li stands it out compared to traditional aluminum alloys. ...
... The simultaneous addition of copper and lithium improves the strength of the alloy [6]. However, challenges such as prohibitive cost, poor fatigue performance, low fracture toughness, poor corrosion resistance and high anisotropic behavior constrained the wide deployment of Al-Li in the aircraft industry [1,2,7,8]. Perhaps, poor fatigue performance is the larger reason that sundry works on different generations of the alloys were focused on fatigue crack behavior [9][10][11][12][13][14][15]. While there is growing research interest in the tensile behavior of the Al-Li, fatigue studies of the alloy are scarce in the open literature. ...
Article
Full-text available
Microstructural characteristics, monotonic and strain-controlled cyclic axial behaviors of AW2099-T83 Aluminum-Lithium alloy were investigated. Grain sizes and structures are not uniform in the different orientations studied. High strength and low ductility characterize the tensile behavior of the alloy under static loading. Strain-controlled fatigue testing was conducted at strain amplitudes ranging from 0.3% to 0.7%. Over this range, macro plastic deformation was only observed at 0.7%. Cyclic stress evolution was found to be dependent on both the applied strain amplitude and the number of cycles. Limited strain hardening was observed at low number of cycles, followed by softening, due probably to damage initiation. With low plastic strain, analytical approach was adopted to profile the damaging mechanism for the different applied strain amplitude. Because of the absence of fatigue ductility parameters due to low plasticity, a three-parameter equation was used to correlate fatigue life. Fractured specimens were studied under SEM to characterize the fracture surface and determine the controlling fracture mechanisms. The fractography analysis revealed that fracture at low strain amplitudes was shear controlled while multiple secondary cracks were observed at high strain amplitude. Intergranular failure was found to be the dominant crack propagation mode.
... The aerospace industry is specially known to depend heavily on them for airplane bodies and components. The presence of aluminum alloys in the aerospace industry is nearly as old as the industry itself [1][2][3][4]. Aluminum-lithium (Al-Li) alloy is particularly sought after in the aerospace industry due to its high strength-to-weight ratio. An addition of 1% lithium to aluminum decreases the weight by 3% while increasing the modulus of elasticity by 6% [3,5]. ...
... The development of the first generation of the alloy started in 1920, but it was not until the 1950s that it was first put into use in the form of Al-Li 2020. The development of the second generation which includes Al-Li 2020, 2090, 2091, 8090 and 8091 alloys spanned over three decades [1][2][3][4]. Up to the mid 90s when the second generation is thought to have reached maturity, challenges such as prohibitive cost, poor fatigue performance, low fracture toughness, poor corrosion resistance and high anisotropic behavior were encountered. ...
... Up to the mid 90s when the second generation is thought to have reached maturity, challenges such as prohibitive cost, poor fatigue performance, low fracture toughness, poor corrosion resistance and high anisotropic behavior were encountered. These drawbacks limited the application of Al-Li alloy in the aircraft industry [1,2,6,7]. However, with further research and development, a third generation of the alloy with better corrosion resistance, fatigue crack growth performance, better mechanical strength and toughness has been introduced [2, 6,8]. ...
... 10% of the initial part) and is made of aluminum-magnesium-lithium alloy. This type of alloy is quite interesting for aeronautic applications due to the enhancement of the structure lightening as well as the strengthening of the alloy compared to classical aluminum alloys [2]. The forming of the part is performed at high temperature (350°C), which corresponds to the solution treatment temperature of the alloy. ...
Conference Paper
Creep forming of Al-Mg-Li alloy sheets is studied. An instrumented bulging machine is used to form a double curvature panel at a reduced scale. The deformation of the work-sheet is ensured by a 7475 aluminum alloy lost sheet deformed by a gas pressure applied on its upper surface. A numerical model using the ABAQUS software is developed in order to obtain the pressure law and to ensure the forming conditions during the cycle. This model is validated by comparing experiments and numerical results in terms of deformed shape and thickness evolution.
... similar to their conventional counterparts, whereas, medium-and high- strength, second generation alloys Al-Li alloys are known to have lower fracture toughness values and R-curve characteristics than their conventional counter- parts. It should be noted, however, that the fracture toughness of third gener- ation Al-Li alloys has been greatly improved, making them attractive for various aerospace applications [7,25]. One such example is AA2043, which exhibits high strength similar to many other legacy 2.63 g/cc (0.095 lbs/in. ...
... From this viewpoint, lithium alloyed aluminum alloys are of interest. Compared with D16, V95, and AMg6 alloys, which are widely used in aerospace hardware, they have density lowered by 7-13% at higher elasticity modulus, ultimate strength, and yield point at room and elevated temper atures (up to 150°C) and low cycle fatigue resistance and corrosion strength [1][2][3][4]. ...
Article
Full-text available
The results of studying the influence of fabrication modes of 1.5-, 2.0-, and 3.0-mm-thick sheets made of V-1461 Al–Li alloy on the microstructure, crystallographic orientation, and anisotropy of properties are presented. It is established that the deformed structure is characteristic of all studied samples, and 3.0-mm-thick sheets have a partially recrystallized structure, 2.0-mm-thick ones have unrecrystallized structure, and 1.5-mm-thick ones have a mainly recrystallized structure. The preferential crystallographic orientation of the 1.5-mm-thick samples is [110](200), that of 2.0-mm-thick samples is [110](110), and that of 3.0-mm-thick samples is [210](110). All the sheets possess insignificant anisotropy of properties irrespective of thickness, and extremely low anisotropy index (μ < 0.4) is characteristic of them. These facts determine the liability of aluminum–lithium alloys to the preferential development of deformation over the sheet thickness, which leads to its premature thinning and lowers formability in the course of drawing and stretching.
... Qualification of an alloy, or alloys, for aerospace structures involves many considerations relating to the engineering properties and also the manufacturing of components and complete structures. As an example, table 15.6 outlines a service qualification programme for an Al-Li sheet/plate alloy (Wanhill 1994) with some updates (Babel and Parrish 2004;Pacchione and Telgkamp 2006;Lequeu et al. 2010;Rioja and Liu 2012). Another example of a qualification programme is included in Chapter 16, which is a review of the procedures for certifying metallic materials to be used in aerospace structures. ...
... Their favourable density, strength, toughness, fatigue behaviour and thermal stability make them attractive candidates in applications requiring both high specific strength and excellent damage tolerance [3][4][5]. Additionally, unlike most conventional aerospace alloys, Al-Cu-Li alloys are fusion weldable, which opens up new opportunities in fuselage construction [6]. ...
Article
Surface modification by excimer laser surface melting (LSM) has been performed with the aim to improve the corrosion resistance of the AA2050-T8 alloy. LSM produced melted surfaces, largely free of precipitates, with both microstructure and corrosion behaviour depending upon the number of laser pulses employed. Increased number of laser pulses resulted in thicker melted layers, but also in greater trapped porosity and formation of micro-cracks at the overlapping area. Nevertheless, the LSM-treated specimens exhibited enhanced corrosion resistance compared to the untreated alloy, which was associated with the formation of a relatively uniform melted layer and a diminished presence of precipitates.
... Qualification of an alloy, or alloys, for aerospace structures involves many considerations relating to the engineering properties and also the manufacturing of components and complete structures. As an example, table 15.6 outlines a service qualification programme for an Al-Li sheet/plate alloy (Wanhill 1994) with some updates (Babel and Parrish 2004;Pacchione and Telgkamp 2006;Lequeu et al. 2010;Rioja and Liu 2012). Another example of a qualification programme is included in Chapter 16, which is a review of the procedures for certifying metallic materials to be used in aerospace structures. ...
Chapter
Full-text available
This chapter starts with a brief overview of the historical development of aerospace aluminium alloys. This is followed by a listing of a range of current alloys with a description of the alloy classification system and the wide range of tempers in which Al alloys are used. A description is given of the alloying and precipitation hardening behaviour, which is the principal strengthening mechanism for Al alloys. A survey of the mechanical properties, fatigue behaviour and corrosion resistance of Al alloys is followed by a listing of some of the typical aerospace applications of Al alloys. The Indian scenario with respect to production of primary aluminium and some aerospace alloys, and the Type Certification process of Al alloys for aerospace applications are described. Finally there is a critical review of some of the gaps in existing aerospace Al alloy technologies.
