《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

T.Dursun.C.Soutis/Materials and Design 56(2014)862-871 867 toughness,and higher tensile yield and compressive yield Table 5 strengths.The ultimate tensile strength,bearing and shear Mechanical properties of some Al-Li alloys [221. strengths for the T8E80 temper are similar to those for 2024,while Al-Li UTS Yield Strength Fracture Elongation for the T8E79 temper,these strengths tend to be lower.However, alloys (MPa) (MPa) Toughness. ) this reduction in tensile yield strength provides the higher spec- Kic (Mpa m2) trum fatigue crack growth performance.Thus,one of the two tem- 2050-T84 540 500 43(LT) NA pers of 2199 may be more suitable for a given application, 2090-T83 531 483 43.9 3 20q8-T92 503 depending on the design criterion [44]. 476 NA 6 2099-T83 543 520 30(LT) 7.6 Al-Li 2099 alloy has low density,high stiffness,superior dam- 27(TL) age tolerance,excellent corrosion resistance and weldability for 2199-T8 400 345 53 10 use in aerospace structures that require high strength.Alloy 2099 8090-T851 500 455 33(LT).30(TL) 12 extrusions can replace 2xxx,6xxx,and 7xxx aluminium alloys in 12.4(sL) applications such as statically and dynamically loaded fuselage structures and lower wing stringers.2nd generation Al-Li alloys were susceptible to cracking and delamination during installation improved stress corrosion resistance without any redesign and of interference fit fasteners as a result of cold working.Low elonga- when strength,stiffness and fatigue properties are taken into ac- tion and work hardening properties were the results of these prob- count,it can lead to weight reduction up to a total of about 10% lems.In the 3rd generation Al-Li alloys elongation and cold depending on the part design drivers. working capability were improved.Alloy 2099 extrusions have Al-Li alloy 2198 was developed to replace 2024 and 2524 in air- good machining.forming.fastening,and surface finishing proper- craft structures where damage tolerance is the critical design fac- ties.The 2099 plate and forgings have better strength,modulus, tor.It has a wt.%Cu composition ranging from 2.9%to 3.3%and density and corrosion performance than the7075-T73 and 7050- respective of Li from 0.9%to 1.1%.Under constant amplitude load- T74 plate products.The T8E67 temper has much higher strength ing and stress ratio R=0.1 the fatigue endurance limit is almost than the 2024-T3511 or 2026-T3511 with better toughness,much 40%below the 2024 yield stress,while for 2198-T351 is only 8% better corrosion resistance (Fig.5)and lower density.The fatigue lower than the respective yield stress.When taking into account crack growth resistance of alloy 2099 also shows improvement density,2198 is superior to 2024 in high cycle fatigue and fatigue with respect to the 2024-T3511.which has been a baseline alloy endurance limit regimes.For the same normalised applied stresses. for fatigue critical components 47. 2198 was observed to absorb 2-3 times more energy to fracture The effects of normal heat treatments and thermomechanical than 2024 [50,51].Comparing the fatigue results in air it was ob- heat treatments on the mechanical properties and fracture tough- served that 2524-T3 presented a higher fatigue strength and fati- ness of the 2A97 new generation Al-Li alloy were studied by Yuan gue limit than the 2198-T851 Al-Li alloy.However,when the et al.[48.The aim was to improve the relationships of strength, alloys were pre-corroded in saline environment they presented ductility and fracture toughness,and make possible their applica- similar fatigue behaviour [521. tions in the aeronautical industries.The Al-Li 2A97 alloy was 2060 and 2055 are the newest 3rd generation Al-Li alloys.2060 developed primarily in an attempt to be used for plates and for- has 0.75 wt.%of Li,3.95 wt.%of Cu and 0.85 wt.%of Mg whereas gings as a promising aerospace material.It was stated that the 2055 has 1.15 wt.%of Li,3.7 wt.%of Cu and 0.4 wt.%of Mg.The problem with this alloy is that it yields low ductility and fracture wt.%of the other alloying elements are approximately same for toughness in T8 temper with a high tensile strength,and it yields these two alloys.These alloys show improved strength/toughness low strength in T6 temper with a high ductility and fracture tough- relationship.Additionally.these alloys exhibit good thermal ness.With 4%deformation after low temperature underaging,the stability.Both 2055 and 2060 have excellent corrosion performance ductility and fracture toughness were improved for the 2A97 alu- compared to that of common aerospace aluminium alloys such as minium-lithium alloy.The Ko value of 43.5 MPaym in the T8 tem- 2024-T3 and 7075-T6.Therefore,these alloys could be alternative per higher than that of 42.5 MPaym in the T6 temper was materials for fuselage,lower wing and upper wing constructions. obtained,by heat-treatment process and thermomechanical heat- Trade study analyses show that implementation of Al-Li alloys treatment process [48]. can save significant weight over the baseline 2000 and 7000 series Another new generation Al-Cu-Li alloy 2050 was developed to aluminium alloys.For instance for fuselage skin applications replace the 2000 series and 7000 series alloys where medium to 2060-T8 can save 7%weight compared to that of 2524-T3,for lower high strength and high damage tolerance are needed [49]. wing skin applications 2060-T8 can save 14%weight compared to Strength,corrosion resistance,fatigue initiation and crack growth that of 2024-T351 and for upper wing skin and stringer resistance properties were compared and according to the test re- applications,2055-T8 can save 10%weight compared to that of sults it was concluded that the 2050-T84 alloy in addition to its 7055-T7751 [47.53].The 3rd generation Al-Li alloys offers up to density benefit,offers improvements over the 2024-T351 in sta- 10%weight savings,lower risk and 30%less expensive to manufac- tic-related properties and corrosion resistance.When compared ture,operate and repair than composite-intensive planes.In to incumbent alloy 7050-T7451,the 2050-T84 offers an improved addition,these alloys can provide passenger comfort features that (strength,toughness)balance,at 5%lower density and significantly are equivalent to composite-intensive planes,such as large Table 4 Chemical composition of some Al-Li alloys [22]. Al-Li alloys Cu Zn Mg Mn e Cr Zr Others 2050 0.7-1.3 3.2-3.9 025 02-0.6 0.2-0.5 0.1 0.08 0.05 0.06-0.14 0.1 02-0.7Ag 2090 19-26 24-30 0.1 025 0.05 0.12 0.10 0.05 008-0.15 0.15 2098 0.8-13 3.2-3.8 035 0.25-0.8 0.35 0.15 0.12 0.04-0.18 0.1 025-0.6Ag 2099 1.6-2.0 2.4-3.0 0.4-1.0 0.1-05 0.1-0.5 0.07 0.05 0.1-0.5 0.05-0.12 0.1 0.0001Be 2199 1.4-1.8 2.0-2.9 02-0.9 0.05-0.4 0.1-0.5 0.07 0.05 0.05-0.12 0.1 0.0001Be 8090 22-2.7 1.0-1.6 025 0.6-1.3 0.10 030 0.20 0.10 0.04-0.16 0.1

