《金属材料强韧化与组织调控》教学资源（参考文献）Recent developments in advanced aircraft aluminium alloys

Materials and Design 56(2014)862-871 Contents lists available at ScienceDirect Materials Materials and Design Design ELSEVIER journal homepage:www.elsevier.com/locate/matdes Review Recent developments in advanced aircraft aluminium alloys CrossMark Tolga Dursun a.*,Costas Soutisb Aselsan Inc,Ankara 06750,Turkey bAerospace Research Institute,University of Manchester,Manchester M13 9PL,UK ARTICLE INFO ABSTRACT Article history: Aluminium alloys have been the primary material for the structural parts of aircraft for more than Received 16 September 2013 80 years because of their well known performance,well established design methods,manufacturing Accepted 2 December 2013 and reliable inspection techniques.Nearly for a decade composites have started to be used more widely Available online 13 December 2013 in large commercial jet airliners for the fuselage,wing as well as other structural components in place of aluminium alloys due their high specific properties,reduced weight,fatigue performance and corrosion Keywords: resistance.Although the increased use of composite materials reduced the role of aluminium up to some Aircraft structures extent,high strength aluminium alloys remain important in airframe construction.Aluminium is a rela- Aluminium alloys Al-Li alloys tively low cost,light weight metal that can be heat treated and loaded to relatively high level of stresses, Composites and it is one of the most easily produced of the high performance materials,which results in lower man- Mechanical properties ufacturing and maintenance costs.There have been important recent advances in aluminium aircraft alloys that can effectively compete with modern composite materials.This study covers latest develop- ments in enhanced mechanical properties of aluminium alloys,and high performance joining techniques The mechanical properties on newly developed 2000,7000 series aluminium alloys and new generation Al-Li alloys are compared with the traditional aluminium alloys.The advantages and disadvantages of the joining methods,laser beam welding and friction stir welding,are also discussed. 2013 Elsevier Ltd.All rights reserved. 1.Introduction increasing tensile strength,elastic modulus or damage tolerance 1].Airframe durability is another parameter that directly affects The cost reduction for aircraft purchase and operation has be- costs.The cost of service and maintenance over the 30-year life come a driving force in many airline companies.Cost reduction of the aircraft are estimated to exceed the original purchase price can be achieved by decreasing the fuel consumption,maintenance by a factor of two[1.Therefore,both material producers and air- cost,operational costs,frequency of periodical controls and craft designers are working in harmony to reduce weight,improve increasing the service life and carrying more passengers at a time. damage tolerance,fatigue and corrosion resistance of the new Therefore aircraft manufacturers are competing to meet the metallic alloys.As a result,near future primary aircraft structures requirements of their airline customers.Weight reduction can im- will show an extended service life and require reduced frequency prove fuel consumption,increase payload and increase range. of inspections. Additionally,improved and optimised mechanical properties of Composite materials are increasingly being used in aircraft pri- the materials can result in increased period between maintenance mary structures (B787,Airbus A380,F35,and Typhoon).Fig.1 and reduce repair costs.Since the material has a great impact on shows the increased usage of composites in several types of Boeing cost reduction,airframe manufacturers and material producers fo- aircraft.The attractiveness of composites in the manufacturing of cus on the development of new materials to meet customer high performance structures relies on their superior mechanical requirements.Hence,a current challenge is to develop materials properties when compared to metals.such as higher specific stiff- that can be used in fuselage and wing construction with improve- ness,specific strength (normalised by density),fatigue and corro- ments in both structural performance and life cycle cost.According sion resistance.Although composites are thought to be the to the design trials it is seen that an effective way of reducing the preferable material for wing and fuselage structures,their higher aircraft weight is by reducing the material density.It is found that certification and production costs,relatively low resistance to im- the decrease in density is about 3-5 times more effective than pact and complicated mechanical behaviour due to change in envi- ronmental conditions (moisture absorption.getting soft/brittle when exposed to hot/cold environments)make designers to ex- Correspondng author.Tel.:+90 312 847 53 00. plore alternative material systems.Fibre metal laminates such as E-mail addresses:tdursun@aselsan.com.tr (T.Dursun).constantinos.soutis@ manchester.ac.uk (C.Soutis). GLARE which combines aluminium layers with glass fibre epoxy 0261-3069/$-see front matter 2013 Elsevier Ltd.All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.12.002

Review Recent developments in advanced aircraft aluminium alloys Tolga Dursun a,⇑ , Costas Soutis b a Aselsan Inc, Ankara 06750, Turkey b Aerospace Research Institute, University of Manchester, Manchester M13 9PL, UK article info Article history: Received 16 September 2013 Accepted 2 December 2013 Available online 13 December 2013 Keywords: Aircraft structures Aluminium alloys Al–Li alloys Composites Mechanical properties abstract Aluminium alloys have been the primary material for the structural parts of aircraft for more than 80 years because of their well known performance, well established design methods, manufacturing and reliable inspection techniques. Nearly for a decade composites have started to be used more widely in large commercial jet airliners for the fuselage, wing as well as other structural components in place of aluminium alloys due their high specific properties, reduced weight, fatigue performance and corrosion resistance. Although the increased use of composite materials reduced the role of aluminium up to some extent, high strength aluminium alloys remain important in airframe construction. Aluminium is a rela￾tively low cost, light weight metal that can be heat treated and loaded to relatively high level of stresses, and it is one of the most easily produced of the high performance materials, which results in lower man￾ufacturing and maintenance costs. There have been important recent advances in aluminium aircraft alloys that can effectively compete with modern composite materials. This study covers latest develop￾ments in enhanced mechanical properties of aluminium alloys, and high performance joining techniques. The mechanical properties on newly developed 2000, 7000 series aluminium alloys and new generation Al–Li alloys are compared with the traditional aluminium alloys. The advantages and disadvantages of the joining methods, laser beam welding and friction stir welding, are also discussed. 2013 Elsevier Ltd. All rights reserved. 