... Under constant amplitude and spectrum fatigue loading, the fatigue crack growth rate (da/dN) in the third generation of Al/Li alloys was reported to be lower when compared to traditional aluminum alloys [5,[25][26][27][28]. On the other hand, the first and second generation Al/Li alloys exhibited no fatigue behavior benefits compared to the traditional AA2024-T3, showing that both alloys exhibited similar crack growth rates [29]; however, Al/Li alloys have the advantage of lower density and higher elastic stiffness. ...
Article
A comparative study on the fatigue crack growth behavior in a typical third generation Al/Li alloy (AA2198-T8) and traditional aerospace aluminum alloy (AA2024-T3) was conducted. Fatigue crack growth history was reconstructed using marker band loading sequences (i.e., applying two different R-ratios). Effect of alloying and loading direction (relative to the rolling direction) was discerned from the fatigue fracture surface morphologies, which were markedly different in the two alloys. The marker band imprints aided in identifying suspected fatigue crack initiation region and estimating crack growth rate. A data reduction scheme was used to estimate crack initiation cycle.
... This is because lithium creates an explosion hazard in the aluminium remelting phase; a consequence of 'chemically produced' inherent properties (Suomalainen et al., 2017). However, the extra technical value imparted by the aluminium-lithium alloys, such as low density, high elastic modulus, high strength and superior fatigue crack growth resistance, is currently an efficient way of reducing material weight and improving longevity, potentially outweighing the environmental cost of preventing recyclability (Wanhill, 1994). In the anaerobic digestion of agricultural wastes, feedstocks with a high degradability, such as cereal grains, poultry and pig manures give a higher ammonium to total nitrogen ratio than feedstocks of low biodegradability (e.g. ...
Article
Full-text available
The growing British waste management sector has consistently voiced the need to improve the quality of waste streams and thus the value of secondary resources produced, in order to achieve higher reprocessing rates. Mismanagement of wastes that may lead to contamination and degradation of the recyclate feedstock constitutes one of the main barriers in the pathway to a circular economy. The sector has also repeatedly called upon manufacturers to collaborate in designing materials, components and products (MCPs) with properties that aid recovery, refurbishing, repair and recycling (e.g. separabilty of materials, clear labelling), as waste managers recognise the value of early engagement well before MCPs enter the supply chain (i.e. before MCPs are produced and distributed to the end user). Nonetheless, progress has been slow with regard to improved design for promoting components and products longevity and segregation at source when they reach their end-of-use or end-of-life stage in order to promote circularity. China's ban on imports of low quality recyclates at the end of 2017 marked the beginning of a new era in waste management. It drew attention to UK's dependence on export of low-value secondary resources, placing 'quality' in the spotlight. This article delves into the notion of quality; how quality is understood and assessed at different parts of the MCPs lifecycle, and how it might be systematically measured. A typology to distinguish avoidable and unavoidable designed and created characteristics at all stages of MCPs lifecycle is proposed to provide industry with a tool to design wastes out of the economy. The typology's application is demonstrated using the single-use plastic bottles as an example.
... Qualification of an alloy, or alloys, for aerospace structures involves many considerations relating to the engineering properties and also the manufacturing of components and complete structures. As an example, table 15.6 outlines a service qualification programme for an Al-Li sheet/plate alloy (Wanhill 1994) with some updates (Babel and Parrish 2004;Pacchione and Telgkamp 2006;Lequeu et al. 2010;Rioja and Liu 2012). Another example of a qualification programme is included in Chapter 16, which is a review of the procedures for certifying metallic materials to be used in aerospace structures. ...
Chapter
Full-text available
The material and manufacturing property requirements for selection and application of 3rd generation aluminium-lithium (Al-Li) alloys in aircraft and spacecraft are discussed. Modern structural concepts using Laser Beam Welding (LBW), Friction Stir Welding (FSW), SuperPlastic Forming (SPF) and selective reinforcement by Fibre Metal Laminates (FMLs) are also considered.Al-Li alloys have to compete with conventional aluminium alloys, Carbon Fibre Reinforced Plastics (CFRPs) and GLAss REinforced FMLs (GLARE), particularly for transport aircraft structures. Thus all these materials are compared before discussing their selection for aircraft. This is followed by a review of the aluminium alloy selection process for spacecraft.Actual and potential applications of 3rd generation Al-Li alloys are presented. For aircraft it is concluded that the competition between different material classes (aluminium alloys, CFRPs and FMLs) has reached a development stage where hybrid structures, using different types of materials, may become the rule rather than the exception. However, aluminium alloys are still the main contenders for spacecraft liquid propellant launchers.
... This is because lithium creates an explosion hazard in the aluminium remelting phase; a consequence of 'chemically produced' inherent properties (Suomalainen et al., 2017). However, the extra technical value imparted by the aluminium-lithium alloys, such as low density, high elastic modulus, high strength and superior fatigue crack growth resistance, is currently an efficient way of reducing material weight and improving longevity, potentially outweighing the environmental cost of preventing recyclability (Wanhill, 1994). In the anaerobic digestion of agricultural wastes, feedstocks with a high degradability, such as cereal grains, poultry and pig manures give a higher ammonium to total nitrogen ratio than feedstocks of low biodegradability (e.g. ...
Article
The growing British waste management sector has consistently voiced the need to improve the quality of waste-streams and thus the value of secondary resources produced, in order to achieve higher reprocessing rates. Mismanagement of wastes that may lead to contamination and degradation of the recyclate feedstock constitutes one of the main barriers in the pathway to a circular economy. The sector has also repeatedly called upon manufacturers to collaborate in designing materials, components and products (MCPs) with properties that aid recovery, refurbishing, repair and recycling (e.g. separabilty of materials, clear labelling), as waste managers recognise the value of early engagement well before MCPs enter the supply chain (i.e. before MCPs are produced and distributed to the end user). Nonetheless, progress has been slow with regard to improved design for promoting components and products longevity and segregation at source when they reach their end of use or life stage in order to promote circularity. China's ban on imports of low quality recyclates at the end of 2017 marked the beginning of a new era in waste management, drawing attention to UK's dependence on export of low-value secondary resources and thus placing ‘quality’ in the spotlight. This article delves into the notion of quality, the way it is understood and assessed at different parts of the MCPs lifecycle, and makes recommendations on how it might be systematically measured. A typology to distinguish avoidable and unavoidable designed and created characteristics at all stages of MCPs lifecycle is proposed to provide industry with a tool to design wastes out of the economy. The typology's application is demonstrated using the single-use plastic bottles as an example.
... Palmberg et al. [22] is one of the pioneers of the concept of damage tolerance, by performing a statistical analysis to control the propagation of fatigue cracks and considering inspection intervals in order to keep low the probability of complete failure. Later, Wanhill [23,24] has examined damage tolerance in the use of aluminium alloys for aircraft structural applications. Newman [25,26] suggested that fatigue damage can be https://doi. ...
Article
This paper focuses on the multiscale fatigue life analysis of an aircraft fuselage panel in order to investigate the damage tolerance performance. The two-dimensional multiscale analyses were carried out under different levels of external loads and based on linear elastic fracture mechanics. For this, a micro model consisting of a square plate with a central circular hole and two cracks, from pre-established initial damages and subjected to uniaxial tension, was modelled using the computer program BemCracker2D–an academic computational package for crack growth analysis based on the dual boundary element method (DBEM). The objective is to compute the number of fatigue cycles for each load increment and relate them to their respective compliance in local micro elements. Then, the results were treated statistically with Monte Carlo simulation to ensure the integrity of the fuselage and, therefore, avoiding reaching a Limit State during the design lifespan. The main advantages of using the DBEM formulation implemented in BemCracker2D are easy pre- and post-processing, simplicity of meshing and satisfactory accuracy.
... Aluminum lithium alloy has been used for aerospace structure because its higher modulus and lower density comparing to conventional aerospace aluminum alloy [1]. The alloy also shows superior mechanical properties in higher specific strength, enhanced resistance to high cycle fatigue, and high fracture toughness at cryogenic temperatures [2]. One of the well-known application of aluminum lithium alloys is for space launch vehicle. ...