toughness, and higher tensile yield and compressive yield strengths. The ultimate tensile strength, bearing and shear strengths for the T8E80 temper are similar to those for 2024, while for the T8E79 temper, these strengths tend to be lower. However, this reduction in tensile yield strength provides the higher spec￾trum fatigue crack growth performance. Thus, one of the two tem￾pers of 2199 may be more suitable for a given application, depending on the design criterion [44]. Al–Li 2099 alloy has low density, high stiffness, superior dam￾age tolerance, excellent corrosion resistance and weldability for use in aerospace structures that require high strength. Alloy 2099 extrusions can replace 2xxx, 6xxx, and 7xxx aluminium alloys in applications such as statically and dynamically loaded fuselage structures and lower wing stringers. 2nd generation Al–Li alloys were susceptible to cracking and delamination during installation of interference fit fasteners as a result of cold working. Low elonga￾tion and work hardening properties were the results of these prob￾lems. In the 3rd generation Al–Li alloys elongation and cold working capability were improved. Alloy 2099 extrusions have good machining, forming, fastening, and surface finishing proper￾ties. The 2099 plate and forgings have better strength, modulus, density and corrosion performance than the7075-T73 and 7050- T74 plate products. The T8E67 temper has much higher strength than the 2024-T3511 or 2026-T3511 with better toughness, much better corrosion resistance (Fig. 5) and lower density. The fatigue crack growth resistance of alloy 2099 also shows improvement with respect to the 2024-T3511, which has been a baseline alloy for fatigue critical components [47]. The effects of normal heat treatments and thermomechanical heat treatments on the mechanical properties and fracture tough￾ness of the 2A97 new generation Al–Li alloy were studied by Yuan et al. [48]. The aim was to improve the relationships of strength, ductility and fracture toughness, and make possible their applica￾tions in the aeronautical industries. The Al–Li 2A97 alloy was developed primarily in an attempt to be used for plates and for￾gings as a promising aerospace material. It was stated that the problem with this alloy is that it yields low ductility and fracture toughness in T8 temper with a high tensile strength, and it yields low strength in T6 temper with a high ductility and fracture tough￾ness. With 4% deformation after low temperature underaging, the ductility and fracture toughness were improved for the 2A97 alu￾minium–lithium alloy. The Kq value of 43.5 MPapm in the T8 tem￾per higher than that of 42.5 MPapm in the T6 temper was obtained, by heat-treatment process and thermomechanical heat￾treatment process [48]. Another new generation Al–Cu–Li alloy 2050 was developed to replace the 2000 series and 7000 series alloys where medium to high strength and high damage tolerance are needed [49]. Strength, corrosion resistance, fatigue initiation and crack growth resistance properties were compared and according to the test re￾sults it was concluded that the 2050-T84 alloy in addition to its density benefit, offers improvements over the 2024-T351 in sta￾tic-related properties and corrosion resistance. When compared to incumbent alloy 7050-T7451, the 2050-T84 offers an improved (strength, toughness) balance, at 5% lower density and significantly improved stress corrosion resistance without any redesign and when strength, stiffness and fatigue properties are taken into ac￾count, it can lead to weight reduction up to a total of about 10%, depending on the part design drivers. Al–Li alloy 2198 was developed to replace 2024 and 2524 in air￾craft structures where damage tolerance is the critical design fac￾tor. It has a wt.% Cu composition ranging from 2.9% to 3.3% and respective of Li from 0.9% to 1.1%. Under constant amplitude load￾ing and stress ratio R = 0.1 the fatigue endurance limit is almost 40% below the 2024 yield stress, while for 2198-T351 is only 8% lower than the respective yield stress. When taking into account density, 2198 is superior to 2024 in high cycle fatigue and fatigue endurance limit regimes. For the same normalised applied stresses, 2198 was observed to absorb 2–3 times more energy to fracture than 2024 [50,51]. Comparing the fatigue results in air it was ob￾served that 2524-T3 presented a higher fatigue strength and fati￾gue limit than the 2198-T851 Al–Li alloy. However, when the alloys were pre-corroded in saline environment they presented similar fatigue behaviour [52]. 2060 and 2055 are the newest 3rd generation Al–Li alloys. 2060 has 0.75 wt.% of Li, 3.95 wt.% of Cu and 0.85 wt.% of Mg whereas 2055 has 1.15 wt.% of Li, 3.7 wt.% of Cu and 0.4 wt.% of Mg. The wt.% of the other alloying elements are approximately same for these two alloys. These alloys show improved strength/toughness relationship. Additionally, these alloys exhibit good thermal stability. Both 2055 and 2060 have excellent corrosion performance compared to that of common aerospace aluminium alloys such as 2024-T3 and 7075-T6. Therefore, these alloys could be alternative materials for fuselage, lower wing and upper wing constructions. Trade study analyses show that implementation of Al–Li alloys can save significant weight over the baseline 2000 and 7000 series aluminium alloys. For instance for fuselage skin applications 2060-T8 can save 7% weight compared to that of 2524-T3, for lower wing skin applications 2060-T8 can save 14% weight compared to that of 2024-T351 and for upper wing skin and stringer applications, 2055-T8 can save 10% weight compared to that of 7055-T7751 [47,53]. The 3rd generation Al–Li alloys offers up to 10% weight savings, lower risk and 30% less expensive to manufac￾ture, operate and repair than composite-intensive planes. In addition, these alloys can provide passenger comfort features that are equivalent to composite-intensive planes, such as large Table 4 Chemical composition of some Al–Li alloys [22]. Al–Li alloys Li Cu Zn Mg Mn Fe Si Cr Zr Ti Others 2050 0.7–1.3 3.2–3.9 0.25 0.2–0.6 0.2–0.5 0.1 0.08 0.05 0.06–0.14 0.1 0.2–0.7 Ag 2090 1.9–2.6 2.4–3.0 0.1 0.25 0.05 0.12 0.10 0.05 0.08–0.15 0.15 – 2098 0.8–1.3 3.2–3.8 0.35 0.25–0.8 0.35 0.15 0.12 – 0.04–0.18 0.1 0.25–0.6 Ag 2099 1.6–2.0 2.4–3.0 0.4–1.0 0.1–0.5 0.1–0.5 0.07 0.05 0.1–0.5 0.05–0.12 0.1 0.0001 Be 2199 1.4–1.8 2.0–2.9 0.2–0.9 0.05–0.4 0.1–0.5 0.07 0.05 – 0.05–0.12 0.1 0.0001 Be 8090 2.2–2.7 1.0–1.6 0.25 0.6–1.3 0.10 0.30 0.20 0.10 0.04–0.16 0.1 – Table 5 Mechanical properties of some Al–Li alloys [22]. Al–Li alloys UTS (MPa) Yield Strength (MPa) Fracture Toughness, KIC (Mpa m1/2) Elongation (%) 2050-T84 540 500 43(LT) NA 2090-T83 531 483 43.9 3 2098-T82 503 476 NA 6 2099-T83 543 520 30 (LT) 7.6 27 (TL) 2199-T8 400 345 53 10 8090-T851 500 455 33 (LT), 30 (TL) 12 12.4 (SL) T. Dursun, C. Soutis / Materials and Design 56 (2014) 862–871 867

868 T.Dursun,C.Soutis /Materials and Design 56(2014)862-871 presented in Fig.6.It is shown that all series of aluminium alloys can be friction stir welded. 2.5 AA2090 The riveting is accepted as the traditional technique of joining AA2091 fuselage and wing structures which are generally made of alumin- 2 AA2196 solubllity limit at 500 ium alloys.However,riveting increases the weight of the airframe AA2099 Riveting also causes stress concentration leading to fatigue crack 1.5 initiation and growth.Another way of joining these structures is AA2098 by welding.Since fuselage and wing parts are made of high 219 1 strength 2000 and 7000 series of aluminium alloys,weldability A2198 of these alloys can be relatively very low.Also in traditional weld- 0.5 1.5 ing techniques metal is heated until melting point which causes a 2.5 3 3.5 large area of heat affected zone(HAZ).HAZ reduces the mechanical Cu concentration(wt%) properties of the metals resulting in reduced strength and reduced Fig.4.Positioning of selected Al-Cu-Li alloys in Li and Cu concentrations [34] resistance to fatigue.The difficulties with the welding of the high strength aluminium alloys can be listed as follows [1]: a.The stable surface oxide must be removed by either chemi- 400 cal methods or by thoroughly wire brushing the joint area. 3rd Generation Al-Li Alloys b.Weld cracking or distortion due to residual stresses resulting 350 from high coefficient of thermal expansion. 300 c.The high thermal conductivity of aluminium requires the Conventional Aerospace Al high heat input during welding further leading to the possi- 250 Alloys bility of distortion or cracking. 200 d.Weld cracking due to aluminium's high solidification shrinkage 150 e.Aluminium's high solubility for hydrogen when in the mol- 100 ten state leads to weld porosity. f.Susceptibility of high strength 2000 and 7000 series alloys to 50 weld cracking. 