1. Introduction The cost reduction for aircraft purchase and operation has be￾come a driving force in many airline companies. Cost reduction can be achieved by decreasing the fuel consumption, maintenance cost, operational costs, frequency of periodical controls and increasing the service life and carrying more passengers at a time. Therefore aircraft manufacturers are competing to meet the requirements of their airline customers. Weight reduction can im￾prove fuel consumption, increase payload and increase range. Additionally, improved and optimised mechanical properties of the materials can result in increased period between maintenance and reduce repair costs. Since the material has a great impact on cost reduction, airframe manufacturers and material producers fo￾cus on the development of new materials to meet customer requirements. Hence, a current challenge is to develop materials that can be used in fuselage and wing construction with improve￾ments in both structural performance and life cycle cost. According to the design trials it is seen that an effective way of reducing the aircraft weight is by reducing the material density. It is found that the decrease in density is about 3–5 times more effective than increasing tensile strength, elastic modulus or damage tolerance [1]. Airframe durability is another parameter that directly affects costs. The cost of service and maintenance over the 30-year life of the aircraft are estimated to exceed the original purchase price by a factor of two [1]. Therefore, both material producers and air￾craft designers are working in harmony to reduce weight, improve damage tolerance, fatigue and corrosion resistance of the new metallic alloys. As a result, near future primary aircraft structures will show an extended service life and require reduced frequency of inspections. Composite materials are increasingly being used in aircraft pri￾mary structures (B787, Airbus A380, F35, and Typhoon). Fig. 1 shows the increased usage of composites in several types of Boeing aircraft. The attractiveness of composites in the manufacturing of high performance structures relies on their superior mechanical properties when compared to metals, such as higher specific stiff￾ness, specific strength (normalised by density), fatigue and corro￾sion resistance. Although composites are thought to be the preferable material for wing and fuselage structures, their higher certification and production costs, relatively low resistance to im￾pact and complicated mechanical behaviour due to change in envi￾ronmental conditions (moisture absorption, getting soft/brittle when exposed to hot/cold environments) make designers to ex￾plore alternative material systems. Fibre metal laminates such as GLARE which combines aluminium layers with glass fibre epoxy 0261-3069/$ - see front matter 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.12.002 ⇑ Correspondng author. Tel.: +90 312 847 53 00. E-mail addresses: tdursun@aselsan.com.tr (T. Dursun), constantinos.soutis@ manchester.ac.uk (C. Soutis). Materials and Design 56 (2014) 862–871 Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes

T.Dursun.C.Soutis /Materials and Design 56(2014)862-871 863 plies to improve tensile strength and more importantly damage and Alclad 2524-T3 sheet and 2524-T351 plate for the fuselage tolerance are finding great use in aerospace applications [3-12]. skin.They also developed 7150-T7751 extrusions for the support- Impact resistance,effect of damage on stiffness/strength especially ing members of the fuselage structure.The application of these when loaded in compression and damage identification and detec- materials saved thousands of pounds of weight for the Boeing tion,in addition to joints,repair and recycling remain big chal- 777[191. lenges for composites with the need of further research [13-18] The aircraft manufacturers are also working to decrease the Aluminium alloys have been the primary structural material for number of parts in new aircraft.These needs could be met by commercial and military aircraft for almost 80 years due to their applying several approaches.The first method is producing large well known mechanical behaviour,easiness with design,mature and thick plates having fatigue and fracture characteristics equiv- manufacturing processes and inspection techniques,and will re- alent to those of a thin plate.The second method is implementa- main so for some time to come.However,the non-metallic mate- tion of joining technologies such as friction stir welding that rials,despite the issues mentioned earlier,due to their superior allows the manufacture of large integrally stiffened panels that specific strength properties provide a very competitive alternative, can be used for wing and fuselage skins [201. so aluminium producers need to keep investing and put great This review article covers the latest developments related to effort in improving the thermo-mechanical properties of the alu- aluminium alloys used as aircraft primary structures and high- minium alloys they produce. lights performance improvements in the 2000,7000 series alumin- Density.strength,Young's modulus,fatigue resistance,fracture ium alloys as well as the new generation of Al-Li alloys.Currently toughness and corrosion resistance are all important parameters the 7000 series Al-Zn alloys are used where the main limiting de- that need to be improved.Depending on the particular component sign parameter is strength:2000 series Al-Cu alloys are used for under consideration,material properties have to outperform those fatigue critical applications since these alloys are more damage tol- offered by polymer composites.Chemical composition and erant,while Al-Li alloys are chosen where high stiffness and lower processing control the microstructural features such as precipi- density are required.The advantages and disadvantages of the tates,dispersoids,degree of recrystallization,grain size and shape, joining techniques,laser beam welding and friction stir welding. crystallographic texture and intermetallic constituent particles. are also discussed. These properties affect the physical,mechanical and corrosion characteristics of aluminium alloys.Therefore material producers 2.Developments in 2000 series Al-Cu aluminium alloys working closely with aircraft designers could design different types of metallic alloys where the physical and mechanical properties The aluminium-copper(2000 series)alloys are the primary al- have been tailored to the specified needs.For instance,the upper loys used in airframe structural applications where the main de- side of the wing is mainly subjected to compression loading during sign criterion is damage tolerance.The 2000 series alloys flight,but also exposed to tension during static weight and taxiing. containing magnesium have higher strength resulting from the while the opposite happens to the lower part of the wing.hence precipitation of Al2Cu and AlzCuMg phases and superior damage careful optimisation of tensile and compressive strength properties tolerance and good resistance to fatigue crack growth compared is required.Damage tolerance,fatigue and corrosion resistance are to other series of aluminium alloys.2024 and 2014 are well known also needed making the selection and optimisation more examples for Al-Cu-Mg alloys.It is well known that due to differ- challenging. ent loading conditions each component of the airframe requires During the design of Boeing 777,aluminium manufacturers different material properties for optimum and reliable design. were asked for improvements in upper-wing structure and fuse- The fuselage is subject to cabin pressure (tension)and shear loads. lage.Higher compressive yield strength was needed for the upper longitudinal stringers are exposed to the longitudinal tension and wing structure.Improved corrosion resistance was also desirable. compression loads due to bending.circumferential frames have For the case of fuselage,higher damage tolerance and durability to sustain the fuselage shape and redistribute loads into the skin. than the incumbent 2024-T3 was needed.Aluminium manufactur- Strength,stiffness,fatigue crack initiation resistance,fatigue crack ers accounting for the designer's needs developed the 7055-T7751 growth rate,fracture toughness and corrosion resistance all are plate and 7055-177511 extrusions for the upper wing structure, important,but fracture toughness(resistance to crack growth)is 49%1%1% 5% 1% 13% 14% Aluminium Steel 747 767 Titanium 81% 80% Composite ■Misc 6% 3% 1% 11% 1% 5% 20% 12% % 777 T87 757 11% 10% 70% 78% 50% 15% Fig.1.Combination of materials used in Boeing Aircrafts.The figure is based on [2]