Article
Full-text available
Al-Li alloys have been extensively used in aerospace vehicle structure since the presence of lithium increases the modulus and reduce the density of the alloy. Especially the third generation Al-Cu-Li alloy shows enhanced fracture toughness at cryogenic temperatures so that the alloy has been used on the fuel tank of space launchers, like Super Lightweight External Tank of the Space Shuttle. Since the commercial size of the plate cannot accommodate the large tank size of the launcher, joining several pieces is required. However, lithium is highly reactive and its compounds can decompose with heat from conventional fusion welding and form different types of gases which result in formation of defects. In this study, the microstructure change is investigated after solid state welding process to join the Al-Cu-Li sheets with optical and transmission electron microscopic analysis of precipitates.
Article
The potential of laser surface melting (LSM) as a pre-treatment prior to conventional anodising has been evaluated on an AA2050-T8 (Al–Cu–Li) aerospace alloy. A KrF excimer laser was utilized, of wavelength of 248nm, with variation of the number of pulses received per unit area. After LSM, the specimens were anodised at a constant voltage of 12V in 0.46M sulphuric acid for 240s. Material characterization, in terms of surface morphology, microstructure and phase transformation, was performed using scanning and transmission electron microscopies, interferometry and scanning Kelvin probe force microscopy (SKPFM). The corrosion behaviour was evaluated based on the standard ASTM G34-01 EXCO test, revealing the distinct improvement in performance of the combined laser and anodising treatments.
Article
A parameter, notch insensitivity factor, has been derived in order to assess the stress concentration sensitivity of Al–Li based alloys. The effect of impurities and Ce element on the stress concentration sensitivity of Al–Li–Cu–Zr, Al–Li–Cu–Mg–Zr and Al–Li–Mg–Zr alloy sheets has been investigated and the dependence of the stress concentration sensitivity on the mechanical properties and microstructural parameters discussed. Deleterious impurities can enhance the stress concentration sensitivity especially for the conventional impurities in Al–Li–Cu–Mg–Zr alloy and for the alkali metal impurities in Al–Li–Cu–Zr alloy. Suitable Ce microalloying can reduce the stress concentration sensitivity to a certain degree and the benefit is more significant to the Al–Li–Cu–Zr alloy rich in the conventional impurities. When varying the composition systems, impurity kinds, impurity content, Ce element concentration and heat treating process makes the yield strength increase, fracture toughness decrease, partial recrystallised volume rise and grain size increase, the stress concentration sensitivity usually tends to increase. Special attention should be paid to the practical application of Al–Li alloys because there is generally high stress concentration sensitivity for the alloys as compared with conventional aluminum alloys.
Article
The unusual susceptibility of the Al-Li alloy AA8090 to sustained macroscopic deviation of fatigue cracks from a nominal mode I path during conventional fatigue testing is discussed. It is demonstrated that the mixed mode crack growth associated with macroscopic deviation may be characterized in terms of elastic strain energy release rates for a range of mixed mode loading conditions. It is specifically shown that this form of mixed mode crack growth may lead materials are subjected to conventional, mode I based structure lifing techniques.
Article
Two principal approaches are available to materials' engineers to improve the overall cost-weight balance of metallic airframe structures: improving alloy performance and optimising materials' utilisation. Although both approaches have been successful in the past, they are most effective when applied concomitantly. The Aluminium industry has a long record of improving aerospace alloys' performance. Nevertheless, even in apparently well-explored alloy systems such as the 7xxx family, products with improved damage tolerance-strength balances have recently been developed, thanks to an improved understanding of the optimum Zn-Mg-Cu combinations for the required property balances but also to developments in casting capability. Novel dispersoids and dispersoid combinations have enabled further improvements of the performance of existing alloy families. For example, appropriate Sc and Zr additions have a significant impact on the grain structure of 2xxx alloys and thus on performance. Another high potential approach for alloy performance improvements is the optimisation of Al-Cu-Li-(Mg-Ag-Zn) alloys. These so-called "third generation Al-Li alloys" were principally developed for military and space applications; in order to meet the demands of future commercial airframes, more damage tolerant variants are being developed. AA2198 and AA2050 are used to illustrate the potential of these higher damage tolerance Al-Cu-Li alloys. However, materials performance improvements are only part of the potential developments of metallic solutions for airframes. Further gains of a similar magnitude in component weight and cost can be achieved by applying new technologies and new design solutions to metallic structures. The future of metallic airframes will depend on the concomitant application of both these approaches.
Article
The objective of the present study is to examine the effect of a rare earth addition, Ce, and some impurities, Fe, Si, Na and K, on the fatigue and fracture properties of 8090 Al-Li alloy sheet by means of the determinations for the fatigue life (N{sub f}) under a constant stress amplitude, fatigue crack propagation (FCP) rates and plane stress fracture toughness. Impurity Fe and Si in 8090 alloy sheets increase the fatigue crack propagation rates and impair the fracture properties although they could not bring about significant effect on the fatigue life under the test conditions maximum cyclic stress of 280 MPa, load ratio of 0.1 and Fe + Si content of 0.24%. Impurity Na and K in 8090 alloy sheets reduce the fracture properties and fatigue life. When the level stress intensity factor is higher, or {Delta} K>10{sup 1.1} Mpam{sup 1/2} in the test, Na and K markedly increase the fatigue crack propagation rates. Ce addition in 8090 alloy sheets containing a certain amount of Fe and Si impurities could suppress the effects of Na and K impurities on the fracture behavior. Adding about 0.28% Ce in 8090 alloy containing trace Fe and Si impurities improves the crack propagation resistance and plane stress fracture toughness. However, adding Ce from 0.10% to 0.29% is unprofitable to the fatigue life of 8090 alloy containing more impurities. There are more and coarser Ce-containing compound particles in the alloy sheets with high Ce content. These particles could produce a detrimental effect on the fatigue properties.
Article
In the present study, 2195 aluminium–lithium (Al–Li) alloy joints were welded by laser-metal inert gas (laser-MIG) hybrid welding. The effect of different laser powers (700, 1000, and 1300 W) on the microstructures and mechanical properties of the weld joints was investigated. The dendritic solidification structure of the weld joint comprised the α-Al, θ-(Al2Cu), and T-(AlLiSi) phases. When the laser power increased, the grains coarsened, and the amount of the precipitated phase decreased. Consequently, the micro-hardness of the weld decreased. The heat input also increased with the increase in laser power. This increased the fluidity and solidification time of the liquid metal at the bottom of the molten pool. Therefore, the deposition of the particles of the refractory metal compounds and the formation of the equiaxed grain zone (EQZ) was hindered.
Chapter
High cycle fatigue properties were studied in different directions within the rolling plane and in the short transverse direction of a hot-cross rolled Al-Li 8090 alloy plate (45 mm thick). The fatigue tests were conducted at constant stress amplitudes ranging from 30% to 100% of the yield strength ay in the rolling direction, R = 0.1, a frequency of 20 Hz, room temperature in air using a self-aligning four-point bend rig. It was found that the fatigue life of a sample loaded in the rolling direction was about ten times longer than that of a sample loaded in the short transverse direction at the same stress amplitude. The fatigue limit for the short transverse samples was measured to be about 30% sigma(y), while it was about 40% ay for the longitudinal samples. Growth of fatigue cracks in the short transverse samples was predominantly intergranular, whereas it was markedly crystallographic (along a 111 plane in each grain) in the samples cut from the plate center when loading axis was parallel to the rolling plane. The inferior high cycle fatigue property of the short transverse samples is believed to be due to the combination of planar grain boundaries (parallel to {110} plane) being perpendicular to the load axis and the existence of trace elements, Na and K, in the alloy. Within the rolling plane, the fatigue behavior was relatively isotropic in the surface region of the plate, while in the plate center it was significantly inferior in the direction of about 35 relative to the rolling direction. Strong texture (mainly {110}<111> and {110}<112>) in the center of this alloy was the main factor that contributed to the in-plane anisotropy.
Chapter
Samples cut from an Al-Li 8090 alloy plate were fatigued in four-point bend at a stress amplitude of 50% sigma(y), R=0.1, a frequency of 20 Hz and room temperature in air. The plate had been hot cross-rolled, solution heat-treated, water-quenched, stretched by 6% and peak-aged. Crack initiation and propagation were monitored in detail using an optical microscope and SEM. The growth rate of short cracks was found to fluctuate greatly mainly due to the variation of crystallographic texture of the neighbouring grain ahead of the crack tip. In most grains the crack propagated along a {111} plane, but in some grains the crack became non-crystallographic and grew roughly along the direction perpendicular to the stress axis. Measurement of the crack growth rate revealed that the non-crystallographic part of the crack grew more slowly than the crystallographic part. TEM studies of the material indicated that beta' particle distribution was markedly non-uniform. In most of grains only a small number of beta' particles were observed, but in some grains they were densely distributed showing the "doughnut-shape" contrast in TEM due to their acting as a substrate for delta' nucleation. Because the beta' particles cannot be sheared by dislocations the slip behaviour in the grains with high concentration of beta' particles is expected to show non-planarity. As a result, the crack growth became non-crystallographic in these grains.
