Table 1. Mechanical Properties and Mechanical Performance Evaluation of the Advanced Wrought Aluminum Alloys [7]
In aerospace applications, materials with high strength to weight ratios along with properties such as excellent corrosion resistance, light weight, creep resistance and high thermal strength are needed. Also cost parameters need to be considered without compromising with quality. In accordance with the properties required; aluminium, titanium, magnesium, nickel and their alloys are mostly used in aerospace industries for making most of its sub components. In this paper, a detailed review has been presented on Al based alloy used in making aircraft structures and components. The characteristics of metallic components for aircraft seats are discussed. It has been found that, the aluminum alloys are the major contributors for aircraft components. The aluminium alloys (2xxx, 6xxx, 7xxx and 8xxx) are found to be the prominent ones. Among these, the 8xxx series is widely used due to its low density.
Today's aircraft industry is demanding high support from its raw material suppliers. On one hand, they expect low cost materials for current aircraft versions while on the other hand, new approaches and advanced materials are desired to face the challenges of next century mass air transportations [1].
Ever since the first day of powered flight, aircraft designers are trying to achieve minimum weight. Starting from 1903 to till date, absolute minimum weight and strength to weight ratio are the major priorities for material selection [2]. In initial times of aircraft development for about 25 years, airframe used was made of wooden structure laced by wires and covered with fabrics.
In the development of next generation aircrafts, important features considered are the lighter, stiffer and stronger, less fatigue sensitive and more damage tolerant of materials for airframes and engines [3]. In early of twenty first century, Titanium alloys and Metal Matrix Composites (MMCs) with continuous fibre reinforcement were used for weight reductions of 40-60% in aircrafts; furthermore, it was expected that, use of fibre reinforced polymers, MMCs and ceramic-matrix composites in aircraft engines and structures would increase the engine performance by more than 50%.
The new advanced processing technologies are also being developed to produce high-performance alloys and composites in a cost effective manner. The aircraft industries have global competition between airlines and manufacturers, forcing them to cut down the life cycle cost of the aircraft [4]. The new lighter metallic materials; aluminium matrix composites, hybrid polymer metal composites for airframes and titanium based metal matrix composites for engine applications are being developed, which are capable of service temperatures beyond the capabilities of polymer based materials [3].
The aerospace materials must carry the structural and aerodynamic loads while being inexpensive and easy to fabricate. The aircraft material should not crack, corrode, oxidize or suffer other forms of damage while operating under adverse conditions that involve high loads, freezing and high temperatures, lightning strikes and hail impact, and exposure to potentially corrosive fluids such as jet fuel, lubricants and paint strippers. In addition to high mechanical properties and long-term durability, it is essential that, the materials must be lighter in weight. The savings in weight by using light materials are structurally efficient and it results in less fuel burn, greater range and speed, and smaller engine requirements [5]. The rising fuel costs and higher performance requirements have resulted in renewed interest in alternative materials, distinguished by greater strength to weight ratios [19].
Materials used for aero engine are generally alloys of steel, aluminium, titanium, magnesium and nickel. Other materials used are carbon, ceramic and composites. Carbon composites are now gaining more recognition due to their improved properties and manufacturing processes [6].
The high strength to weight ratio of aluminium favors the selection of aluminium alloys in critical weight applications. Aluminium alloys are the prime choice for fuselages, wings and support structure of commercial airlines and military cargo. The performance characteristics, fabrication cost, design experience and well known fabrication techniques are a few reasons for continuous use of aluminium alloys in significant quantities for future aircrafts [2]. The first superplastic aluminium production aircraft parts were made from aluminium alloy 2004 and the produced parts were seat ejector components.
The 2000, 6000, 7000 and 8000 series are extensively used in aircraft structures. The experimental investigation was carried out on a series of aircraft structural wrought aluminium alloys, for characterization. The potential for mechanical performance was compared with the respective conventional alloys for aircraft structural applications and ranking was given on the basis of its potential for mechanical performance by involving the quality indices. These advanced alloys are currently in use for aircraft structural applications. The alloys made from 2xxx series (2024 and 2091), 6xxx series (6013), 7xxx series (7050, 7075 and 7175) and 8xxx series (8090) were kept in first group. The second group contained newly developed wrought aluminium alloys for aircraft industry; 2024, 2024 HP (High purity), Al-Mg-Li (Bare - B), Al-Mg-Li (Stretched formed – SF), Al-Mg-Sc (Bare - B), Al-Mg-Sc (Cold formed - CF). These alloys along with their mechanical properties are illustrated in Table 1. In Table 1, data has been generated by cutting the specimens in both longitudinal (L) and longitudinal transverse (LT) directions. Tensile specimens were machined according to the ASTM E8M [7].
Table 1. Mechanical Properties and Mechanical Performance Evaluation of the Advanced Wrought Aluminum Alloys [7]
The creep behavior of 30 newly developed aluminium alloys (2000, 6000 and 8000 series) were investigated experimentally for structural applications for the next generation Supersonic Civil Transport Aircrafts. All creep tests were conducted according to ASTM E139 standards using constant load creep machines. The creep results obtained from the plates were slightly better than from the sheets. The creep resistance is sensitive even for slight variations of the alloy chemical compositions [8] .
The 2024 alloy sheets have excellent fracture toughness and fatigue crack propagation characteristics. They also posses higher damage tolerance and longer term durability for aerospace applications. The fracture toughness testing was made according to ASTM E651 and ASTM B646 standards. The fatigue crack propagation testing was made according to the ASTM E647 standard. The dispersoid spacing had little or no effect on fracture toughness. At high ΔK level, ie. 15 MPa m1/2 or higher dispersoid spacing, had no effect on crack propagation rate. At low ΔK level, equal to 15 MPa m1/2 or lower, the fatigue crack propagation rate decreased. At high and low ΔK levels, the propagation rate decreased as the constituent spacing was broadened [9].
