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 RemainderZn (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