Article
Low cycle fatigue (LCF) resistance data from binary Al–Li, ternary Al–Li–Cu, and quaternary Al–Li–Cu–Mg alloys have been compiled and discussed. The LCF resistance is measured in terms of the variation of the number of reversals to failure 2N fwith the plastic strain amplitude Δɛ p /2 as well as a modified average plastic strain energy per cycle (ΔW p )modified , obtained at different applied total strain amplitudes (Δɛ t /2). The data show the effects of microstructural features, namely dominant strengthening precipitates and the degree of recrystallisation as well as crystallographic texture. Lithium content, when in excess of 2·5 wt-%in aluminium decreases the low cycle fatigue resistance the most. The degree of aging, the degree of recrystallisation, and the degree of texture also influence the LCF resistance; among which the effect of the degree of aging is the most pronounced. The effects of lithium content in aluminium solid solution and strengthening precipitates obtainable by the change in the Li/Cu ratio are found to be marginal. Based on a modified total cyclic plastic strain energy till fracture, it is shown that most of the Al–Li alloys exhibit degradation in their LCF resistance in both hypotransition (higher fatigue lives) and hypertransition (lower fatigue lives) regions. Such degradation is attributed to the combined effects of mechanical fatigue, strain localisation through dislocation–precipitate interaction, environmental effects, and finally strain localisation through the high angle grain boundaries. In comparison with the currently used 2XXX and 7XXX series aluminium alloys, Al–Li alloys require substantial improvement before they can be considered for fatigue critical applications.
Article
The effects of different cerium contents on the mechanical properties of a novel Al-Cu-Li alloy under the peak aged conditions of T6 and T8 were studied by conventional tensile test and Kahn Tear Test. In addition, the grain boundary precipitates and fractography were observed by transmission electron microscopy (TEM) and scanning electron microscope (SEM). The results show that the UIE and the strength for alloys under T6 condition are generally lower than those for the alloys under T8 condition, while strength-toughness combination has been obtained in alloy 2#(0.15% Ce) under T8 (pre-stretched + artificially aged) condition. Meanwhile, a statistic analysis of the microscopic data by Image-Pro Plus (IPP) software, taking into account the effect of cerium content and pre-stretch was carried out. The analysis shows that the larger area fraction of the combination of dimples and intermetallic particles is corresponding to the higher value of UIE. The changes in the precipitate free zone (PFZ) width, grain boundary precipitate number density and grain boundary precipitate effective size govern the fracture resistance and the crack path of the test alloy under the conditions of the different Ce contents and heat treatment.
Article
The tensile strength of notch specimens has been investigated for alloy 2090 sheets with various Ce contents. The notch strength (N) has been quantitatively analyzed. By comparison with Ce-free alloy, N of Ce-containing alloy sheets exhibits an insignificantly change. The statistical estimation shows that N evaluated from the theoretical expression is in better agreement with the test data. Accordingly, the strength of notched specimens can be conveniently predicted by means of the conventional tensile properties of smooth specimens. The notch insensitivity factor (KN) has been derived for the alloy 2090 sheets with various Ce contents. KN can be applied to assess the stress concentration sensitivity to structural notches in practical structures by comparison with the theoretical stress concentration factor (Kt). Increasing in Ce content in the alloy 2090 sheets can slightly enhances KN or reduces the stress concentration sensitivity. However, Ce microalloying can not still essentially decrease the stress concentration sensitivity of the alloy 2090 sheets because the ductility can not be improved to a greater degree only by adding Ce element. Therefore, high stress concentration sensitivity may be a potential obstruction to the practical application of high-strength Al-Li based alloys in primary aircraft structures containing structural notches.
Article
In the present article, the high strength Al-Li-Cu-Zr alloy sheets modified by a rare earth element, Ce, are considered for possible application in practical aircraft products containing structural notches or stress concentrations; accordingly, a study has been made on the effects of stress concentration levels and Ce contents on the tension strength of notched specimens for the alloy sheets. Moreover, a discussion has been set off on the theoretical predictability on the basis of a theoretical expression for the notch strength by means of the mechanical properties of the smooth specimens. The test results show that when the stress concentration level increases, the notch strength linearly decreases in the double logarithmic coordinate; by comparison with the Ce-free alloy, the Ce-containing alloy sheets exhibit an insignificantly varying notch strength when the Ce content changes from 0.13 to 0.31 wt pct in the transverse orientation specimens or is 0.21 wt pct in the longitudinal orientation specimens even though their ductility for the smooth specimens can be improved to a certain degree by the Ce modification. The test data of notched specimens under the theoretical stress concentration factor (K t), from 2.0 to 8.0 agree better with the predicted values of notch strength. Therefore, in accordance with some engineering properties such as the ultimate tensile strength (UTS), percentage elongation (EL), and Young’s modulus (E) of the smooth specimens, the notch strength of the alloy sheets under plane strain state can be easily estimated in a certain range of stress concentration levels.
Article
Flight simulation fatigue crack growth tests are necessary for verification of aircraft damage tolerance analyses and crack growth prediction methods, and also for comparing candidate materials for aircraft structural applications. However, such tests involve complicating issues that have emerged from many investigations on aluminium alloys since the late 1960s. These issues are reviewed to provide guidelines for further testing.
Article
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The influence of sole and joint Zr and Mn additions on the recrystallization behaviour of an AA2198-base alloy has been compared during extended annealing at 535 °C, following hot rolling to sheet. With a constant Zr level recrystallization resistance was seen to diminish with the addition of Mn and became progressively worse with a decrease in Zr content, as more Mn was added. It has been found that this behaviour arises because the additional pinning pressure of Al20Cu2Mn3 dispersoids does not adequately compensate for the expansion in width of the Al3Zr-free bands that occurs on the addition of Mn to the base alloy, even with the same Zr level. The lower potency of Mn, relative to Zr, in inhibiting recrystallization has been attributed to the poorer coherency and higher aspect ratio of the Al20Cu2Mn3 dispersoids, which reduces their Zener pinning pressure by a factor of four relative to that of the Al3Zr phase. In addition, the presence of the coarser Al20Cu2Mn3 dispersoids was found to increase the stored energy after hot rolling. The recrystallization mechanism was dependent on the dispersoid type. The addition of Zr led to the dominance of broad front strain-induced boundary migration (SIBM), whereas the addition of Mn-containing dispersoids favoured particle-stimulated nucleation (PSN). Texture measurements have verified this observation, with SIBM favouring the growth of recrystallized grains of orientations typical of the deformation texture and PSN promoting the growth of randomly orientated grains during recrystallization.
Article
This paper reviews the recent series of research work conducted at Oxford on the effects of macro- and micro-texture on the fatigue properties of two Al-Li 8090 alloys. One alloy was a direct-chill cast and hot-cross rolled plate, and the other was a spray-cast and extruded bar. Both were solution heat-treated, water quenched, stretched and peak aged. Samples cut from different positions through the thickness of the plate and the bar were fatigued at a stress amplitude of 50% to 80% σy, frequency of 20 Hz, R=0.1 and -1 and room temperature, in air. The texture profile through the plate thickness and the local texture ahead of the tip of a short fatigue crack in the sample were characterized using EBSD. Both the type and intensity of the texture varied sharply through the plate thickness. It was found that the global and local textures have significant effects on fatigue crack initiation, short crack growth and fatigue lives of the alloys. In the spray-cast alloy cracks were often formed in grains with orientations close to Goss orientations ({110}<001>), while trace elements (Na and K) could be responsible for crack initiation on {001} planes which were perpendicular to the stress axis in the direct chill cast alloy. It was found that the twist angle on the grain boundary plane between the crack planes across the grain boundary was the key factor that retarded crack growth through the boundary, and that a sample with more random texture had higher resistance to short crack growth.