2024-T3 7075-T6 7050-T742099-T862199-T8E802060-T8 After the invention of Friction Stir Welding(FSW)in 1991 as an alternative way of welding,research effort on the applications of Fig.5.Comparison of corrosion resistance of Al-Li alloys with 2000 and 7000 series FSW in aircraft manufacturing technology increased substantially. alloys.The figure is based on [47.531. Friction stir welding(FSW)is a solid-state process that operates by windows,higher humidity and higher cabin pressure,due to their generating frictional heat between a rotating tool and the work- improved fatigue behaviour.According to the test results in addi- piece.A rotating tool with a shoulder and a threaded pin moves tion to the improvements in material properties,the application along the butting surfaces of two rigidly clamped plates placed of advanced structural design concept resulted in up to 10 times on a backing plate as shown in Fig.7.The shoulder makes firm con- improved damage tolerance performance in critical areas.Beside tact with the top surface of the work piece.Heat generated by fric- these advantages aluminium-lithium alloys have fusion welding tion at the shoulder softens the material being welded.Higher capacity and standardised use of tooling.mature assembly tech- plastic deformation on the metal occurs as the tool is moved along niques,repair and maintenance procedures and ease of recycling the welding direction.Material is transported from the front of the at the end of the aircraft's life make the Al-Li alloys compete with tool to the trailing edge where it is forged into a joint.Although the polymer composites currently used.While Al-Li alloys offer Fig.7 shows a butt joint for illustration,other types of joints such improvements,delaminations in these alloys play a significant role as lap joints and fillet joints can also be fabricated by FSW [57]. in their fracture processes.Therefore,a more complete understand- FSW offers several advantages compared to traditional welding ing of the factors that affect the behaviour of these delaminations techniques.FSW process takes place in the solid phase below the and their corresponding effect on the primary crack behaviour melting point of the metals to be joined.Problems related to the especially near holes need to be well understood [45,541. solidification of a fused material are eliminated.Difficult to fusion weld materials like the high strength 2000 and 7000 series alumin- ium alloys,could be joined with minor loss in strength. 5.Developments in joining techniques The main advantages of friction stir welding can be listed as follows: Aircraft manufacturers have been continuing their research activities in the field of the construction of aircraft fuselage struc- a.Welding of butt,lap and T joint configurations are possible. tures because of the increasing demands on damage tolerance of b.No special need for joint preparation is required. fuselage structures,increased cost pressure among aircraft manu- c.2000 and 7000 series alloys could be welded. facturers,and the requirements of airlines for lower aircraft d.Dissimilar alloys could be welded. inspection and maintenance costs.New trends in the construction e.No crack formation occurs during the fusion and HAZs. and manufacture of aircraft fuselage have therefore emerged in f.No weld porosity occurs. which welding,bonding,and extrusion are increasingly replacing g.No filler metals needed. the use of rivets[55.The trend of building larger structures with h.For aluminium no requirement for shielding gases. fewer parts has led to demands for thicker and longer plate from which more complex sections can be machined.Alternatively. In general,mechanical properties obtained by FSW are better smaller parts can be joined together and welding appears as the than for many other welding processes.For example the static most suitable solution [13].Weldability of aluminium alloys is properties of the friction stir welded 2024-T351 are between 80%

windows, higher humidity and higher cabin pressure, due to their improved fatigue behaviour. According to the test results in addi￾tion to the improvements in material properties, the application of advanced structural design concept resulted in up to 10 times improved damage tolerance performance in critical areas. Beside these advantages aluminium–lithium alloys have fusion welding capacity and standardised use of tooling, mature assembly tech￾niques, repair and maintenance procedures and ease of recycling at the end of the aircraft’s life make the Al–Li alloys compete with the polymer composites currently used. While Al–Li alloys offer improvements, delaminations in these alloys play a significant role in their fracture processes. Therefore, a more complete understand￾ing of the factors that affect the behaviour of these delaminations and their corresponding effect on the primary crack behaviour especially near holes need to be well understood [45,54]. 5. Developments in joining techniques Aircraft manufacturers have been continuing their research activities in the field of the construction of aircraft fuselage struc￾tures because of the increasing demands on damage tolerance of fuselage structures, increased cost pressure among aircraft manu￾facturers, and the requirements of airlines for lower aircraft inspection and maintenance costs. New trends in the construction and manufacture of aircraft fuselage have therefore emerged in which welding, bonding, and extrusion are increasingly replacing the use of rivets [55]. The trend of building larger structures with fewer parts has led to demands for thicker and longer plate from which more complex sections can be machined. Alternatively, smaller parts can be joined together and welding appears as the most suitable solution [13]. Weldability of aluminium alloys is presented in Fig. 6. It is shown that all series of aluminium alloys can be friction stir welded. The riveting is accepted as the traditional technique of joining fuselage and wing structures which are generally made of alumin￾ium alloys. However, riveting increases the weight of the airframe. Riveting also causes stress concentration leading to fatigue crack initiation and growth. Another way of joining these structures is by welding. Since fuselage and wing parts are made of high strength 2000 and 7000 series of aluminium alloys, weldability of these alloys can be relatively very low. Also in traditional weld￾ing techniques metal is heated until melting point which causes a large area of heat affected zone (HAZ). HAZ reduces the mechanical properties of the metals resulting in reduced strength and reduced resistance to fatigue. The difficulties with the welding of the high strength aluminium alloys can be listed as follows [1]: a. The stable surface oxide must be removed by either chemi￾cal methods or by thoroughly wire brushing the joint area. b. Weld cracking or distortion due to residual stresses resulting from high coefficient of thermal expansion. c. The high thermal conductivity of aluminium requires the high heat input during welding further leading to the possi￾bility of distortion or cracking. d. Weld cracking due to aluminium’s high solidification shrinkage. e. Aluminium’s high solubility for hydrogen when in the mol￾ten state leads to weld porosity. f. Susceptibility of high strength 2000 and 7000 series alloys to weld cracking. After the invention of Friction Stir Welding (FSW) in 1991 as an alternative way of welding, research effort on the applications of FSW in aircraft manufacturing technology increased substantially. Friction stir welding (FSW) is a solid-state process that operates by generating frictional heat between a rotating tool and the work￾piece. A rotating tool with a shoulder and a threaded pin moves along the butting surfaces of two rigidly clamped plates placed on a backing plate as shown in Fig. 7. The shoulder makes firm con￾tact with the top surface of the work piece. Heat generated by fric￾tion at the shoulder softens the material being welded. Higher plastic deformation on the metal occurs