Currently, the 7000 series Al–Zn alloys are being used for 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 densities are required. The 2024-T3 has been the most widely used alloys in fuselage construction. The 7055- T7751 has a yield stress that may exceed 620 MPa and the estimated weight saving is high (upto 635kg) in the Boeing aircraft 777. The most effective way to reduce the structural weight of the aircraft is by reducing the density of materials. Li has very less density of approximately 0.54 g/cm3 and it is highly soluble in aluminium and increases the elastic modulus by 6% for each 1% Li addition. The new generation of 2199 Al-Li alloy sheets and plates are capable to be used in aircraft fuselages and lower wing applications. The 2060 and 2050 are the newest 3rd generation Al-Li alloys. Alloy 2060 has 0.75 wt. % of Li and Alloy 2050 has 1.15 wt.% of Li; these alloys have excellent properties like thermal stability, corrosion resistance, high strength and light weight. These alloys offer 10% weight saving and 30% less expense to manufacture, operate and repair [10].
The aluminium alloy 7050-T7451 is used in flight critical airframe structural components mainly due to the technical characteristics associated with its fabrication, service and maintenance. The modelling framework has been applied to predict the cyclic lifetime of the 7050 alloy using process variants based on the populations of life limiting microstructural features, thereby allowing the effects of material pedigree to be predictively linked with the structural integrity of end components [11].
Depending on the seats used in long range aircrafts, they contain up to 44.1 - 42.7 wt% of aluminium. The major metallic components of seats include backrests and armrest, rear and front legs, seat spreaders and beams; each with slight different requirements in strength, ductility and bending properties; and should have sufficient level of fatigue and corrosion resistance. The 7055-T77511 and 3rd generation Al-Li alloy (2099-T83) are used for seat structures requiring high loads [12].
The fatigue tests were conducted for fatigue crack nucleation in 7075-T6 and 7079-T6 alloy sheets according to the ASTM E446 standard. The uncoated thin (1.6mm) and thick (4 mm) specimens were loaded in transverse direction to 310 MPa and 303 MPa, respectively. The 7075- T6511 (old, unused, thick 4.1 mm, anodized both sides, shot peened on one side interior side) and 7079-T6 (old, used, thin 1.7 mm, anodized both sides, shot peened on one side interior side) specimens were loaded in longitudinal direction to a maximum stress of 207 MPa. The fatigue performance of old and coated specimens were inferior as compared to the new uncoated specimens. In designing the fins of the Airbus A310, horizontal stabilizer of Airbus A340 and the Boeing 777, aluminium alloys was the primary material choice for airbus. The 7079-T6 material had seen long service as part of the fuselage skin of a transport aircraft [13].
The impurities Fe and Si coarse constituents in 2xxx, 7xxx and 8xxx alloys, results in lower fracture toughness and effect on both fatigue crack initiation and fatigue crack growth resistance. The Al-Mg-Li alloy 1420 and the Al-Li-Cu- X alloys 2090 and 8090 are now in service in the MIG 29 and the EH1 helicopter [14].
Future structural research issues involve integrating existing and new materials into functional systems with high-quality and low-cost features. The future efforts should address innovative materials processing, low-cost fabrication and other technology challenges to enable more affordable, lighter, higher, stronger and stiffer, safer and more durable vehicles for different flight regimes, for planetary atmospheric entry and for flights throughout the solar system [16]. Figure 1 illustrates some critical parts in an aircraft structure and Table 2 illustrates the materials used in these aircraft components with their grades and alloying elements.
Figure1. Critical Parts in Aircraft Structure
Table 2. Application of Aluminium alloys in Aircraft Components
Giummara et al. (2007) have compared the Al-Li alloys (2099 and 2199) for aerospace applications, with 2024. As compared to 2024, the 2199 (T8E80 and T8E79 tempers) plates have less density, better corrosion resistance, better toughness and yield strengths. The 2099-T83 extrusion alloys have high tensile, compression, shear strength compared to 2024-T83 and also have better corrosion resistance, higher modulus and lower density. In comparison to 2024- T3, the 2199-T8 prime sheets have higher toughness, modulus, yield strengths and improved fatigue crack growth resistance. After conducting the trade study, the alloy 2199 plates are suitable for lower wing skin, 2199 sheets for the fuselage skin and 2099 extruded sections for the lower wing stringers. The use of composites materials in airframes is growing rapidly. This is seen in commercial aircrafts such as Airbus A380, A350XWB and Boeing 787 as well as business aircrafts such as Raytheon and Dassault. The increased usage of composites is being driven by its performance improvement compared to conventional aluminium alloys. The composites offer benefits in both reduced weight and maintenance costs (longer inspection intervals, better corrosion resistance) [17] .
The Al-Li alloy (8090-T651) with melting range of 600-655o C, could be used to build the extendable nose. The future supersonic/hypersonic aircrafts are being designed to cruise at speeds beyond 5.0 mach; which may heat up the tructure to a temperature above 400 oC. This operating condition demands the development of advanced composites that can withstand high temperature gradients [18].
In the last few years, number of researches have been carried out on materials based on processing of aluminium based alloys. It has been found that, these alloys offer tremendous opportunities for savings in the aircraft weight and further improvements in the engine performance are possible.
The 2000 series alloys offer strength and damage tolerance, the 6000 series are conducive to good corrosion resistance and improved machinability, the 7000 series alloys offer higher strength potential and 8000 series alloys provide opportunities for high temperature performance.
Through these published findings, it can be concluded that, the exact set of required properties depend on the specific requirements and applications.