Article
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In this paper, the application of photoacoustic methods to study thermal properties of Al-Li alloy is described. The photoacoustic measurements are carried out for thermal properties on Al-Li alloy and studied for various thicknesses. The theoretical basis for quantitative measurements is discussed together with the advantages and limitations of these methods as compared with conventional measurements. Applications to spectroscopic and depth-profile analysis and also to thermal property measurements in Al-Li samples heat-treated at 755K per hour and annealed at 505K are discussed. The results are compared with literature values, and discussed.
Chapter
Wrought aluminium alloys are important engineering materials which account for some 85% of all aluminium that is produced worldwide. Following a brief introduction to the conventional practices of direct-chill casting, ingot homogenisation, fabrication and heat treatment by age hardening, attention is directed to the general role of microstructure in strengthening aluminium alloys. The significance of microstructure/property relationships is then discussed for different commercial and experimental wrought alloys, and for some of the applications in which they are used. Special consideration is given to the physical basis for the design of various novel compositions and processing methods that have resulted in materials with properties which are unattainable in existing alloys.
Article
This paper presents a comparative study on the fatigue behavior of a typical third generation Aluminum–Lithium alloy, AA2198-T8, to that of a traditional aerospace aluminum–copper alloy, AA2024-T3 with a focus on developing a methodology for reconstructing the crack growth history. Fatigue testing was conducted using a lap joint configuration designed to induce large amounts of secondary bending for the purpose of accentuating any unique behaviors in these two material systems. Fatigue fracture surface morphology was examined in order to determine the effect of alloying and loading direction relative to plate rolling direction on fatigue crack growth behavior. Crack initiation sites occurred in the vicinity of the fastener hole, at multiple sites along the faying surface, yielding crack tunneling in all specimens, prior to the ductile catastrophic fracture. The fracture surface morphologies in the two alloys were markedly different: the AA2024-T3 exhibited substantial meandering fracture surface, with localized fatigue crack progressing along multiple directions and at different rates. The AA2198-T8 alloy exhibited more uniform but shallower fatigue striations, with numerous micro and macro interlaminar cracks. Crack growth rate ( ) measurements along the width, thickness and the striation ridge lines indicated that the latter provides the most reliable results in terms of estimating fatigue crack initiation cycle, crack initiation site, and crack growth rate.
Article
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Al-Li alloys play a significant role in current aerospace components due to their exceptional strength and stiffness-to-density ratios and are most competitive or superior to a composite based design in an aircraft or space launcher. Since integral components of aerospace vehicle requires very large sheets, which size is not available commercially, joining of smaller pieces are required. It is known that fusion welding of Al-Li alloys is difficult due to the low melting temperature of element Li and causes porosity, cracking and low joint efficiency. Therefore, a solid-state joining process, like friction stir welding, is generally appropriate for joining of Al-Li alloys. In friction stir welding process, a non-consumable rotating welding tool is used to generate the frictional heat and plastic deformation of the material in the welding zone, which is a combination of extrusion and forging. In this article, experimental investigation provided the influence of rotating and welding speed on microstructure, mechanical properties, and joint quality. Valuable information about processing window which results in a successful friction welded joining of this alloy is presented.
Article
Unacceptably low through-thickness fracture toughness values and their large disparity with those values corresponding to the in-plane orientations has been the principal limitation for the use of thicker sections of aluminium - lithium alloys for aerostructural applications. Optimisation of fracture toughness among the three orthogonal directions is one of the primary requirements of this class of advanced alloys if they are to become realistic aerostructural materials to displace the traditionally used aluminium alloys. In the present paper, we report the mode-I (tensile) and mixed-mode I/III fracture behaviour in the through-thickness orientations of S-L and S-T in quaternary Al-Li-Cu-Mg alloy in the peak aged (T6) temper condition. Results reported here include microstructural characteristics, tensile properties, mode-I and mixed-mode I/III fracture toughness properties. The alloy plate exhibited significant anisotropy in through-thickness fracture resistance with fracture toughness values that are less than half of those in in-plane orientations. A two-step aging treatment suggested in the literature involving short time, high temperature thermal exposure, was evaluated in case of the present alloy. In line with the reported observations, two-step aging treatment resulted in significant improvement in fracture resistance with a moderate decrease (2-5%) in the tensile strength properties. The alloy plate also exhibited enhanced total fracture resistance with imposed mode-III components and hence, the structural design of these alloys in the through-thickness orientations could be based on mode-I fracture toughness, KIc.
Article
Fully reversed, total axial strain controlled low cycle fatigue tests were conducted on aluminium-lithium alloy plate of AA8090 composition in underaged (UA, T3) condition at strain amplitudes ranging from 0.0045 to 0.015. The properties studied include cyclic stress response behaviour, cyclic stress-strain data and fatigue life variation with plastic strain amplitude (Δεp/2), average stress amplitude (Δσ/2) or average plastic strain energy per cycle (ΔWp). The fatigue data obtained for the underaged alloy have been compared with those of a peakaged (PA, T8E51) temper alloy plate of similar composition. The alloys in the two ageing conditions exhibit similar cyclic stress response behaviour, which varies with applied strain amplitude. Initial cyclic hardening was followed by cyclic stability till fracture at lower strain amplitudes, while cyclic softening followed initial hardening at higher strain amplitudes. A comparison of the fatigue life data of the two alloys with reference to Δεp/ 2, Δσ/2 or ΔWp, reveals that the underaged alloy possesses lower fatigue resistance than the peakaged alloy. The UA alloy exhibits bilinear fatigue life power-law relationships with power-law constants at lower strain amplitudes being higher than those at higher strain amplitudes, in a manner similar to that reported earlier for the PA condition. The observed transition in the fatigue life power-law relationships is attributable to changes in the deformation and/or deformation-assisted fracture modes.
Article
Results of investigations into the formation of the crystallographic orientation of the structure and anisotropy of properties during rolling sheets of the aluminum–lithium 1420 alloy of the Al–Mg–Li system are given. Hot-rolled billets of the 1420 alloy were cold-rolled with intermediate quenching according to the following schedule: 7.3 mm → 4.8 mm → 3.0 mm → 1.8 mm. The samples were selected after each passage to perform mechanical testing and analyze the structure using optical microscopy and diffractometry. A deformed fibrous structure and considerable anisotropy of mechanical properties is characteristic of sheets of all considered states. Herewith, the maximal plasticity is observed at an angle of 45° to the rolling direction. The character of anisotropy of properties formed at the hot-rolling stage is not varied during cold rolling. Sheets of the 1420 alloy have a sharp deformation texture at all rolling stages due to the conservation of the unrecrystallized structure. For example, when analyzing pole figures and preferential orientations, an increase in volume fractions of rolling texture is revealed (the slow one of the brass type and more rapid of the S type) with the rise of summary deformations of cold rolling. The recrystallization texture (of the R type) is present in small amounts only after hot rolling. The volume fraction of the texture-free component decreases with an increase in summary deformations. It is concluded based on these results that, in order to decrease the fraction of the deformation texture and lower anisotropy of properties in sheets of the 1420 alloy, it is first and foremost necessary to provide the running of recrystallization at the hot-rolling stage in order to fabricate the recrystallized hot-rolled billet for subsequent cold rolling.
Article
The fracture behavior has been investigated for alloy 1420 sheets contained various Ce contents under the conditions of aging for 30 h at 120°C and for 6 h at 170°C. The results indicate that adding minor Ce into alloy 1420 could decrease the crack propagation resistance, starting fracture resistance and plane stress fracture toughness to a certain degree. The secondary cracks can be also reduced and the crack-dividing delamination becomes insignificant because of Ce microalloying. The reason why Ce microalloying could not be of benefit to the fracture toughness of alloy 1420 sheets might be supposed to be that the level of extrinsic toughness of the alloy is reduced by adding Ce element.
Article
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The results of studying the influence of manufacturing modes of sheets 1,5, 2,0 and 3,0 mm thick made of V-1461 Al–Li alloy on the microstructure, crystallographic orientation, and anisotropy of properties are presented. It is established that the deformed structure is characteristic of all studied samples, and sheets 3.0 mm thick have partially recrystallized structure, those 2,0 mm thick have unrecrystallized structure, and those 1,5 mm thick have mainly recrystallized structure. The preferential crystallographic orientation of the samples 1,5 mm thick is [110](200), that of samples 2,0 mm thick is [110](110), and that of 3,0 mm thick is [210](110). All the sheets possess insignificant anisotropy of properties irrespective of thickness, and extremely low anisotropy index (μ < 0,4) is characteristic of them. These facts determine the liability of aluminum–lithium alloys to the preferential development of deformation over the sheet thickness, which leads to its premature thinning and lowers admissible forming in the course of drawing and stretching.