as the tool is moved along the welding direction. Material is transported from the front of the tool to the trailing edge where it is forged into a joint. Although Fig. 7 shows a butt joint for illustration, other types of joints such as lap joints and fillet joints can also be fabricated by FSW [57]. FSW offers several advantages compared to traditional welding techniques. FSW process takes place in the solid phase below the melting point of the metals to be joined. Problems related to the solidification of a fused material are eliminated. Difficult to fusion weld materials like the high strength 2000 and 7000 series alumin￾ium alloys, could be joined with minor loss in strength. The main advantages of friction stir welding can be listed as follows: a. Welding of butt, lap and T joint configurations are possible. b. No special need for joint preparation is required. c. 2000 and 7000 series alloys could be welded. d. Dissimilar alloys could be welded. e. No crack formation occurs during the fusion and HAZs. f. No weld porosity occurs. g. No filler metals needed. h. For aluminium no requirement for shielding gases. In general, mechanical properties obtained by FSW are better than for many other welding processes. For example the static properties of the friction stir welded 2024-T351 are between 80% Fig. 4. Positioning of selected Al–Cu–Li alloys in Li and Cu concentrations [34]. 0 50 100 150 200 250 300 350 400 2024-T3 7075-T6 7050-T74 2099-T86 2199-T8E80 2060-T8 ST SCC Threshold Stress, MPa 3rd Generation Al-Li Alloys Conventional Aerospace Al Alloys Fig. 5. Comparison of corrosion resistance of Al–Li alloys with 2000 and 7000 series alloys. The figure is based on [47,53]. 868 T. Dursun, C. Soutis / Materials and Design 56 (2014) 862–871

T.Dursun.C.Soutis /Materials and Design 56(2014)862-871 869 Weldability of Aluminium Alloys 1XXX 2XXX 3XXX 4XXX 5XXX 6XXX 7XXX 8XXX Traditional Welding Friction Stir Welding Mostly weldable Mostly non-weldable Fig.6.Weldability of various aluminium alloys.The figure is based on [56). weld,researches have been invested to understand the effect of these parameters 61-63]. Another welding technique under interest is the laser beam welding of high strength aluminium alloys where relatively small aerospace production of parts is required.With this welding pro- cess good weld properties can be obtained at high production Welding direction speeds.No electrode or filler metal is required and narrow welds Shoulder. with small HAZs are produced.Laser welding produces a concen- trated high energy density heat source that results in very narrow Tool Pin Retreating side heat affected zones,minimising both distortion and loss of strength in HAZ [1]. Advancing side In laser beam welding radiant energy is used to produce the heat required to melt the materials to be joined.A concentrated beam of coherent,monochromatic light is guided by optical de- vices and focused to a small spot,for higher power density,on Fig.7.Schematic of FSW [57] the abutting surfaces of the parts being joined.Dissimilar alloys could be joined in a noncontact process.Pulsed or continuous wave mode lasers are used to join the metals.The main advantages of la- and 90%of the parent metal,and the fatigue properties approach ser welding are the shape of the weld and good penetration,high those of the parent metal [11. precision,high mechanical properties of the weld,high welding Joints produced by FSW have higher strengths than riveted speed,low heat input,high flexibility and possibility of automa- joints and much lower residual stresses than typical fusion welded tion.Both very local welds and heat-affected zones occur with joints.In welding 7000 series aluminium alloys,post weld ageing is the help of the high energy density beam,and therefore good necessary to stabilise the microstructure in the friction stir welded mechanical properties with relatively low distortion of the work- regions.The selected overaging treatments also improve corrosion piece could be achieved.The main disadvantages are the relatively resistance of these alloys [58. high cost of investment and the important requirements related to Due to the high strength of FSW joints,it allows considerable the machining of the parts to assure a precise groove (reduced weight savings in lightweight construction compared to conven- dimensional tolerances).Higher product quality,in terms of im- tional joining technologies.The use of welded instead of riveted proved in-service properties,could be achieved through:improved joints is also advantageous because of the lower production costs. tolerances;accurate control of process parameters:selection of Therefore,the FSW process has recently been identified as key new materials;and product redesign.EADS Airbus has invested technology for fuselage and wing manufacturing by leading aircraft in laser welding as a replacement for riveting in non-critical appli- manufacturers. cations.In double-sided laser beam welding of T-joints the incident As the large aircraft experience higher stresses and shorter fati- beam position,incident beam angle,and beam separation distance gue life,the technology should be applied carefully.There exist are the most important welding parameters.The incident beam several parameters which have influence on the quality and position has great impact on joint quality.6xxx series Al-Mg-Si al- strength of the friction stir weld.This process must be optimised loys are susceptible to hot cracking.Aluminium filler (such as for each specific application.In order to optimise the performance AA4047)wire containing excess silicon is recommended for 6xxx of the FSW joint,it is important to identify the welding parameters. series alloys.It is reported that the crack sensitivity decreases if The main FSW process parameters are the followings [59,60]: the Silicon content exceeds 1.5%64].One application involves joining stiffening stringers to the skin of the fuselage.The dam- a.Tool geometry (shoulder,probe). age-tolerant alloy 6013 is the base material and 4047 is the filler b.Clamping system. material.The stringers are welded from two sides at 10 m/min. c.Axial Load. using two 2.5 kW CO2 laser beams.The joint is designed such that d.Tool rotational direction. the HAZ is contained in the stringer,and does not impinge on the e.Plunge depth of probe in workpieces. skin.The process was first used in series production of the Airbus f.Plunge speed of the probe in the workpieces at the start 318,and was then implemented successfully in other aircraft mod- position. els [651. g.Dwell time at start of the weld. h.Tilt angle. i.Preheating/interpass temperature of workpieces. 6.Conclusions j.Control during plunge,dwell and weld periods. k.Welding speed versus rotation speed. Aluminium alloys have been successfully used as primary mate- rial for the structural parts of aircraft for more than 80 years.Air- As mentioned above since there are several tools and operating craft designers possess considerable experience in the design, parameters that affect the quality and strength of the friction stir production,operation and maintenance of aluminium airframes