plies to improve tensile strength and more importantly damage tolerance are finding great use in aerospace applications [3–12]. Impact resistance, effect of damage on stiffness/strength especially when loaded in compression and damage identification and detec￾tion, in addition to joints, repair and recycling remain big chal￾lenges for composites with the need of further research [13–18]. Aluminium alloys have been the primary structural material for commercial and military aircraft for almost 80 years due to their well known mechanical behaviour, easiness with design, mature manufacturing processes and inspection techniques, and will re￾main so for some time to come. However, the non-metallic mate￾rials, despite the issues mentioned earlier, due to their superior specific strength properties provide a very competitive alternative, so aluminium producers need to keep investing and put great effort in improving the thermo-mechanical properties of the alu￾minium alloys they produce. Density, strength, Young’s modulus, fatigue resistance, fracture toughness and corrosion resistance are all important parameters that need to be improved. Depending on the particular component under consideration, material properties have to outperform those offered by polymer composites. Chemical composition and processing control the microstructural features such as precipi￾tates, dispersoids, degree of recrystallization, grain size and shape, crystallographic texture and intermetallic constituent particles. These properties affect the physical, mechanical and corrosion characteristics of aluminium alloys. Therefore material producers working closely with aircraft designers could design different types of metallic alloys where the physical and mechanical properties have been tailored to the specified needs. For instance, the upper side of the wing is mainly subjected to compression loading during flight, but also exposed to tension during static weight and taxiing, while the opposite happens to the lower part of the wing, hence careful optimisation of tensile and compressive strength properties is required. Damage tolerance, fatigue and corrosion resistance are also needed making the selection and optimisation more challenging. During the design of Boeing 777, aluminium manufacturers were asked for improvements in upper-wing structure and fuse￾lage. Higher compressive yield strength was needed for the upper wing structure. Improved corrosion resistance was also desirable. For the case of fuselage, higher damage tolerance and durability than the incumbent 2024-T3 was needed. Aluminium manufactur￾ers accounting for the designer’s needs developed the 7055-T7751 plate and 7055-T77511 extrusions for the upper wing structure, and Alclad 2524-T3 sheet and 2524-T351 plate for the fuselage skin. They also developed 7150-T7751 extrusions for the support￾ing members of the fuselage structure. The application of these materials saved thousands of pounds of weight for the Boeing 777 [19]. The aircraft manufacturers are also working to decrease the number of parts in new aircraft. These needs could be met by applying several approaches. The first method is producing large and thick plates having fatigue and fracture characteristics equiv￾alent to those of a thin plate. The second method is implementa￾tion of joining technologies such as friction stir welding that allows the manufacture of large integrally stiffened panels that can be used for wing and fuselage skins [20]. This review article covers the latest developments related to aluminium alloys used as aircraft primary structures and high￾lights performance improvements in the 2000, 7000 series alumin￾ium alloys as well as the new generation of Al–Li alloys. Currently the 7000 series Al–Zn alloys are used where the main limiting de￾sign parameter is strength; 2000 series Al–Cu alloys are used for fatigue critical applications since these alloys are more damage tol￾erant, while Al–Li alloys are chosen where high stiffness and lower density are required. The advantages and disadvantages of the joining techniques, laser beam welding and friction stir welding, are also discussed. 2. Developments in 2000 series Al–Cu aluminium alloys The aluminium–copper (2000 series) alloys are the primary al￾loys used in airframe structural applications where the main de￾sign criterion is damage tolerance. The 2000 series alloys containing magnesium have higher strength resulting from the precipitation of Al2Cu and Al2CuMg phases and superior damage tolerance and good resistance to fatigue crack growth compared to other series of aluminium alloys. 2024 and 2014 are well known examples for Al–Cu–Mg alloys. It is well known that due to differ￾ent loading conditions each component of the airframe requires different material properties for optimum and reliable design. The fuselage is subject to cabin pressure (tension) and shear loads, longitudinal stringers are exposed to the longitudinal tension and compression loads due to bending, circumferential frames have to sustain the fuselage shape and redistribute loads into the skin. Strength, stiffness, fatigue crack initiation resistance, fatigue crack growth rate, fracture toughness and corrosion resistance all are important, but fracture toughness (resistance to crack growth) is Fig. 1. Combination of materials used in Boeing Aircrafts. The figure is based on [2]. T. Dursun, C. Soutis / Materials and Design 56 (2014) 862–871 863

864 T.Dursun,C.Soutis/Materials and Design 56(2014)862-871 often the limiting design parameter [21.The wing can be 3.Developments in 7000 series Al-Zn aluminium alloys considered as a cantilever type of beam that is loaded in bending during flight but also torsion.The wing supports both the static The 7000 series of aluminium alloys show higher strength when weight of the aircraft and any additional loads subjected in service. compared to other classes of aluminium alloys and are selected in Additional wing loads also come from the landing gear during the fabrication of upper wing skins,stringers and horizontal/verti- taxiing.take-off and landing and from the leading and trailing cal stabilizers.The compressive strength and the fatigue resistance edge the flaps and slats that are deployed during take-off and are the critical parameters in the design of upper wing structural landing to create additional low speed lift.The upper surface components.The tail of the airplane,also called the empennage, of the wing is primarily loaded in compression because of the consists of a horizontal stabilizer,a vertical stabilizer or fin,and upward bending moment during flight but can be loaded in control surfaces e.g.elevators and rudder.Structural design of both tension while taxiing[21].Chemical compositions and mechan- the horizontal and vertical stabilizers is essentially the same as for ical properties of some of 2000 series aluminium alloys widely the wing.Both the upper and lower surfaces of the horizontal sta- used in airframe design are given in Tables 1 and 2 bilizer are often critical in compression loading due to bending respectively. 