ReviewRecent developments in advanced aircraft aluminium alloys
Introduction
The cost reduction for aircraft purchase and operation has become 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 improve 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 focus 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 improvements 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 aircraft 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 primary 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 stiffness, specific strength (normalised by density), fatigue and corrosion 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 impact and complicated mechanical behaviour due to change in environmental conditions (moisture absorption, getting soft/brittle when exposed to hot/cold environments) make designers to explore alternative material systems. Fibre metal laminates such as GLARE which combines aluminium layers with glass fibre epoxy plies to improve tensile strength and more importantly damage tolerance are finding great use in aerospace applications [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. Impact resistance, effect of damage on stiffness/strength especially when loaded in compression and damage identification and detection, in addition to joints, repair and recycling remain big challenges for composites with the need of further research [13], [14], [15], [16], [17], [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 remain so for some time to come. However, the non-metallic materials, 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 aluminium 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 precipitates, 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 fuselage. 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 manufacturers 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 supporting 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 equivalent to those of a thin plate. The second method is implementation 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 highlights performance improvements in the 2000, 7000 series aluminium alloys as well as the new generation of Al–Li alloys. Currently the 7000 series Al–Zn alloys are used where the main limiting design parameter is strength; 2000 series Al–Cu alloys are used for fatigue critical applications since these alloys are more damage tolerant, 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.
Section snippets
Developments in 2000 series Al–Cu aluminium alloys
The aluminium–copper (2000 series) alloys are the primary alloys used in airframe structural applications where the main design 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
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/vertical 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
Developments in aluminium–lithium alloys
Reducing the density of materials is accepted as the most effective 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 unique 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,
Developments in joining techniques
Aircraft manufacturers have been continuing their research activities in the field of the construction of aircraft fuselage structures because of the increasing demands on damage tolerance of fuselage structures, increased cost pressure among aircraft manufacturers, 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
Conclusions
Aluminium alloys have been successfully used as primary material for the structural parts of aircraft for more than 80 years. Aircraft designers possess considerable experience in the design, production, operation and maintenance of aluminium airframes. The infrastructure and knowledge base has become mature. However, with the introduction of high performance polymer composites in the application of airframe designs reduced the role of aluminium alloys up to some extent due composites’ high
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