and 90% of the parent metal, and the fatigue properties approach those of the parent metal [1]. Joints produced by FSW have higher strengths than riveted joints and much lower residual stresses than typical fusion welded joints. In welding 7000 series aluminium alloys, post weld ageing is necessary to stabilise the microstructure in the friction stir welded regions. The selected overaging treatments also improve corrosion resistance of these alloys [58]. Due to the high strength of FSW joints, it allows considerable weight savings in lightweight construction compared to conven￾tional joining technologies. The use of welded instead of riveted joints is also advantageous because of the lower production costs. Therefore, the FSW process has recently been identified as key technology for fuselage and wing manufacturing by leading aircraft manufacturers. As the large aircraft experience higher stresses and shorter fati￾gue life, the technology should be applied carefully. There exist several parameters which have influence on the quality and strength of the friction stir weld. This process must be optimised for each specific application. In order to optimise the performance of the FSW joint, it is important to identify the welding parameters. The main FSW process parameters are the followings [59,60]: a. Tool geometry (shoulder, probe). b. Clamping system. c. Axial Load. d. Tool rotational direction. e. Plunge depth of probe in workpieces. f. Plunge speed of the probe in the workpieces at the start position. g. Dwell time at start of the weld. h. Tilt angle. i. Preheating/interpass temperature of workpieces. j. Control during plunge, dwell and weld periods. k. Welding speed versus rotation speed. As mentioned above since there are several tools and operating parameters that affect the quality and strength of the friction stir weld, researches have been invested to understand the effect of these parameters [61–63]. Another welding technique under interest is the laser beam welding of high strength aluminium alloys where relatively small aerospace production of parts is required. With this welding pro￾cess good weld properties can be obtained at high production speeds. No electrode or filler metal is required and narrow welds with small HAZs are produced. Laser welding produces a concen￾trated high energy density heat source that results in very narrow heat affected zones, minimising both distortion and loss of strength in HAZ [1]. In laser beam welding radiant energy is used to produce the heat required to melt the materials to be joined. A concentrated beam of coherent, monochromatic light is guided by optical de￾vices and focused to a small spot, for higher power density, on the abutting surfaces of the parts being joined. Dissimilar alloys could be joined in a noncontact process. Pulsed or continuous wave mode lasers are used to join the metals. The main advantages of la￾ser welding are the shape of the weld and good penetration, high precision, high mechanical properties of the weld, high welding speed, low heat input, high flexibility and possibility of automa￾tion. Both very local welds and heat-affected zones occur with the help of the high energy density beam, and therefore good mechanical properties with relatively low distortion of the work￾piece could be achieved. The main disadvantages are the relatively high cost of investment and the important requirements related to the machining of the parts to assure a precise groove (reduced dimensional tolerances). Higher product quality, in terms of im￾proved in-service properties, could be achieved through: improved tolerances; accurate control of process parameters; selection of new materials; and product redesign. EADS Airbus has invested in laser welding as a replacement for riveting in non-critical appli￾cations. In double-sided laser beam welding of T-joints the incident beam position, incident beam angle, and beam separation distance are the most important welding parameters. The incident beam position has great impact on joint quality. 6xxx series Al–Mg–Si al￾loys are susceptible to hot cracking. Aluminium filler (such as AA4047) wire containing excess silicon is recommended for 6xxx series alloys. It is reported that the crack sensitivity decreases if the Silicon content exceeds 1.5% [64]. One application involves joining stiffening stringers to the skin of the fuselage. The dam￾age-tolerant alloy 6013 is the base material and 4047 is the filler material. The stringers are welded from two sides at 10 m/min, using two 2.5 kW CO2 laser beams. The joint is designed such that the HAZ is contained in the stringer, and does not impinge on the skin. The process was first used in series production of the Airbus 318, and was then implemented successfully in other aircraft mod￾els [65]. 6. Conclusions Aluminium alloys have been successfully used as primary mate￾rial for the structural parts of aircraft for more than 80 years. Air￾craft designers possess considerable experience in the design, production, operation and maintenance of aluminium airframes. Fig. 6. Weldability of various aluminium alloys. The figure is based on [56]. Fig. 7. Schematic of FSW [57]. T. Dursun, C. Soutis / Materials and Design 56 (2014) 862–871 869

870 T.Dursun,C.Soutis/Materials and Design 56(2014)862-871 The infrastructure and knowledge base has become mature.How- It is believed that developments of advanced hybrid materials. ever,with the introduction of high performance polymer compos- like fibre metal laminates could provide additional opportunities ites in the application of airframe designs reduced the role of for aluminium alloys and new material options for the airframe aluminium alloys up to some extent due composites'high specific industry. properties,reduced weight,fatigue performance and corrosion resistance(Boeing 787.Airbus A350).In order for aluminium alloys References to remain attractive in the airframe construction and compete with and/or be compatible with currently used polymer composites,re- [1]Campbell FC.Manufacturing technology for aerospace structural search activities on the improvement of structural performance, materials.Elsevier:2006. weight and cost reductions are needed.Recent developments in [2]Warren AS.Developments and challanges for aluminium -A Boeing perspective.Mater Forum 2004:28:24-31. high strength Al-Zn and Al-Li alloys,damage tolerant Al-Cu and [3]Hombergsmeier E Development of advanced laminates for aircraft structures. Al-Li alloys,have been successful in improving the static strength, In:25th International congress of the aeronautical sciences,Hamburg. fracture toughness,fatigue and corrosion resistance through the Germany:2006. [4]Vlot A.Vogelesang LB.De Vries TJ.Towards application of fibre metal design and control of chemical composition,and/or through the laminates in large aircraft.Aircr Eng Aerosp Technol 1999:7:558-70. development of more effective heat treatments.It has been seen [5]Gunnink JW.Vlot A.De Vries TJ.Van Der Hoeven W.GLARE technology from this review that major improvements of aerospace alumin- development 1997-2000.Appl Compos Mater 2002:9:201-19. [6]Vogelesang LB.Vlot A.Development of fibre metal laminates for advanced ium alloys are due to optimised solute content and solute ratios aerospace structures.J Mater Process Technol 2000:103:1-5. in order to achieve better property balance.The use of new disper- 17]Vermeeren CAJR.An historic overview of the development of fibre metal soid-processing combinations results in desired grain structures laminates.Appl Compos Mater 2003:10:189-205. that provide better damage tolerance.Improvements in under- [8]Wu G.Yang JM.The mechanical behavior of GLARE laminates for aircraft structures.JOM 2005:57:72-9. standing and modelling of the hardening system and especially [9]Alderliesten RC.Homan J.Fatigue and damage tolerance issues of Glare in the effect of minor element addition will help improvements in aircraft structures.Int J Fatigue 2006:28:1116-23. mechanical properties. [10]Alderliesten RC,Benedictus R.Fiber/metal composite technology for future Current research activities for both composites and aluminium include:improvement on mechanical properties,reduction of 2007. manufacturing,maintenance and repair costs,prevention of corro- [11]Vermeeren CAJR,Beumler T.De Kanter JLCG.Van Der Jagt OC.Out BCL Glare design aspects and philosophies.Appl Compos Mater 2003:10:257-76. sion and fatigue and ability to perform reliably throughout its ser- [12]Soltani P.Keikhosravy M,Oskouei RH.Soutis C Studying the tensile behaviour vice life. of GLARE laminates:a finite element modelling approach.Appl Compos Mater In order to use