21l. The 2024-T3 has been one of the most widely used alloys in High strength aluminium alloys such as the 7075-T6 are widely fuselage construction.It has moderate yield strength,very good used in aircraft structures due to their high strength-to-weight ra- resistance to fatigue crack growth and good fracture toughness. tio,machinability and relatively low cost.However,due to their The 2024 aluminium alloy remains as an important aircraft struc- compositions,these alloys are susceptible to corrosion.It is well tural material due to its extremely good damage tolerance and known that corrosion reduces the life of aircraft structures consid- high resistance to fatigue crack propagation in 13 aged condition. erably.During normal operation aircraft are subjected to natural The low yield stress level and relatively low fracture toughness. corrosive environments due to humidity,rain,temperature,oil, limit the application of this alloy in the highly stressed regions hydraulic fluids and salt water.Among the issues facing ageing air- [23].Microstructural effects on the fatigue properties of alumin- craft,corrosion in combination with fatigue is extremely undesir- ium alloys are being investigated intensively.Improvements in able[27]. compositional control and processing have continually produced The 7000 series alloys are also heat treatable,and the Al-Zn- new alloys.It is known that inclusions have substantial effects Mg-Cu versions provide the highest strengths of all aluminium al- on the fatigue crack propagation.Higher fracture toughness values loys.Some of the 7000 series alloys contain about 2%copper in and better resistance to fatigue crack initiation and crack growth combination with magnesium and zinc to improve their strength. were achieved by reducing impurities,especially iron and silicon These alloys although are the strongest they are the least corrosion It has been announced that for the fuselage applications the alloy resistant of the 7000 series.However,newer 7000 series alloys 2524-T3 has a 15-20%improvement in fracture toughness and introduced have higher fatigue and corrosion resistance which twice fatigue crack growth resistance of 2024-T3 24].This may result in weight savings.Newer alloys such as the 7055-T77, improvement leads to weight savings and 30-40%longer service have higher strength and damage tolerance than the 7075-T6 [1. life [25].The 2524 aluminium alloy has replaced the 2024 as fuse- The 7475 (Al-Zn-Mg-Cu)aluminium alloy is a modified version lage skin in the Boeing 777 aircraft.Fatigue tests on the 2524 alloy of 7075 alloy.The 7475 alloy is developed for applications that re- showed that fatigue strength of this alloy is 70%of the yield quire a combination of higher strength,fracture toughness and strength whereas for 2024-T351 fatigue strength is about 45%of resistance to fatigue crack propagation both in air and corrosive the yield strength[26].For the lower wing skin applications[27] environment.Both strength and fracture toughness properties of the 2224-T351 and 2324-T39 alloys offer higher strength values 7075 alloy are improved by decreasing its contents of iron and sil- compared to incumbent 2024-T351 with similar fracture tough- icon,and changing both quenching and ageing conditions.The to- ness and corrosion resistance.Compared to 2024,both composi- tal iron and silicon content in 7075 is 0.90%whereas in 7475 the tional and processing changes for 2224-T351 and 2324-T39 total content is limited to 0.22%.These changes in the 7075 alloy alloys resulted in improved properties.A lower volume fraction resulted in the development of the 7475 alloy which is having a of intermetallic compounds improved fracture toughness.For in- fine grain size,optimum dispersion and highest toughness value stance the maximum iron content is 0.12%and silicon is 0.10%in among the aluminium alloys available at high strength level.It is 2224-T351 whereas in 2024 0.50%for both impurities.A newly also reported that the corrosion resistance and corrosion fatigue developed aluminium alloy 2026 is based on 2024 but it contains behaviour of the 7475 alloy are excellent.In general,its perfor- fewer impurities such as iron and silicon.Additionally,2026 con- mance is better than that of much commercially available high tains a small amount of zirconium which inhibits recrystallization strength aerospace aluminium alloys such as 7050 and 7075 alloys 28].2026 has higher damage tolerance,higher tensile strength, 23].Yield strength,elongation,and Kic properties of widely used higher fatigue performance and acceptable fracture toughness 2024 and 7075 alloys are compared with 7050 and 7475 in Fig.2. compared to 2024 and 2224 291. It may be seen in Fig.2 that the 2024-T351 alloy has high duc- Although the contribution of Cu and Mg in intermetallic phases tility and good fracture toughness (both in TL and LT orientations) results in high strength however,due to the intermetallic phase but has relatively low yield strength.On the other hand,the 7075 particles the corrosion resistance of the alloy significantly drops alloy under T651 temper condition has yield strength of over Several investigations have been done in order to increase both 500 MPa.The reported fracture toughness of this alloy (7075- corrosion and fatigue resistance of 2000 series alloys [30-32] T651)in TL and LT orientations is nearly 24 MPavm and Table 1 Chemical composition of some 2000 series aerospace aluminium alloys [221. 2000 Series Cu Zn Mg Mn Fe Si Cr Zr Ti 2024 4.4 1.5 0.6 ≤0.5 ≤0.5 0.1 0.15 Remainder 2026 3.6-4.3 0.1 1.0-1.6 0.3-0.8 0.07 0.05 0.05-0.25 0.06 Remainder 2224 4.1 1.5 0.6 ≤0.15 ≤0.12 Remainder 2324 3.8-4.4 0.25 12-1.8 03-0.9 012 0.1 0.1 0.15 Remainder 2524 4.0-4.5 0.15 1.2-1.6 0.45-0.7 0.12 0.06 0.05 0.1 Remainder

often the limiting design parameter [21]. The wing can be considered as a cantilever type of beam that is loaded in bending during flight but also torsion. The wing supports both the static weight of the aircraft and any additional loads subjected in service. Additional wing loads also come from the landing gear during taxiing, take-off and landing and from the leading and trailing edge the flaps and slats that are deployed during take-off and landing to create additional low speed lift. The upper surface of the wing is primarily loaded in compression because of the upward bending moment during flight but can be loaded in tension while taxiing [21]. Chemical compositions and mechan￾ical properties of some of 2000 series aluminium alloys widely used in airframe design are given in Tables 1 and 2 respectively. The 2024-T3 has been one of the most widely used alloys in fuselage construction. It has moderate yield strength, very good resistance to fatigue crack growth and good fracture toughness. The 2024 aluminium alloy remains as an important aircraft struc￾tural material due to its extremely good damage tolerance and high resistance to fatigue crack propagation in T3 aged condition. The low yield stress level and relatively low fracture toughness, limit the application of this alloy in the highly stressed regions [23]. Microstructural effects on the fatigue properties of alumin￾ium alloys are being investigated intensively. Improvements in compositional control and processing have continually produced new alloys. It is known that inclusions have substantial effects on the fatigue crack propagation. Higher fracture toughness values and better resistance to fatigue crack initiation and crack growth were achieved by reducing impurities, especially iron and silicon. It has been announced that for the fuselage applications the alloy 2524-T3 has a 15–20% improvement in fracture toughness and twice fatigue crack growth resistance of 2024-T3 [24]. This improvement leads to weight savings and 30–40% longer service life [25]. The 2524 aluminium alloy has replaced the 2024 as fuse￾lage skin in the Boeing 777 aircraft. Fatigue tests on the 2524 alloy showed that fatigue strength of this alloy is 70% of the yield strength whereas for 2024-T351 fatigue strength is about 45% of the yield strength [26]. For the lower wing skin applications [27] the 2224-T351 and 2324-T39 alloys offer higher strength values compared to incumbent 2024-T351 with similar fracture tough￾ness and corrosion resistance. Compared to 2024, both composi￾tional and processing changes for 2224-T351 and 2324-T39 alloys resulted in improved properties. A lower volume fraction of intermetallic compounds improved fracture toughness. For in￾stance the maximum iron content is 0.12% and silicon is 0.10% in 2224-T351 whereas in 2024 0.50% for both impurities. A newly developed aluminium alloy 2026 is based on 2024 but it contains fewer impurities such as iron and silicon. Additionally, 2026 con￾tains a small amount of zirconium which inhibits recrystallization [28]. 