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 Remainderoften 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