the advantage of improvements in mechanical 2011·18271-82. [13]Flower HM,Soutis C.Materials for airframes.Aeronat J 2003:331-41 properties of advanced aluminium alloys and sustain the structural [14]Soutis C.Recent advances in building with composites.Plast Rubber Compos: integrity in mechanically fastened aircraft structures a special Macromol Eng 2009:38:359-66. attention should be paid on the fretting fatigue.There is need to [15]Diamanti K.Soutis C.Structural health monitoring techniques for aircraft composite structures.Prog Aerosp Sci 2010:46:343-52. understand the fretting behaviour of recently developed Al-Li al- [16]Giurgiutiu V,Soutis C.Enhanced composites integrity through structural loys such as 2050 and 2099 and fibre metal laminates in mechan- health monitoring.Appl Compos Mater 2012:1-17. ically fastened aircraft joints. [17]Soutis C.Mohamed G.Hodzic A Performance of GLARE panels subjected to In addition to weight reduction and improvement on the struc- intense pressure pulse loading.Aeronaut J 2012:116:667-79. [18]Mohamed G.Soutis C.Hodzic A.Multi-material arbitrary-lagrangian eulerian tural performance the materials,cost reduction through the devel- formulation for blast-induced fluid-structure interaction in fibre metal opment on the manufacturing techniques is also a key issue. laminates.AlAA 2012:50:1826-33. Manufacturing constitutes the biggest portion of the cost of the air- [19]Cassada W.Liu J.Staley J.Aluminium alloys for aircraft structures.Adv Mater Processes 2002:27-9. frame.Therefore great effort is being spent to reduce the produc- [20]Starke EA.Staley JT.Application of modern aluminium alloys to aircraft.Prog tion costs and part count via introducing high-speed machining. Aerosp Sci1996:32:131-72. novel assembly techniques such as laser beam welding and fric- [21]Williams JC,Starke EA.Progress in structural materials for aerospace systems. Acta Mater2003:51:5775-99. tion-stir welding.For example,unlike most conventional aerospace [22]Merati A.Materials replacement for aging aircraft.RTO-AG-AVT-140 [Chapter alloys,the fusion weldability of Al-Cu-Li alloys could introduce 241. new opportunities in the fabrication of fuselage.Therefore,in addi- [23]Verma BB.Atkinson JD.Kumar M.Study of fatigue behaviour of 7475 aluminium alloy.Bull Mater Sci 2001:24:231-6. tion to metallurgical developments with the combination of other [24]Smith B.The Boeing 777.Adv Mater Processes 2003:41-4. manufacturing techniques than the riveting will help reach opti- [25]Chen YQ,Pan SP,Zhou MZ,Yi DQ,Xu DZ.Xu Y.Effects of inclusions,grain mised damage tolerant designs. boundaries and grain orientations on the fatigue crack initiation and High strain-rate superplastic forming and casting are also draw- propagation behavior of 2524-T3 Al alloy.Mater Sci Eng A 2013:580:150-8. [26]Zheng ZQ.Cai B.Zhai T.Li SC.The behavior of fatigue crack initiation and ing attention as cost effective solutions.Advanced joining tech- propagation in AA2524-T34 alloy.Mater Sci Eng A 2011:528:2017-22. niques will also make aluminium structures more affordable. [27]Necsulescu DA.The effects of corrosion on the mechanical properties of The airframes and other structural parts will continue to be aluminum alloy 7075-T6.UPB Sci Bull 2011:73. [28]Lam FD,Menzemer CC,Srivatsan TS.A study to evaluate and understand the composed of different materials including aluminium,titanium response of aluminum alloy 2026 subjected to tensile deformation.Mater Des steel,polymer composites and fibre metal laminates depending 2010:31:166-75. on the balance of structural and economical factors.Weight sav- [29]Li IX,Zhai T.Garratt MD.Bray GH.Four point bend fatigue of AA2026 aluminum alloy.Metull Mater Trans A 2005:36A:2529-39 ing through increased specific strength and/or stiffness and (30]Pantelakis SG.Chamos AN,Kermanidis A.A critical consideration of use of Al- affordability (procurement,maintenance and repair costs)are cladding for protecting aircraft aluminum alloy 2024 against corrosion.Theor the major drivers for the development and selection of materials Appl Fract Mec 2012:57:36-42 [31]Ziemian CW,Sharma MM,Bouffard BD.Nissley T,Eden TI.Effect of substrate for civil airframes.In selecting new materials for aircraft applica- surface roughening and cold spray coating on the fatigue life of AA2024 tions,there should be no reduction on the levels of safety that is specimens.Mater Des 2014:54:212-21. already reached with conventional alloys.Fatigue resistance,cor- [32]Shi H.Han EH,Liu F.Kallip S.Protection of 2024-T3 aluminium alloy by corosIon resistant phytic acid conversion coating.Appl Surf Sci rosion resistance and damage tolerance are all very important 2013280325-31. mechanical properties of airframe materials that affect the [33]Kim ST.Tadjiev D.Yang HT.Fatigue life prediction under random loading inspection,maintenance and repair costs and this is where mod- conditions in 7475-T7351 aluminum alloy using the RMS model.Int J Damage Mech2006:15:89-102. ern aluminium alloys could compete effectively with polymer [34]Warner T.Recently-developed aluminium solutions for aerospace composites. applications.Mater Sci Forum 2006:519-521:1271-8

The infrastructure and knowledge base has become mature. How￾ever, with the introduction of high performance polymer compos￾ites in the application of airframe designs reduced the role of aluminium alloys up to some extent due composites’ high specific properties, reduced weight, fatigue performance and corrosion resistance (Boeing 787, Airbus A350). In order for aluminium alloys to remain attractive in the airframe construction and compete with and/or be compatible with currently used polymer composites, re￾search activities on the improvement of structural performance, weight and cost reductions are needed. Recent developments in high strength Al–Zn and Al–Li alloys, damage tolerant Al–Cu and Al–Li alloys, have been successful in improving the static strength, fracture toughness, fatigue and corrosion resistance through the design and control of chemical composition, and/or through the development of more effective heat treatments. It has been seen from this review that major improvements of aerospace alumin￾ium alloys are due to optimised solute content and solute ratios in order to achieve better property balance. The use of new disper￾soid-processing combinations results in desired grain structures that provide better damage tolerance. Improvements in under￾standing and modelling of the hardening system and especially the effect of minor element addition will help improvements in mechanical properties. Current research activities for both composites and aluminium include: improvement on mechanical properties, reduction of manufacturing, maintenance and repair costs, prevention of corro￾sion and fatigue and ability to perform reliably throughout its ser￾vice life. In order to use the advantage of improvements in mechanical properties of advanced aluminium alloys and sustain the structural integrity in mechanically fastened aircraft structures a special attention should be paid on the fretting fatigue. There is need to understand the fretting behaviour of recently developed Al–Li al￾loys such as 2050 and 2099 and fibre metal laminates in mechan￾ically fastened aircraft joints. In addition to weight reduction and improvement on the struc￾tural performance the materials, cost reduction through the devel￾opment on the manufacturing techniques is also a key issue. Manufacturing constitutes the biggest portion of the cost of the air￾frame. Therefore great effort is being spent to reduce the produc￾tion costs and part count via introducing high-speed machining, novel assembly techniques such as laser beam welding and fric￾tion-stir welding. For example, unlike most conventional aerospace alloys, the fusion weldability of Al–Cu–Li alloys could introduce new opportunities in the fabrication of fuselage. Therefore, in