2026 has higher damage tolerance, higher tensile strength, higher fatigue performance and acceptable fracture toughness compared to 2024 and 2224 [29]. Although the contribution of Cu and Mg in intermetallic phases results in high strength however, due to the intermetallic phase particles the corrosion resistance of the alloy significantly drops. Several investigations have been done in order to increase both corrosion and fatigue resistance of 2000 series alloys [30–32]. 3. Developments in 7000 series Al–Zn aluminium alloys The 7000 series of aluminium alloys show higher strength when compared to other classes of aluminium alloys and are selected in the fabrication of upper wing skins, stringers and horizontal/verti￾cal stabilizers. The compressive strength and the fatigue resistance are the critical parameters in the design of upper wing structural components. The tail of the airplane, also called the empennage, consists of a horizontal stabilizer, a vertical stabilizer or fin, and control surfaces e.g. elevators and rudder. Structural design of both the horizontal and vertical stabilizers is essentially the same as for the wing. Both the upper and lower surfaces of the horizontal sta￾bilizer are often critical in compression loading due to bending [21]. High strength aluminium alloys such as the 7075-T6 are widely used in aircraft structures due to their high strength-to-weight ra￾tio, machinability and relatively low cost. However, due to their compositions, these alloys are susceptible to corrosion. It is well known that corrosion reduces the life of aircraft structures consid￾erably. During normal operation aircraft are subjected to natural corrosive environments due to humidity, rain, temperature, oil, hydraulic fluids and salt water. Among the issues facing ageing air￾craft, corrosion in combination with fatigue is extremely undesir￾able [27] . The 7000 series alloys are also heat treatable, and the Al–Zn– Mg–Cu versions provide the highest strengths of all aluminium al￾loys. Some of the 7000 series alloys contain about 2% copper in combination with magnesium and zinc to improve their strength. These alloys although are the strongest they are the least corrosion resistant of the 7000 series. However, newer 7000 series alloys introduced have higher fatigue and corrosion resistance which may result in weight savings. Newer alloys such as the 7055-T77, have higher strength and damage tolerance than the 7075-T6 [1]. The 7475 (Al–Zn–Mg–Cu) aluminium alloy is a modified version of 7075 alloy. The 7475 alloy is developed for applications that re￾quire a combination of higher strength, fracture toughness and resistance to fatigue crack propagation both in air and corrosive environment. Both strength and fracture toughness properties of 7075 alloy are improved by decreasing its contents of iron and sil￾icon, and changing both quenching and ageing conditions. The to￾tal iron and silicon content in 7075 is 0.90% whereas in 7475 the total content is limited to 0.22%. These changes in the 7075 alloy resulted in the development of the 7475 alloy which is having a fine grain size, optimum dispersion and highest toughness value among the aluminium alloys available at high strength level. It is also reported that the corrosion resistance and corrosion fatigue behaviour of the 7475 alloy are excellent. In general, its perfor￾mance is better than that of much commercially available high strength aerospace aluminium alloys such as 7050 and 7075 alloys [23]. Yield strength, % elongation, and KIC properties of widely used 2024 and 7075 alloys are compared with 7050 and 7475 in Fig. 2. It may be seen in Fig. 2 that the 2024-T351 alloy has high duc￾tility and good fracture toughness (both in TL and LT orientations) but has relatively low yield strength. On the other hand, the 7075 alloy under T651 temper condition has yield strength of over 500 MPa. The reported fracture toughness of this alloy (7075- T651) in TL and LT orientations is nearly 24 MPapm and Table 1 Chemical composition of some 2000 series aerospace aluminium alloys [22]. 2000 Series Cu Zn Mg Mn Fe Si Cr Zr Ti Al 2024 4.4 – 1.5 0.6 60.5 60.5 0.1 – 0.15 Remainder 2026 3.6–4.3 0.1 1.0–1.6 0.3–0.8 0.07 0.05 – 0.05–0.25 0.06 Remainder 2224 4.1 – 1.5 0.6 60.15 60.12 – – – Remainder 2324 3.8–4.4 0.25 1.2–1.8 0.3–0.9 0.12 0.1 0.1 – 0.15 Remainder 2524 4.0–4.5 0.15 1.2–1.6 0.45–0.7 0.12 0.06 0.05 – 0.1 Remainder 864 T. Dursun, C. Soutis / Materials and Design 56 (2014) 862–871

T.Dursun.C.Soutis/Materials and Design 56(2014)862-871 865 Table 2 Mechanical properties of some 2000 series aerospace aluminium alloys [221. 2000 Series UTS(MPa) Yield Strength(MPa) Fracture Toughness.Kic(MPa m'R) Elongation ( 2024-T351 428 324 37 1 2026-T3511 496 365 NA 11 2224-T39 476 345 0 2324-T39 475 370 38.5-44.0 8 2524-T3 434 306 40(TL) 24 27 MPaym,respectively which corresponds to low level of ductil- 350 ity.The 7475-T7351 alloy has higher fracture toughness ◆7475-7351 ▲7075T6 (42 MPaym and 52 MPay/m in TL and LT orientations,respec- 300 02024-T3 tively)whereas,in comparison to the 7075-T651 alloy,the 7475- T7351 alloy has marginally inferior yield strength but slightly superior ductility.In view of these facts,the use of appropriately EdW'ssans 250 treated 7475 alloy is expected to safely reduce the overall weight of aerospace structure,an important criterion for such applications [231. 200 In Fig.3 fatigue crack growth rates for different aluminium al- loys are compared.It is shown that the 7475 has higher fatigue 150 resistance compared to the 2024,while the 7075-T6 has the lowest fatigue resistance. Corrosion resistance and fatigue behaviour of alloy 7475 are 100 equal to/or better than many of the high strength aluminium alloys 10 105 105 10 10 such as the 7075,7050 and 2024.Alloy 7475 plate and sheet are No.of stress cycles to failure currently being selected for fracture critical components of high performance aircraft applications [33]. Fig.3.S-N curves for different aluminium alloys [23]. Alloy 7050 is another important alloy having the good balance of strength,stress corrosion cracking (SCC)resistance and A recent alloy,the 7055-T7751 (Al-8Zn-2.05Mg-2.3Cu- toughness.It is particularly suited for plate applications in the 0.16Zr).has a yield stress that may exceed 620 MPa and the esti- 76-152 mm thickness range.Alloy 7050 exhibits better tough- mated weight saving attributed to its use for components in the ness/corrosion resistance characteristics than alloy 7075 because Boeing aircraft 777 is 635 kg 341.This alloy provided a nearly it is less quench sensitive than most aerospace aluminium alloys. 