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 0.1toughness, 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