addi￾tion to metallurgical developments with the combination of other manufacturing techniques than the riveting will help reach opti￾mised damage tolerant designs. High strain-rate superplastic forming and casting are also draw￾ing attention as cost effective solutions. Advanced joining tech￾niques will also make aluminium structures more affordable. The airframes and other structural parts will continue to be composed of different materials including aluminium, titanium, steel, polymer composites and fibre metal laminates depending on the balance of structural and economical factors. Weight sav￾ing through increased specific strength and/or stiffness and affordability (procurement, maintenance and repair costs) are the major drivers for the development and selection of materials for civil airframes. In selecting new materials for aircraft applica￾tions, there should be no reduction on the levels of safety that is already reached with conventional alloys. Fatigue resistance, cor￾rosion resistance and damage tolerance are all very important mechanical properties of airframe materials that affect the inspection, maintenance and repair costs and this is where mod￾ern aluminium alloys could compete effectively with polymer composites. It is believed that developments of advanced hybrid materials, like fibre metal laminates could provide additional opportunities for aluminium alloys and new material options for the airframe industry. References [1] Campbell FC. Manufacturing technology for aerospace structural materials. Elsevier; 2006. [2] Warren AS. Developments and challanges for aluminium – A Boeing perspective. Mater Forum 2004;28:24–31. [3] Hombergsmeier E. Development of advanced laminates for aircraft structures. In: 25th International congress of the aeronautical sciences, Hamburg, Germany; 2006. [4] Vlot A, Vogelesang LB, De Vries TJ. Towards application of fibre metal laminates in large aircraft. Aircr Eng Aerosp Technol 1999;7:558–70. [5] Gunnink JW, Vlot A, De Vries TJ, Van Der Hoeven W. GLARE technology development 1997–2000. Appl Compos Mater 2002;9:201–19. [6] Vogelesang LB, Vlot A. Development of fibre metal laminates for advanced aerospace structures. J Mater Process Technol 2000;103:1–5. [7] Vermeeren CAJR. An historic overview of the development of fibre metal laminates. Appl Compos Mater 2003;10:189–205. [8] Wu G, Yang JM. The mechanical behavior of GLARE laminates for aircraft structures. JOM 2005;57:72–9. [9] Alderliesten RC, Homan JJ. Fatigue and damage tolerance issues of Glare in aircraft structures. Int J Fatigue 2006;28:1116–23. [10] Alderliesten RC, Benedictus R. Fiber/metal composite technology for future primary aircraft structures. In: 48th AIAA/ASME/ASCE/AHS/ASC structures, structural dynamics, and materials conference. Honolulu, Hawaii, 23–26 April 2007. [11] Vermeeren CAJR, Beumler T, De Kanter JLCG, Van Der Jagt OC, Out BCL. Glare design aspects and philosophies. Appl Compos Mater 2003;10:257–76. [12] Soltani P, Keikhosravy M, Oskouei RH, Soutis C. Studying the tensile behaviour of GLARE laminates: a finite element modelling approach. Appl Compos Mater 2011;18:271–82. [13] Flower HM, Soutis C. Materials for airframes. Aeronat J 2003:331–41. [14] Soutis C. Recent advances in building with composites. Plast Rubber Compos: Macromol Eng 2009;38:359–66. [15] Diamanti K, Soutis C. Structural health monitoring techniques for aircraft composite structures. Prog Aerosp Sci 2010;46:343–52. [16] Giurgiutiu V, Soutis C. Enhanced composites integrity through structural health monitoring. Appl Compos Mater 2012:1–17. [17] Soutis C, Mohamed G, Hodzic A. Performance of GLARE panels subjected to intense pressure pulse loading. Aeronaut J 2012;116:667–79. [18] Mohamed G, Soutis C, Hodzic A. Multi-material arbitrary-lagrangian eulerian formulation for blast-induced fluid-structure interaction in fibre metal laminates. AIAA J 2012;50:1826–33. [19] Cassada W, Liu J, Staley J. Aluminium alloys for aircraft structures. Adv Mater Processes 2002:27–9. [20] Starke EA, Staley JT. Application of modern aluminium alloys to aircraft. Prog Aerosp Sci 1996;32:131–72. [21] Williams JC, Starke EA. Progress in structural materials for aerospace systems. Acta Mater 2003;51:5775–99. [22] Merati A. Materials replacement for aging aircraft. RTO-AG-AVT-140 [Chapter 24]. [23] Verma BB, Atkinson JD, Kumar M. Study of fatigue behaviour of 7475 aluminium alloy. Bull Mater Sci 2001;24:231–6. [24] Smith B. The Boeing 777. Adv Mater Processes 2003:41–4. [25] Chen YQ, Pan SP, Zhou MZ, Yi DQ, Xu DZ, Xu Y. Effects of inclusions, grain boundaries and grain orientations on the fatigue crack initiation and propagation behavior of 2524-T3 Al alloy. Mater Sci Eng A 2013;580:150–8. [26] Zheng ZQ, Cai B, Zhai T, Li SC. The behavior of fatigue crack initiation and propagation in AA2524-T34 alloy. Mater Sci Eng A 2011;528:2017–22. [27] Necsulescu DA. The effects of corrosion on the mechanical properties of aluminum alloy 7075–T6. UPB Sci Bull 2011:73. [28] Lam FD, Menzemer CC, Srivatsan TS. A study to evaluate and understand the response of aluminum alloy 2026 subjected to tensile deformation. Mater Des 2010;31:166–75. [29] Li JX, Zhai T, Garratt MD, Bray GH. Four point bend fatigue of AA2026 aluminum alloy. Metull Mater Trans A 2005;36A:2529–39. [30] Pantelakis SG, Chamos AN, Kermanidis A. A critical consideration of use of Al￾cladding for protecting aircraft aluminum alloy 2024 against corrosion. Theor Appl Fract Mec 2012;57:36–42. [31] Ziemian CW, Sharma MM, Bouffard BD, Nissley T, Eden TJ. Effect of substrate surface roughening and cold spray coating on the fatigue life of AA2024 specimens. Mater Des 2014;54:212–21. [32] Shi H, Han EH, Liu F, Kallip S. Protection of 2024-T3 aluminium alloy by corrosion resistant phytic acid conversion coating. Appl Surf Sci 2013;280:325–31. [33] Kim ST, Tadjiev D, Yang HT. Fatigue life prediction under random loading conditions in 7475–T7351 aluminum alloy using the RMS model. Int J Damage Mech 2006;15:89–102. [34] Warner T. Recently-developed aluminium solutions for aerospace applications. Mater Sci Forum 2006;519–521:1271–8. 870 T. Dursun, C. Soutis / Materials and Design 56 (2014) 862–871

T.Dursun,C.Soutis /Materials and Design 56 (2014)862-871 871 [35]Chen S.Chen K.Peng G.Jia L.Dong P.Effect of heat treatment on strength, [50]Alexopoulos ND.Migklis E.Stylianos A.Myriounis DP.Fatigue behavior of the exfoliation corrosion and electrochemical behavior of 7085 aluminum alloy. aeronautical Al-Li(2198)aluminum alloy under constant amplitude loading. Mater Des2012:35:93-8. nt I Fatigue2013:56:95-105. [36]Zhang JB.Zhang YA.Zhu BH.Liu RQ.Wang F,Liang QM.Characterization of [51]Decreus B.Deschamps A.Donnadieu P.Ehrstrom JC.On the role of microstructure and mechanical properties of Al-Cu-Mg-Ag-(Mn/Zr)alloy microstructure in governing fracture behavior of an aluminum-copper- with high Cu:Mg.Mater Des 2013:49:311-7. lithium alloy.Mat Sci Eng A 2013:586:418-27. [37]Chakherlou TN.Razavi MJ.Aghdam AB.Abazadeh B.An experimental [52]Moreto JA.Gamboni O,Ruchert COFT.Romagnoli F,Moreira MF.Beneduce Fea. investigation of the bolt clamping force and friction effect on the fatigue Corrosion and fatigue behavior of new Al alloys.Proc Eng 2011:10:1521-6. behavior of aluminum alloy 2024-T3 double shear lap joint.Mater Des [53]Rioj RJ.Liu J.The evolution of Al-Li base products for aerospace and space 20】1324641-9 applications.Metull Mater Trans A 2012:43A:3325-37. with shot and aser peened 0-T61 3]Vazquez J.Navarro C.Dominguez J.Exp ental results in fretting fatigue [54]Hamel SF.A parametric study of delaminations in an aluminium-lithium alloy specimens.Int J Fatigue In:MS Thesis.Champaign:University of Illinois at Urbana:2010. 