10%gain in strength,with higher toughness and significantly im- The 7050 retains its strength properties in thicker sections while proved corrosion resistance [24].T77 temper consists of three step maintaining good stress corrosion cracking resistance and fracture ageing process that produces a higher strength and damage toler- toughness levels.Typical applications for alloy 7050 plates include ance combinations compared to 7050-T76 and 7150-T651 or fuselage frames and bulkheads where section thicknesses are 17751.The improved fracture toughness is a result of controlled 50-152 mm.On the other hand alloy 7050 sheets are used in wing volume fraction of coarse intermetallic particles and uncrystallized skins applications.Long-term controlled and in-service evaluations grain structure.Good combination of strength and corrosion resis- have shown that alloy 7050 plate and sheet products remain tance is attributed to the size and spatial distribution and the cop- equal exfoliation and stress corrosion resistance at higher stress per content of the strengthening precipitates. levels compared with other high strength aluminium alloys such There exists a continuous improvement in the mechanical prop- as7075. erties of aerospace aluminium alloys.This has resulted in the development of high strength 7xxx alloys(e.g.7075,7150,7055. 7449,in chronological order of application).These high strength al- 600 loys are generally used in compression-dominated parts such as upper wing skins where damage tolerance considerations are sec- 500 ondary.However,recent developments show that modifications in solute content and in particular in Zn/Mg/Cu ratios can enable the development of high strength products with significant improve- 400 ments in damage tolerance such as AA7040,AA7140 and AA7085.7085 has been developed as the new generation high 300 strength thick plate alloy to be alternative for 7050/7010 products. Due to the higher Zinc and lower Cu contents,higher fracture 200 toughness and slow quench sensitivity were obtained.This product was selected for wing spar applications on the Airbus A380.There (edw) is also an effort to obtain a good combination of high strength and 100 good corrosion resistance through the applications of different heat treatment methods [35].Two important metallurgical princi- ples resulting in improvements are:a decrease in the Mg/Zn ratio. 2024.T351 7050-T73651 7075-T651 7475-T7351 and an overall reduction in saturation of the composition with re- Yield Strength%Elongation KIc TL Direction Kic LT Direction spect to the theoretical maximum solubility.The strong impact of Mg concentration increases on strength(beneficial)and on tough- Fig.2.Comparative representation of yield strength.%elongation,and Kic in ness (detrimental)is well known.The basis of the Mg/Zn adjust- different aluminium alloys.The figure is based on [23]. ments is the observation that a partial replacement of Mg with

27 MPapm, respectively which corresponds to low level of ductil￾ity. The 7475-T7351 alloy has higher fracture toughness (42 MPapm and 52 MPapm in TL and LT orientations, respec￾tively) whereas, in comparison to the 7075-T651 alloy, the 7475- T7351 alloy has marginally inferior yield strength but slightly superior ductility. In view of these facts, the use of appropriately treated 7475 alloy is expected to safely reduce the overall weight of aerospace structure, an important criterion for such applications [23]. In Fig. 3 fatigue crack growth rates for different aluminium al￾loys are compared. It is shown that the 7475 has higher fatigue resistance compared to the 2024, while the 7075-T6 has the lowest fatigue resistance. Corrosion resistance and fatigue behaviour of alloy 7475 are equal to/or better than many of the high strength aluminium alloys such as the 7075, 7050 and 2024. Alloy 7475 plate and sheet are currently being selected for fracture critical components of high performance aircraft applications [33]. Alloy 7050 is another important alloy having the good balance of strength, stress corrosion cracking (SCC) resistance and toughness. It is particularly suited for plate applications in the 76–152 mm thickness range. Alloy 7050 exhibits better tough￾ness/corrosion resistance characteristics than alloy 7075 because it is less quench sensitive than most aerospace aluminium alloys. The 7050 retains its strength properties in thicker sections while maintaining good stress corrosion cracking resistance and fracture toughness levels. Typical applications for alloy 7050 plates include fuselage frames and bulkheads where section thicknesses are 50–152 mm. On the other hand alloy 7050 sheets are used in wing skins applications. Long-term controlled and in-service evaluations have shown that alloy 7050 plate and sheet products remain equal exfoliation and stress corrosion resistance at higher stress levels compared with other high strength aluminium alloys such as 7075. A recent alloy, the 7055-T7751 (Al–8Zn–2.05Mg–2.3Cu– 0.16Zr), has a yield stress that may exceed 620 MPa and the esti￾mated weight saving attributed to its use for components in the Boeing aircraft 777 is 635 kg [34]. This alloy provided a nearly 10% gain in strength, with higher toughness and significantly im￾proved corrosion resistance [24]. T77 temper consists of three step ageing process that produces a higher strength and damage toler￾ance combinations compared to 7050-T76 and 7150-T651 or T7751. The improved fracture toughness is a result of controlled volume fraction of coarse intermetallic particles and uncrystallized grain structure. Good combination of strength and corrosion resis￾tance is attributed to the size and spatial distribution and the cop￾per content of the strengthening precipitates. There exists a continuous improvement in the mechanical prop￾erties of aerospace aluminium alloys. This has resulted in the development of high strength 7xxx alloys (e.g. 7075, 7150, 7055, 7449, in chronological order of application). These high strength al￾loys are generally used in compression-dominated parts such as upper wing skins where damage tolerance considerations are sec￾ondary. However, recent developments show that modifications in solute content and in particular in Zn/Mg/Cu ratios can enable the development of high strength products with significant improve￾ments in damage tolerance such as AA7040, AA7140 and AA7085. 7085 has been developed as the new generation high strength thick plate alloy to be alternative for 7050/7010 products. Due to the higher Zinc and lower Cu contents, higher fracture toughness and slow quench sensitivity were obtained. This product was selected for wing spar applications on the Airbus A380. There is also an effort to obtain a good combination of high strength and good corrosion resistance through the applications of different heat treatment methods [35]. Two important metallurgical princi￾ples resulting in improvements are: a decrease in the Mg/Zn ratio, and an overall reduction in saturation of the composition with re￾spect to the theoretical maximum solubility. The strong impact of Mg concentration increases on strength (beneficial) and on tough￾ness (detrimental) is well known. The basis of the Mg/Zn adjust￾ments is the observation that a partial replacement of Mg with Table 2 Mechanical properties of some 2000 series aerospace aluminium alloys [22]. 2000 Series UTS (MPa) Yield Strength (MPa) Fracture Toughness, KIC (MPa m1/2) Elongation (%) 2024-T351 428 324 37 21 2026-T3511 496 365 NA 11 2224-T39 476 345 53 10 2324-T39 475 370 38.5–44.0 8 2524-T3 434 306 40 (TL) 24 0 100 200 300 400 500 600 2024-T351 7050-T73651 7075-T651 7475-T7351 YS (MPa), %Elx0.1, Kıcx0.1MPa.m1/2 Yield Strength % Elongation Kıc TL Direction Kıc LT Direction Fig. 2. Comparative representation of yield strength, % elongation, and KIC in different aluminium alloys. The figure is based on [23]. Fig. 3. S–N curves for different aluminium alloys [23]. T. Dursun, C. Soutis / Materials and Design 56 (2014) 862–871 865