Article
The effects of the contents of Ce and deleterious impurities on the ductility and recrystallized microstructure have been investigated for Al-Li based alloys 2090 and 8090 sheets. The relationships between the parameters of recrystallized microstructure and percentage elongation (delta) in the transverse orientation of the sheets have been obtained. The beneficial behavior of Ce microalloying can be markedly shown in the alloy 2090 containing a certain amount of impurities Fe and Si or in the alloy 8090 containing a certain amount of impurities Na and K. Ce microalloying can still improve delta to a certain degree, even though impurities Fe, Si, Na and K coexist and their concentration is rather high in the alloy 8090. When the recrystallized volume, coarse-grained volume and grain average width increase because of the variation in the contents of Ce element and impurities, delta decreases for the alloys 2090 and 8090. When the grain aspect ratio increases, delta increases for the alloy 2090 and decreases for the alloy 8090.
Article
New alloy developments driven by aircraft industry have identified aluminium lithium (Al–Li) alloys as potential candidates for substitution of incumbent high strength aluminium alloys used for manufacturing spacecraft and launchers. Whereas properties like specific stiffness, strength and toughness are proven as superior when compared to those of currently adopted Al alloys, the Stress Corrosion Cracking (SCC) characteristics are still an open aspect if advanced Al–Li alloys are considered for space structural applications. The present paper provides a comprehensive characterisation of the Al–Li 2099-T86 SCC performances.
Technical Report
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This report reviews currently available information on corrosion and fatigue in aircraft as a background to discussing the assessment of corrosion and fatigue damage. The main topics are: j , aircraft operating environments and loads j , corrosion 4 fatigue * corrosion and fatigue * corrosion and fatigue damage assessment. Aircraft structures are susceptible to corrosion and fatigue damage, which concentrate at structural joints. There is a likelihood of interactions between corrosion and fatigue, especially as aircraft become older. However, it is uncertain whether in-service corrosion typically leads to fatigue cracking, and it is also difficult to be specific about the relations between the environments and load histories that the aircraft sees in service. There is a need for ad hoc investigations to determine the in-service environmental contributions to fatigue initiation and crack growth. Structural areas removed from ageing aircraft should be invaluable sources of information for these investigations. At present there is no established methodology for assessing the effects of corrosion on fatigue in order to estimate service lives and specify inspection intervals. This report suggests approaches to corrosion and fatigue damage assessment that are based on current methods of fatigue analysis. DATE 940914 P!J ref 60 77
Article
Purpose In summary, it can be found that the current research on the simulation of natural atmospheric dry–wet alternating accelerated corrosion mainly focused on the study of electrochemical corrosion process and the study of corrosion rate; the micro-pre-corrosion mechanism of materials in this environment, especially for materials. The specific effects of fatigue and fracture performance still lack detailed research. Accordingly, this study aims to more realistically simulate the effect of natural atmospheric corrosion environment on the corrosion resistance and fatigue performance of aircraft skin. Design/methodology/approach In this study, the uniaxial strain control method was used to test the fatigue performance of pre-corrosion samples under simulated natural atmospheric corrosion using MTS809 tensile-torque composite fatigue machine. Scanning electron microscopy, X-ray energy spectrum analysis, atomic force microscopy and X-ray diffraction analysis were used. Fatigue fracture, corrosion morphology and corrosion products were analyzed. Findings The results show that the deep corrosion pit caused by pre-corrosion environment leads to multi-source initiation of crack; the fatigue life of pre-corroded sample decreases by about one-half, chloride ion invades the material and promotes intergranular corrosion; life prediction results show that the natural atmospheric corrosive environment mainly affects the plastic term in the Manson–Coffin formula resulting in a decrease in fatigue life. Originality/value Innovative experimental schemes and materials are used and the test temperature and relative humidity are strictly controlled. The corrosion failure mechanism of 2A70-T6 aluminum alloy under alternating wet and dry accelerated corrosion environment and its influence on fatigue behavior were obtained.
Chapter
Within the last century aluminum alloys have played a strategic role in the manufacturing and development of lightweight aircraft structures. Years of continuous research has led to significant improvement in mechanical properties in the form of advanced 2xxx and 7xxx series alloys and the opportunity to produce more lightweight materials with advanced properties such as the last-generation Al–Li alloys. An overview of the evolution of aircraft aluminum alloys from the original Al–Cu alloys to modern nanocrystalline and hybrid aluminum alloys is presented. Basic properties and processes are featured, that define the material performance and determine their main applications in aircraft industry. Finally, novel trends in the design of aluminum alloys are considered in order to meet the future challenges of modern aircraft applications.
Article
The current status of the effects of impurities and rare earth elements on the microstructure and the mechanical properties of Al−Li alloys has been surveyed in the present paper. Particular attention has been paid to the mechanical properties in respect of different conditions and the mechanisms responsible for both impurity-induced embrittlement and rare earth element microalloying. Al−Li alloys usually show a high sensitivity to impurity-induced embrittlement. High hydrogen concentration in Al−Li alloys results in severe hydrogen embrittlement. Impurities Fe and Si could form some brittle compound particles to produce the negative effects on the microstructure. Alkali metal impurities such as Na and K could result in the formation of grain boundary liquid, recrystallized grains, coarse particles distributed along grain boundaries and δ′ (Al3Li) precipitation-free zones (δ′-PFZ). By reducing the tendency for brittle intergranular fracture, the effects of rare earth elements, such as cerium and scandium, as alloying additions, on the microstructure and the mechanical properties are usually beneficial. Rare earth modification for Al−Li alloys improves ductility, fracture toughness and possibly reduces their anisotropy to a certain degree. These microalloying effects of rare earth additions might suppress the impurity impairment. Adding an amount of rare earth elements somewhat reduces the stress concentration sensitivity in high strength Al−Li alloys while the notch strength is hardly affected. Though the microalloying of rare earth elements improves the mechanical properties, in particular, the ductility of smooth specimens, high strength Al−Li alloys still have high sensitivity to stress concentrations. This might be a possible obstruction to the widespread application of high strength Al−Li alloys in primary aerospace structures containing geometry notch design. In order to achieve an essential improvement in the ductility and fracture toughness of Al−Li alloys, it is necessary to find further efficient alloying approaches associated with rare earth constitutents.
Article
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This paper describes the approach and results of recent work in our laboratories on identifying and solving the ductility problems which have plagued Al-Li alloys. In addition, specific property goals for developing new Al-Li alloys have been established. These include a) a 30% increase in modulus:density ratio, compared to 7075-T76, without significant loss in other properties, and b) a 20% increase in modulus:density ratio and a 20% increase in strength:density ratio compared to 7075-T76, without significant loss in other properties. Two approaches are being pursued, one using conventional ingot metallurgy (I/M) and the other using rapid solidification of fine particulate plus powder metallurgy (P/M) consolidation. The rapid solidification approach is designed to reduce or eliminate segregation effects, reduce the grain size, extend solid solubility of additional elements, and refine the dispersoid particle size. In some cases, we have made a comparison of the structure and properties of similar alloys fabricated by the two methods.
Article
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In the present paper, the fatigue crack growth retardation following a single tensile overload applied to the 8090 alloy is studied and compared with results obtained from conventional aluminium alloys. The 8090 alloy exhibits a high retardation effect to overloads ; reasons for this good behaviour are given considering mechanical and fractographic aspects.
Article
Recently, interest has once again turned toward the aluminum-lithium system for aerospace applications. These alloys are attractive because they have to lower density and higher elastic modulus than the conventional 2XXX and 7XXX alloys currently being used. This paper is divided into two parts. The first part concentrates on the fundamental precipitation mechanisms in binary alloys; the influence of magnesium, copper, and combined magnesium and copper additions; and the influence of the ancillary elements chromium, manganese and zirconium.
Article
Reducing the weight of aircraft structure through the use of lighter advanced materials has become a major goal. Pechiney's extensive Research and Development program has resulted in a new family of aluminium-lithium alloys. Thanks to the combination of efficient D. C. casting, alloy design coprecipitation strengthening and structural control, these CP 27X alloys overcome the previously identified brittleness of Al-Li alloys and already match most current goals for a wide use of aircraft.