2012:40:143-53. [55]Lenczowski B.New lightweight alloys for welded aircraft structure.In:ICAS [39]Chakherlou TN.Shakouri M.Akbar A.Aghdam AB.Effect of cold expansion and ongress:2002. bolt clamping on fretting behavior of Al 2024-T3 in double shear lap joints.Eng [56]ESAB Technical.Friction Stir Welding.laccessed Pa1Ana2012.2529=41 25.01121 [40]Oskouei RH.Ibrahim RN.Improving fretting fatigue behaviour of Al 7075-T6 [57]Nandan R.DebRoy T,Bhadeshia HKDH.Recent advances in friction stir bolted plates using electroless Ni-P coatings.Int J Fatigue 2012:44:157-67. welding-process,weldment structure and properties.Prog Mater Sci [41]Oskouei RH.Ibrahim RN.An investigation on the fatigue behaviour of Al 7075 2008:53:980-1023 T6 coated with titanium nitride using physical vapour deposition process. [58]Burford D.Widener C.Tweedy B.Advances in friction stir welding for Mater Des2012:39:294-302. aerospace application.In:6th AlAA Aviation Technology.Integration and [42]Sarhan AAD.Zalnezhad E.Hamdi M.The influence of higher surface hardness Operations Conference,Wichita,USA:2006. on fretting fatigue life of hard anodized aerospace Al 7075-T6 alloy.Mater Sci [59]Colegrove P.Airbus evaluates friction stir welding.[accessed 20.02.12]. [43]Polmear Il.Aluminium alloys -a century of age hardening.Mater Forum [60]Vilaca P.Thomas W.Friction stir welding technology.Adv Struct Mater. 2004:28:1-14. Heidelbe Berlin:Springer-Verlag:2011. [44]Giummarra C.Thomas B.Rioja RJ.New aluminium lithium alloys for aerospace [61]Lertora E.Gambaro C.AA8090 Al-Li alloy fsw parameters to minimize defects applications.In:Light metals technology conference:2007. and increase fatigue life Int I Mater Form 20103:1003-6. [45]Kalyanam S.Beaudoin AJ.Dodds Jr RH,Barlat F.Delamination cracking in [62]Liu H.Zhang H,Pan Q.Yu L.Effect of friction stir welding parameters on advanced aluminium-lithium alloys-experimental and computational studies. microstructural characteristics and mechanical properties of 2219-T6 Eng Fract Mech 2009:76:2174-91 aluminium alloy joints.Int J Mater Form 2011:201:1048-5. [46]Soboyejo WO.Srivatsan TS.Properties,design optimization and [63]Buffa G.Campanile G.Fratini L,Prisco A.Friction stir welding of lap joints: applications.Taylor Francis Group,LLC:2006. influence of process parameters on the metallurgical and mechanical [47]Bodily B.Heinimann M,Bray G.Colvin E,Witters J.Advanced aluminum and properties.Mater Sci Eng A 2009:519:19-26. aluminum-lithium solutions for derivative and next generation aerospace [64]Yang ZB.Tao W.Li LQ,Chen YB.Li FZ,Zhang YL Double-sided laser beam structures.SAE paper no 2012-01-1874. welded T-joints for aluminum aircraft fuselage panels: process [48]Yuan Z.Lu Z.Xie Y.Wu X.Dai S.Liu C.Mechanical properties of a novel high- microstructure,and mechanical properties.Mater Des 2012:33:652-8. strength aluminium-lithium alloy.Mater Sci Forum 2011:689:385-9. [65]Quintino L Miranda R.Dilthey U.Laser welding of structural aluminium.Adv [49]Lequeu PH.Smith K.Danie'lou A.Aluminium-copper-lithium alloy 2050 Struct Mater.Heidelberg.Berlin:Springer-Verlag:2011. developed for medium to thick plate.JMEPEG 2010:19:841-7

[35] Chen S, Chen K, Peng G, Jia L, Dong P. Effect of heat treatment on strength, exfoliation corrosion and electrochemical behavior of 7085 aluminum alloy. Mater Des 2012;35:93–8. [36] Zhang JB, Zhang YA, Zhu BH, Liu RQ, Wang F, Liang QM. Characterization of microstructure and mechanical properties of Al–Cu–Mg–Ag–(Mn/Zr) alloy with high Cu:Mg. Mater Des 2013;49:311–7. [37] Chakherlou TN, Razavi MJ, Aghdam AB, Abazadeh B. An experimental investigation of the bolt clamping force and friction effect on the fatigue behavior of aluminum alloy 2024-T3 double shear lap joint. Mater Des 2011;32:4641–9. [38] Vazquez J, Navarro C, Dominguez J. Experimental results in fretting fatigue with shot and laser peened Al 7075-T651 specimens. Int J Fatigue 2012;40:143–53. [39] Chakherlou TN, Shakouri M, Akbar A, Aghdam AB. Effect of cold expansion and bolt clamping on fretting behavior of Al 2024-T3 in double shear lap joints. Eng Fail Anal 2012;25:29–41. [40] Oskouei RH, Ibrahim RN. Improving fretting fatigue behaviour of Al 7075-T6 bolted plates using electroless Ni–P coatings. Int J Fatigue 2012;44:157–67. [41] Oskouei RH, Ibrahim RN. An investigation on the fatigue behaviour of Al 7075- T6 coated with titanium nitride using physical vapour deposition process. Mater Des 2012;39:294–302. [42] Sarhan AAD, Zalnezhad E, Hamdi M. The influence of higher surface hardness on fretting fatigue life of hard anodized aerospace Al 7075-T6 alloy. Mater Sci Eng A 2013;560:377–87. [43] Polmear IJ. Aluminium alloys – a century of age hardening. Mater Forum 2004;28:1–14. [44] Giummarra C, Thomas B, Rioja RJ. New aluminium lithium alloys for aerospace applications. In: Light metals technology conference; 2007. [45] Kalyanam S, Beaudoin AJ, Dodds Jr RH, Barlat F. Delamination cracking in advanced aluminium–lithium alloys-experimental and computational studies. Eng Fract Mech 2009;76:2174–91. [46] Soboyejo WO, Srivatsan TS. Properties, design optimization and applications. Taylor & Francis Group, LLC; 2006. [47] Bodily B, Heinimann M, Bray G, Colvin E, Witters J. Advanced aluminum and aluminum–lithium solutions for derivative and next generation aerospace structures. SAE paper no 2012-01-1874. [48] Yuan Z, Lu Z, Xie Y, Wu X, Dai S, Liu C. Mechanical properties of a novel high￾strength aluminium–lithium alloy. Mater Sci Forum 2011;689:385–9. [49] Lequeu PH, Smith K, Danie’lou A. Aluminium–copper–lithium alloy 2050 developed for medium to thick plate. JMEPEG 2010;19:841–7. [50] Alexopoulos ND, Migklis E, Stylianos A, Myriounis DP. Fatigue behavior of the aeronautical Al–Li (2198) aluminum alloy under constant amplitude loading. Int J Fatigue 2013;56:95–105. [51] Decreus B, Deschamps A, Donnadieu P, Ehrström JC. On the role of microstructure in governing fracture behavior of an aluminum–copper– lithium alloy. Mat Sci Eng A 2013;586:418–27. [52] Moreto JA, Gamboni O, Ruchert COFT, Romagnoli F, Moreira MF. Beneduce Fea. Corrosion and fatigue behavior of new Al alloys. Proc Eng 2011;10:1521–6. [53] Rioj RJ, Liu J. The evolution of Al–Li base products for aerospace and space applications. Metull Mater Trans A 2012;43A:3325–37. [54] Hamel SF. A parametric study of delaminations in an aluminium–lithium alloy. In: MS Thesis. Champaign: University of Illinois at Urbana; 2010. [55] Lenczowski B. New lightweight alloys for welded aircraft structure. In: ICAS Congress; 2002. [56] ESAB Technical. Friction Stir Welding. [accessed 25.01.12]. [57] Nandan R, DebRoy T, Bhadeshia HKDH. Recent advances in friction stir welding-process, weldment structure and properties. Prog Mater Sci 2008;53:980–1023. [58] Burford D, Widener C, Tweedy B. Advances in friction stir welding for aerospace application. In: 6th AIAA Aviation Technology, Integration and Operations Conference, Wichita, USA; 2006. [59] Colegrove P. Airbus evaluates friction stir welding. [accessed 20.02.12]. [60] Vilaça P, Thomas W. Friction stir welding technology. Adv Struct Mater. Heidelberg, Berlin: Springer-Verlag; 2011. [61] Lertora E, Gambaro C. AA8090 Al–Li alloy fsw parameters to minimize defects and increase fatigue life. Int J Mater Form 2010;3:1003–6. [62] Liu H, Zhang H, Pan Q, Yu L. Effect of friction stir welding parameters on microstructural characteristics and mechanical properties of 2219–T6 aluminium alloy joints. Int J Mater Form 2011;201:1048–5. [63] Buffa G, Campanile G, Fratini L, Prisco A. Friction stir welding of lap joints: influence of process parameters on the metallurgical and mechanical properties. Mater Sci Eng A 2009;519:19–26. [64] Yang ZB, Tao W, Li LQ, Chen YB, Li FZ, Zhang YL. Double-sided laser beam welded T-joints for aluminum aircraft fuselage panels: process, microstructure, and mechanical properties. Mater Des 2012;33:652–8. [65] Quintino L, Miranda R, Dilthey U. Laser welding of structural aluminium. Adv Struct Mater. Heidelberg, Berlin: Springer-Verlag; 2011. T. Dursun, C. Soutis / Materials and Design 56 (2014) 862–871 871