866 T.Dursun,C.Soutis/Materials and Design 56(2014)862-871 Zn (a slightly less effective hardener per wt.%)enables an increase fracture toughness problem being one of primarily low strength in in toughness while maintaining adequate strength.The overall the short transverse direction [1.21,44,451. reduction in solute saturation directly affects the quench sensitiv- The pressure for higher strength and improved fracture tough- ity,which is critical for damage tolerance properties of high solute ness with reduced weight in aircraft applications have resulted in alloys.AA7056-T79,developed for the upper wing skin of large the development of new generation of Al-Li alloys.The new gener- commercial aircraft is good example of the improvements in ation of Al-Li alloys provides not only weight savings,due to lower strength-toughness balance [34.On the other hand the addition density,but also overcomes the disadvantage of the previous prob- of Mn and Zr in aluminium alloys can form fine dispersoids which lems with increased corrosion resistance,good spectrum fatigue affect recrystallization characteristics and grain structure.These crack growth performance,a good strength and toughness combi- dispersoids retards recrystallization and grain growth.Zr content nation and compatibility with standard manufacturing techniques. in aluminium alloys can form A13Zr dispersoid,which have a rela- This results in well-balanced,light weight and high performance tionship with the matrix and significantly refines the grain size aluminium alloys [1.44,46.In the new generation (3rd)Al-Li The addition of Zn increases the strength of the alloy,whereas alloys Li concentration was reduced to 0.75-1.8 wt.%.The addition the addition of Mn increases the fracture toughness of the alloy of alloying elements in the 3rd generation Al-Li alloys is used to due to the formation of the secondary phase containing Mn and improve the mechanical properties.Poor corrosion resistance of Fe,which decreases the adverse effects of Fe on fracture toughness 2nd generation Al-Li alloys is eliminated in 3rd generation Al-Li [36.Chemical composition of some of the important 7000 series alloys by optimising alloy composition and temper.Also Zn aluminium alloys are given in Table 3. additions improved corrosion resistance.The additions of Cu,Li Fretting.a special type of wear process that occurs at the con- and Mg form the strengthening precipitates and small additions tact area between two materials under load and subject to very of the dispersoid-forming elements Zr and Mn control the grain small amount of relative motion,is another important issue structure and crystallographic texture during thermo-mechanical needed to be understood in bolted/pinned aircraft joints.There is processing.Crack deviation occurs due to high crystallographic a current focus on the prevention of fretting in the aerospace texture in addition with slip planarity.Deviation from expected industry since due to fretting,cracks can initiate at stresses (fret- direction of crack propagation makes it difficult to define inspec- ting zone),well below the fatigue limit of non-fretted materials tion points and the positioning of crack arresters.It was found that and the structure's resistance to fatigue can be decreased by 50- in addition to reduction of the texture components,the severity of 70%.Introduction of compressive residual stresses at the surface slip planarity had to be decreased.This reduction was achieved by of hole,reduction in coefficient of friction,increased surface hard- decreasing the amount of (AlsLi)phase.This can be achieved by ness,changing the surface chemistry and increasing the surface keeping the amount of Li additions below 1.8 wt ptc.The fracture roughness are the main methods that are applied to reduce the toughness of 2nd generation Al-Li alloys was often lower than the nucleation and growth of fretting cracks and improve the fatigue incumbent 2024 alloy products for designs where damage toler- life of aerospace joints and improve fretting resistance 37-42. ance is the driving parameter.It was determined that fracture toughness is affected only by insoluble second-phase particles.In 4.Developments in aluminium-lithium alloys 3rd generation Al-Li alloys like 2199 this disadvantageous condi- tion was eliminated by composition optimisation,thermal- Reducing the density of materials is accepted as the most effec- mechanical processing and precipitate microstructure control. tive way of lowering the structural weight of aircraft.Li (density Chemical compositions and mechanical properties of some of 0.54 g/cm)is one of the few elements that have a high solubility the widely used Al-Li alloys are shown in Tables 4 and 5 in aluminium.This is significant because,for each 1%added,the respectively. density of an aluminium alloy is reduced by 3%.Lithium is also un- Alloy 2195,a new generation Al-Li alloy,has a lower copper ique amongst the more soluble alloying elements in that it causes a content and has replaced the 2219 for the cryogenic fuel tank on considerable increase in the elastic modulus (6%for each 1%Li the space shuttle where it provides a higher strength,higher mod- added).Additional advantage is that,aluminium alloys containing ulus and lower density than the 2219.Other alloys,including the Li respond to age hardening [43]. 2096,2097 and 2197,also have lower copper contents but also The use of aluminium-lithium(Al-Li)alloys in aerospace appli- have slightly higher lithium contents than 2195 [1].New genera- cations goes back to 1950s with the development of alloy 2020.In tion of Al-Li alloys have higher Cu/Li ratio than the second gener- the 1980s,2nd generation of Al-Li alloys were developed.The sec- ation alloys (2090 and 2091)as illustrated in Fig.4. ond generation alloys included the 2090,2091,8090 and 8091.The The new generation of 2199 Al-Li alloys sheet and plates found Al-Li alloys 2090,2091,8090 and 8091 contain 1.9-2.7%lithium. applications in the aircraft for fuselage and lower wing applica- which results in an about 10%lower density and 25%higher spe- tions,respectively and the 2099 extrusions for internal structure. cific stiffness than the 2000 and 7000 series alloys.However,due It was determined that the 2199-T8E79 plate for the lower wing to technical problems such as anisotropy in the mechanical prop- skin,the 2099-T83 extrusions for lower wing stringers and the erties,low toughness,poor corrosion resistance,manufacturing is- 2199-T8 prime sheet for fuselage skin would provide the most sues (hole cracking and delamination during drilling),2nd benefit for the given applications examined.It is stated that com- generation Al-Li alloys did not find wide use in aircraft industry. pared to 2024,the 2199 plates have lower density.significantly The anisotropy experienced by these alloys is a result of the strong better stress corrosion and exfoliation corrosion resistance,signif- crystallographic textures that develop during processing,with the icantly better spectrum fatigue crack growth performance,better Table 3 Chemical composition of some 7000 series aerospace aluminium alloys [221. 7000 Series Cu Zn Mg Mn Fe Si Cr Zr Ti 7050 2.3 6.2 225 ≤0.15 ≤0.12 0.1 Remainder 7055 2.0-2.6 7.6-8.4 1.8-2.3 0.05 0.15 0.1 0.04 0.08-0.25 0.06 Remainder 7075 1.2-2.0 5.1-6.1 2.1-2.9 03 0.5 0.4 0.18-0.28 02 Remainder 7150 1.9-2.5 5.9-6.9 2.0-2.7 0.1 0.15 0.12 0.04 0.08-0.15 0.06 Remainder 7475 1.2-1.9 5.2-6.2 19-2.6 0.06 0.12 0.10 0.18-0.25 0.06 Remainder