Article
Alithalite Al-Li alloys are being developed by Alcoa to meet four first-generation objectives: low-density replacements for 2024-T3X, 7075-T6X, and 7075-T73X products, and a moderate-strength, minimum density alloy. One alloy, recently registered as 2090 (Al-2. 7wt%Cu-2. 2%Li-0. 12%Zr), has been introduced as a replacement for 7075-T6X plate. sheet, and extrusion product forms. This paper will concentrate on discussing the metallurgical and engineering property characteristics of this alloy. Progress toward developing alloys to meet the other objectives also will be outlined, with emphasis given to property achievements.
Article
The influence of lithium and copper compositions (1. 1 Li/4. 5 Cu, 2. 1 Li/3 Cu and 2. 9 Li/1 Cu in wt pct) on constant amplitude fatigue crack growth rates and post overload crack growth retardation is examined in alloys belonging to the Al-Li-Cu-Zr system tested in vacuo and moist air environments. Substantial improvements in fatigue crack growth resistance, threshold level and retardation are observed with increasing Li content. Several intrinsic mechanisms associated with intrinsic microstructural and environmental effects contribute to the differences in fatigue behavior among the alloys studied. An important aspect of such beneficial crack growth resistance stems from the crystallographic crack growth mechanism promoted by the presence of delta prime precipitates in alloys of high lithium content.
Article
The physical metallurgy of alloys based on Al-Mg-Li, Al-Cu-Li and Al-Cu-Mg-Li is described in terms of precipitation sequences during age hardening, grain structure, texture and the effects of grain refining additions. A simple foundry technique was used and the resulting properties are compared with direct chill cast material. The physical, mechanical and corrosion properties are compared with design requirements emphasising fracture toughness and resistance to fatigue crack growth. The mechanical properties are explained with reference to deformation characteristics and fracture behaviour. 9 refs.
Article
Aluminium-lithium alloys are likely to have considerable impact on the future design, manufacture and operating economics of aircraft. This article describes the development of this promising range of alloys, their metallurgy and likely future role in aerospace applications, particularly in the face of competition from non-metallic composite materials. The authors emphasise that aluminium-lithium is capable of offering comparable weight savings to fibre reinforced materials whilst avoiding a radical departure from well-established technology and manufacturing practices.
Article
Micromechanisms influencing the propagation of long (>10 mm) fatigue cracks in aluminum-lithium alloys are examined by specifically comparing crack-growth kinetics in a peak-aged Al-Li-Cu-Zr alloy 2090, processed as 1.6-mm thin (T83) sheet and 12.7-mm thick (T81) plate. It is found that in general crack-growth rates are significantly faster in the sheet material at equivalent stress-intensity levels, due to differences in the role of crack-tip shielding, resulting from crack deflection and consequent crack closure from wedging of fracture-surface asperities. Microstructurally, such differences are related to variations in the degree of recrystallization, grain structure and deformation texture in the two wrought-product forms. 14 refs., 4 figs.
Article
The influence of wrought product form on the fatigue-crack propagation resistance of aluminum-lithium alloys was examined; specifically, results on the growth kinetics of long (greater than 10 mm) fatigue cracks in peak-aged AlLiCuZr 2090-T8X and AlLiCuMgZr 2091-T8 alloys, fabricated as sheet and plate, are presented as a function of microstructure, load ratio and specimen orientation. Contrary to popular belief, it was found that fatigue-crack growth rates at equivalent stress intensity levels are significantly faster and less dependent on specimen orientation in sheet than in plate. Such differences are attributed to the prominent role of crack-tip shielding during fatigue in these alloys, resulting from crack deflection and consequent crack closure from wedging of fracture-surface asperities, which microstructurally is related to variations in the degree of recrystallization, grain morphology and texture between the two product forms.
Article
The EH101 is a long range, large capacity helicopter developed and built jointly by Westland Helicopters of the UK and Agusta of Italy to meet the needs of civil, naval and utility operations and in which many new aerodynamic, electronic and structural design concepts are successfully combined. In order to effect significant structural weight reductions, extensive use will be made of aluminium-lithium based alloys which are now becoming commercially available. The results of a weight-saving cost analysis favour the use of aluminium-lithium sheet, extruded profiles and forgings on the EH101, while corresponding plate and extruded bar will not be used due to the combined effects of low utilisation rate and the inherently higher material cost of aluminium-lithium. Keywords: Alloys, Great Britain, EH101 Helicopters, Metallurgy, Uses and properties. (JG)
Article
The advantages to be gained by weight reduction of aerospace structures have encouraged the aluminum industry to develop a family of aluminum alloys which contain lithium as one of the alloying elements. When alloyed with aluminum, lithium can reduce the density by approximately three percent and increase the elastic modulus by six percent for every weight percent added. A new series of aluminum alloys, typified by 2090, 2091, 8090, and 8091, have been developed and are currently being produced in commercial quantities. These alloys have densities between 7% and 10% lower than the conventional alloy 7075 with correspondingly higher stiffness. Although a combined set of specific properties of the (Aluminum-Lithium-X) alloys often exceeds those of the conventional aluminum materials used in aerospace, these properties seem to be much more sensitive to processing parameters. The strong processing-property relationship is associated with sharp crystallographic textures that are developed during primary processing and very complex precipitate microstructures whose distributions are sensitive to quench rates and degree of deformation prior to aging. This paper describes the processing-microstructure-property relationships of the new Al-Li-X alloys and focuses on strength, ductility, fracture toughness, fatigue and stress corrosion properties. Keywords: Symposia, Metallurgy, Microstructure, Alloys, Mechanical properties. (JG)
Article
Mechanisms 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 ΔK. 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 tip ‘driving force’ from crack deflection and, more importantly, from the consequent crack closure induced by the wedging of fracture surface asperities. Since microstructurally small cracks are unable to develop the same level of shielding from crack closure by virtue of their limited wake, small crack growth rates are significantly accelerated. Unlike fracture toughness behaviour, artificial aging of commercial Al–Li alloys to peak strength has a mixed influence on the (long crack) resistance. Although behaviour at higher growth rates is relatively unaffected, in 2091 nominal threshold ΔKTH values are increased by 17%, whereas in 8090 and 8091 they are decreased by 16–17%. However, all alloys show reduced effective fatigue thresholds at peak strength after correcting for crack closure.MST/926b
Article
A study has been made of the mechanics and mechanisms of fatigue crack propagation in a commercial plate of aluminum-lithium alloy 2090-T8E41. In Part I, the crack growth and crack shielding behavior of long (≳5 mm) through-thickness cracks is examined as a function of plate orientation and load ratio, and results compared to traditional high strength aluminum alloys. It is shown that rates of fatigue crack extension in 2090 are, in general, significantly slower (at a given stress intensity range) than in traditional alloys, although behavior is strongly anisotropic. Differences in growth rates of up to 4 orders of magnitude are observed between the L-T, T-L, and T-S orientations, which show the best crack growth resistance, and the S-L, S-T, and L + 45, which show the worst. Such behavior is attributed to the development of significant crack tip shielding (i.e., a reduction in local crack driving force), primarily resulting from the role of the crack path morphology in inducing crack deflection and crack closure from the consequent asperity wedging. Whereas crack advance perpendicular to the rolling plane (e.g., L-T,etc.) involves marked crack path deflection and branching, thereby promoting very high levels of shielding to cause the slowest growth rates, fatigue fractures parallel to the rolling plane (e.g., S-L,etc.) occur by an intergranular, delamination-type separation, with much lower shielding levels to give the fastest growth rates. The implications of such “extrinsic toughening” effects on the fracture and fatigue properties of aluminum-lithium alloys are discussed in detail.
Article
“Low density” AlLi alloys are attractive replacements for conventional aluminium alloys but can have a low resistance to intergranular fracture under overload and creep conditions. The effects of aging treatments and testing temperature on the short transverse fracture toughnesss of AlLiCuMgZr 8090 alloy plate are reported in this paper (Part I). Studies of creep crack growth are reported in a companion paper (Part II). In Part I, double-aging treatments which substantially increase the toughness with only a small loss of strength are described in particular. Observations made using a variety of metallographic and fractographic techniques, consideration of the effects of the temperature on toughness, and determination of the activation energy for re-embrittlement of toughened material by subsequent aging suggest that toughness is mainly determined by the extent of segregation of lithium at grain boundaries. The area fraction of grain boundary precipitates and the extent of strain localization in precipitate-free zones at grain boundaries are also significant factors.
Article
Mechanisms for the transient crack growth retardation of fatigue cracks following single tensile overloads are examined in a high-strength aluminium-lithium alloy 2090-T8E41, with specific emphasis on the role of fatigue crack closure. Mechanically, it is found that following the application of the overload, an immediate acceleration in fatigue crack growth is observed, which results from a reduction in (far-field) crack closure levels due to crack tip blunting. Subsequent crack growth retardation, as the crack penetrates the overload plastic zone, is associated with enhanced crack tip shielding prompted by compressive residual stresses ahead of the crack tip, crack deflection and consequent (near-tip) crack closure induced by asperity wedging in the immediate crack wake. Aluminum-lithium alloys, by virtue of their marked anisotropy and planar slip characteristics which promote meandering crack paths and hence high levels of shielding, are found to show large load interaction effects, and thus rank superior to other high strength aluminum alloys under tension-dominated variable amplitude loading spectra. However, under conditions where specimen surface layers are removed, or where cycling is performed at high (positive) load ratios, or involves compressive overload cycles (all processes which tend to limit the development of crack closure), the superior performance of aluminum-lithium alloys under variable amplitude fatigue loading may be compromised.
Article
The fatigue crack propagation behavior of naturally-occurring, microstructurally-small (1–1000 μm), surface cracks is examined as a function of microstructure in commercial aluminum-lithium alloys 2090-T8E41, 8091-T351 and 2091-T351, and results compared with behavior in traditional high-strength aluminum alloys 2124 and 7150. Despite large differences in the fatigue crack propagation behavior of long (≳ 10 μm) cracks in these alloys, little difference is observed in the small crack growth resistance, with the small crack growth rates for all microstructures lying within a scatterband some 2–4 orders of magnitude higher than the near-threshold fatigue behavior of long cracks. Such results are attributed primarily to a lack of crack tip shielding (developed from crack deflection and resulting crack closure from asperity wedging) with small cracks of limited wake. Since the well-known superior fatigue crack growth resistance of aluminum-lithium alloys can be traced principally to such shielding, promoted by the branched and tortuous nature of their crack paths, the anomaly between long and small crack behavior appears to be most significant in these alloys.
Article
Useful information and guidelines are presented for engineers who wish to minimize fracture problems in aluminum engineering structures through better application of materials knowledge and optimum alloy choice. Consideration is given to methods and examples of characterizing fracture resistance of aluminum alloys including interpretations of data addressing basic design approaches. Specific attention is given to trade-offs among properties of strength, fracture toughness, corrosion, stress corrosion cracking, and fatigue in structural aluminum materials.
Article
The fatigue crack propagation resistances of 7010-T7651, 7010-T73651 and 7075-T7351 thick plate were compared for flight simulation (gust spectrum) loading conditions. The alloy 7075-T7351 was found to be slightly superior, and this superiority was not essentially due to its higher fracture toughness. The influence of specimen thickness on the crack propagation resistance was significant. Thinner specimens gave longer crack propagation lives and, generally, lower crack propagation rates and longer delays in crack growth following severe flights.
Article
The fatigue resistance of metals can be profoundly affected by the environmental reactions that change crack initiation and propagation processes. These corrosion fatigue processes can occur in high strength aluminium alloys in aircraft, which frequently encounter salt spray and high humidity environments. For this reason, the fatigue behaviour of 8090-T6 alloy in the form of 25mm thick plate has been investigated in both air and an aerated 2.5% NaCl solution. Results are compared to a conventional high strength aluminium alloy, 2014-T6. Fatigue crack initiation and propagation data are presented for different crack orientations and mean stress levels. The slightly lower fatigue strength of 8090-T6 in air is attributed to earlier crack initiation. The fatigue crack propagation behaviour of 8090-T6 is shown to be superior to that of 2014-T6 at low to intermediate levels of ƊK, under both environmental conditions. The mechanisms associated with fatigue crack initiation and propagation, and environmental effects, are discussed.
Article
Throughout the past decade extensive research and development has been carried out on aluminium-lithium base alloys because of the attractive combination of lower density and higher modulus that can be achieved in this system compared with "conventional" aluminium alloys. Much of this effort has been directed at understanding and overcoming their "Achilles heel" of low ductility and poor fracture toughness (particularly for crack planes perpendicular to the short transverse direction). This study reviews the metallurgical features which affect ductility and damage tolerance in the 8090 and 8091 alloys developed at Alcan and the Royal Aircraft Establishment. The paper shows that control of these metallurgical features can be used to markedly improve these properties. Three examples will be given. In the first case the production of material with a high damage tolerance will be discussed. It will be shown that by control of grain structure and ageing practice, 8090 sheet with a plane stress fracture toughness (Kc) of >130 MPa/[MATH] can be achieved at similar strength levels to 2024-T3 sheet. There is also a decrease in fatigue crack growth rate. This results in sheet with an overall damage tolerance capability comparable with that of 2024-T3. The other examples concern the processing of 8090 and 8091 plate to meet medium and high strength airframe requirements respectively. 8090 plate (<100 mm thick) can achieve the property requirements of 7010-T7651, with short transverse ductilities in excess of 3% and short transverse fracture toughness of 218 MPa/[MATH]. This is accomplished by controlled thermomechanical treatment. Similar treatments applied to the 8091 alloy result in a material competitive with 7150-T651 for specific apflications (e.g.upper wing skins, where short transverse performance is sacrificed in favour of in-plane properties).
Article
Aluminium alloys containing up to 3wt% lithium and cast using an ingot metallurgy route are currently being developed for aerospace applications. The objective of this paper is to review the current status of the metallurgical understanding of these alloys. Particular emphasis is placed on the alloy system Al-Li-Cu-Mg-Zr, which includes the alloys 8090 and 8090 developed in the United Kingdom by the Royal Aircraft Establishment and Alcan. Comparison is made with other alloy systems, where appropriate, e.g. Al-Li, Al-Li-Mg, Al-Cu-Li.
Article
This paper discusses the microstructural features of aluminium-lithium alloys that influence their corrosion and stress corrosion behaviour. Emphasis has been given to the effect of microstructure on stress corrosion crack initiation and propagation in Al-Li-Cu-Mg-Zr alloys (e.g. 8090 and 8091). In particular, the use of both isothermal and duplex ageing routes to develop overaged microstructures with increased resistance to stress corrosion crack initiation has been assessed and significantly, crack propagation plateau velocities of both under and peak aged 8090 were found to compare favourably with both 2024-T3 and 7075-T651. The applicability of accepted test methods developed for conventional medium and high strength aluminium alloys for determining exfoliation resistance of Al-Li-Cu-Mg-Zr alloys has been evaluated and results have been obtained which illustrate the hazards of relying on visual ratings to compare exfoliation performance. Underaged tempers of Al-Li-Cu-Mg-Zr alloys exhibit greatest resistance to both exfoliation and intergranular corrosion attack.
Article
Initiation and microcrack propagation stages were investigated by optical microscopic observations of fatigue darnage. The differences observed at high fatigue lives on S-N curves are found to correlate with the nature of initiation sites. It is shown that microcrack growth rates are equivalent for both 2024 T351 and 2091 T8 alloys, though long crack growth rates for 2091 T8 are lower as compared to those of 2024 T351.
Article
The principal mechanisms which govern the fatigue crack propagation resistance of aluminum-lithium alloys are investigated, with emphasis on their behavior in controlled gaseous and aqueous environments. Extensive data describe the growth kinetics of fatigue cracks in ingot metallurgy Al-Li alloys 2090, 2091, 8090, and 8091 and in powder metallurgy alloys exposed to moist air. Results are compared with data for traditional aluminum alloys 2024, 2124, 2618, 7075, and 7150. Crack growth is found to be dominated by shielding from tortuous crack paths and resultant asperity wedging. Beneficial shielding is minimized for small cracks, for high stress ratios, and for certain loading spectra. While water vapor and aqueous chloride environments enhance crack propagation, Al-Li-Cu alloys behave similarly to 2000-series aluminum alloys. Cracking in water vapor is controlled by hydrogen embrittlement, with surface films having little influence on cyclic plasticity.
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