Zn (a slightly less effective hardener per wt.%) enables an increase in toughness while maintaining adequate strength. The overall reduction in solute saturation directly affects the quench sensitiv￾ity, which is critical for damage tolerance properties of high solute alloys. AA7056-T79, developed for the upper wing skin of large commercial aircraft is good example of the improvements in strength-toughness balance [34]. On the other hand the addition of Mn and Zr in aluminium alloys can form fine dispersoids which affect recrystallization characteristics and grain structure. These dispersoids retards recrystallization and grain growth. Zr content in aluminium alloys can form A13Zr dispersoid, which have a rela￾tionship with the matrix and significantly refines the grain size. The addition of Zn increases the strength of the alloy, whereas the addition of Mn increases the fracture toughness of the alloy due to the formation of the secondary phase containing Mn and Fe, which decreases the adverse effects of Fe on fracture toughness [36]. Chemical composition of some of the important 7000 series aluminium alloys are given in Table 3. Fretting, a special type of wear process that occurs at the con￾tact area between two materials under load and subject to very small amount of relative motion, is another important issue needed to be understood in bolted/pinned aircraft joints. There is a current focus on the prevention of fretting in the aerospace industry since due to fretting, cracks can initiate at stresses (fret￾ting zone), well below the fatigue limit of non-fretted materials and the structure’s resistance to fatigue can be decreased by 50– 70%. Introduction of compressive residual stresses at the surface of hole, reduction in coefficient of friction, increased surface hard￾ness, changing the surface chemistry and increasing the surface roughness are the main methods that are applied to reduce the nucleation and growth of fretting cracks and improve the fatigue life of aerospace joints and improve fretting resistance [37–42]. 4. Developments in aluminium–lithium alloys Reducing the density of materials is accepted as the most effec￾tive way of lowering the structural weight of aircraft. Li (density 0.54 g/cm3 ) is one of the few elements that have a high solubility in aluminium. This is significant because, for each 1% added, the density of an aluminium alloy is reduced by 3%. Lithium is also un￾ique amongst the more soluble alloying elements in that it causes a considerable increase in the elastic modulus (6% for each 1%Li added). Additional advantage is that, aluminium alloys containing Li respond to age hardening [43]. The use of aluminium–lithium (Al–Li) alloys in aerospace appli￾cations goes back to 1950s with the development of alloy 2020. In the 1980s, 2nd generation of Al–Li alloys were developed. The sec￾ond generation alloys included the 2090, 2091, 8090 and 8091. The Al–Li alloys 2090, 2091, 8090 and 8091 contain 1.9–2.7% lithium, which results in an about 10% lower density and 25% higher spe￾cific stiffness than the 2000 and 7000 series alloys. However, due to technical problems such as anisotropy in the mechanical prop￾erties, low toughness, poor corrosion resistance, manufacturing is￾sues (hole cracking and delamination during drilling), 2nd generation Al–Li alloys did not find wide use in aircraft industry. The anisotropy experienced by these alloys is a result of the strong crystallographic textures that develop during processing, with the fracture toughness problem being one of primarily low strength in the short transverse direction [1,21,44,45]. The pressure for higher strength and improved fracture tough￾ness with reduced weight in aircraft applications have resulted in the development of new generation of Al–Li alloys. The new gener￾ation of Al–Li alloys provides not only weight savings, due to lower density, but also overcomes the disadvantage of the previous prob￾lems with increased corrosion resistance, good spectrum fatigue crack growth performance, a good strength and toughness combi￾nation and compatibility with standard manufacturing techniques. This results in well-balanced, light weight and high performance aluminium alloys [1,44,46]. In the new generation (3rd) Al–Li alloys Li concentration was reduced to 0.75–1.8 wt.%. The addition of alloying elements in the 3rd generation Al–Li alloys is used to improve the mechanical properties. Poor corrosion resistance of 2nd generation Al–Li alloys is eliminated in 3rd generation Al–Li alloys by optimising alloy composition and temper. Also Zn additions improved corrosion resistance. The additions of Cu, Li and Mg form the strengthening precipitates and small additions of the dispersoid-forming elements Zr and Mn control the grain structure and crystallographic texture during thermo-mechanical processing. Crack deviation occurs due to high crystallographic texture in addition with slip planarity. Deviation from expected direction of crack propagation makes it difficult to define inspec￾tion points and the positioning of crack arresters. It was found that in addition to reduction of the texture components, the severity of slip planarity had to be decreased. This reduction was achieved by decreasing the amount of (Al3Li) phase. This can be achieved by keeping the amount of Li additions below 1.8 wt ptc. The fracture toughness of 2nd generation Al–Li alloys was often lower than the incumbent 2024 alloy products for designs where damage toler￾ance is the driving parameter. It was determined that fracture toughness is affected only by insoluble second-phase particles. In 3rd generation Al–Li alloys like 2199 this disadvantageous condi￾tion was eliminated by composition optimisation, thermal– mechanical processing and precipitate microstructure control. Chemical compositions and mechanical properties of some of the widely used Al–Li alloys are shown in Tables 4 and 5 respectively. Alloy 2195, a new generation Al–Li alloy, has a lower copper content and has replaced the 2219 for the cryogenic fuel tank on the space shuttle where it provides a higher strength, higher mod￾ulus and lower density than the 2219. Other alloys, including the 2096, 2097 and 2197, also have lower copper contents but also have slightly higher lithium contents than 2195 [1]. New genera￾tion of Al–Li alloys have higher Cu/Li ratio than the second gener￾ation alloys (2090 and 2091) as illustrated in Fig. 4. The new generation of 2199 Al–Li alloys sheet and plates found applications in the aircraft for fuselage and lower wing applica￾tions, respectively and the 2099 extrusions for internal structure. It was determined that the 2199-T8E79 plate for the lower wing skin, the 2099-T83 extrusions for lower wing stringers and the 2199-T8 prime sheet for fuselage skin would provide the most benefit for the given applications examined. It is stated that com￾pared to 2024, the 2199 plates have lower density, significantly better stress corrosion and exfoliation corrosion resistance, signif￾icantly better spectrum fatigue crack growth performance, better Table 3 Chemical composition of some 7000 series aerospace aluminium alloys [22]. 7000 Series Cu Zn Mg Mn Fe Si Cr Zr Ti Al 7050 2.3 6.2 2.25 – 60.15 60.12 – 0.1 – Remainder 7055 2.0–2.6 7.6–8.4 1.8–2.3 0.05 0.15 0.1 0.04 0.08–0.25 0.06 Remainder 7075 1.2–2.0 5.1–6.1 2.1–2.9 0.3 0.5 0.4 0.18–0.28 – 0.2 Remainder 7150 1.9–2.5 5.9–6.9 2.0–2.7 0.1 0.15 0.12 0.04 0.08–0.15 0.06 Remainder 7475 1.2–1.9 5.2–6.2 1.9–2.6 0.06 0.12 0.10 0.18–0.25 – 0.06 Remainder 866 T. Dursun, C. Soutis / Materials and Design 56 (2014) 862–871