The low cycle fatigue (LCF) and high cycle fatigue (HCF) properties of Al–Li alloys are influenced by alloy composition, microstructural characteristics, tensile stretching prior to artificial aging, and crystallographic texture. In general the fatigue properties, notably the notched HCF resistances, of Al–Li alloys are similar to those of conventional aerospace aluminium alloys. Alloy development programs on newer Al–Li alloys aim to study further the effects of minor alloying additions (rare earths, beryllium, silver and TiB); various thermomechanical treatments; alloy microstructure, notably crystallographic texture and grain size; and the fatigue load history and environment on the mechanical behavior, including the fatigue properties. It is important to note that the occurrence of bilinearity in LCF life-dependence on strain amplitude in most Al–Li alloys engenders the overestimation of the LCF lives in both the hypo-transition (lower strain amplitudes; longer fatigue lives) and hyper-transition (higher strain amplitudes; shorter fatigue lives) regions if the lives are estimated by extrapolation from either of these regions. Further, in cases such as in Al-Li alloys where there are large differences in strength-based (Basquin-like) and plastic strain – based (Coffin-Manson) power-law relationships, it is appropriate to develop an alloy design philosophy based on either plastic strain energy per cycle (Halford-Morrow) or fatigue toughness (total plastic strain energy to fracture). All of these aspects are discussed in detail in this chapter.
The material and manufacturing property requirements for selection and application of 3rd generation aluminium-lithium (Al–Li) alloys in aircraft and spacecraft are discussed. Modern structural concepts using Laser Beam Welding (LBW), Friction Stir Welding (FSW), SuperPlastic Forming (SPF) and selective reinforcement by Fibre Metal Laminates (FMLs) are also considered. Al–Li alloys have to compete with conventional aluminium alloys, Carbon Fibre Reinforced Plastics (CFRPs) and GLAss REinforced FMLs (GLARE), particularly for transport aircraft structures. Thus all these materials are compared before discussing their selection for aircraft. This is followed by a review of the aluminium alloy selection process for spacecraft. Actual and potential applications of 3rd generation Al–Li alloys are presented. For aircraft it is concluded that the competition between different material classes (aluminium alloys, CFRPs and FMLs) has reached a development stage where hybrid structures, using different types of materials, may become the rule rather than the exception. However, aluminium alloys are still the main contenders for spacecraft liquid propellant launchers.
The structural and engineering property requirements for widespread deployment of aluminium-lithium (Al-Li) alloys in aircraft are discussed, particularly with respect to commercial transport aircraft. The development of Al-Li alloys has been driven mainly by the fact that additions of lithium to aluminium alloys lowers the density and increases the elastic modulus, thereby offering the potential of significant weight savings with respect to conventional (non-lithium containing) alloys. The first use of Al-Li alloys in aircraft goes back to the late 1950s (alloy AA 2020) and mid-1960s (alloys 1420 and 1421). These materials are referred to as the 1st generation Al-Li alloys. Subsequently there have been two major development programmes resulting in the 2nd and 3rd generation alloys. Development of the 2nd generation alloys began in the 1970s and continued through the 1980s. Attempts were made to develop families of Al-Li alloys for widespread replacement of conventional alloys. Ultimately this was unsuccessful except for ‘niche’ applications. The failure to find widespread application was associated largely with the too-high lithium contents of the alloys (typically more than 2 wt%). This resulted in serious disadvantages, including mechanical property anisotropy, low short-transverse ductility and fracture toughness, and thermal instability. Development of the 3rd generation Al-Li alloys began in the late 1980s and is ongoing. These alloys have significantly reduced lithium contents (0.75 – 1.8 wt%) and there are other important compositional changes. Silver and zinc have been added for strength, and zinc improves the corrosion resistance; and manganese is added besides zirconium, which was already present in 2nd generation alloys, to control recrystallization and texture. These differences and improved knowledge about thermomechanical processing and heat-treatment have resulted in a family of alloys with significant property advantages covering all major structural areas and applications for transport aircraft.
Most aluminium-lithium (Al–Li) alloy fatigue crack growth (FCG) data have been obtained for 2nd generation alloys, specifically under constant amplitude (CA) and constant stress ratio (CR) loading, and for long/large cracks. These data show the alloys in a favourable light, but this FCG ‘advantage’ essentially disappears under realistic flight simulation loading, and is also absent for short/small cracks. Furthermore, the FCG advantage is due to inhomogeneous plastic deformation, which has undesirable consequences for other important properties. These consequences have greatly restricted the use of 2nd generation alloys in aerospace structures. FCG data for 3rd generation Al–Li alloys are becoming more available. Many of the issues associated with 2nd generation alloys have been eliminated or greatly alleviated as a result of several changes, including reduced Li contents and innovative thermomechanical processing. Consequently, the FCG behaviour of 3rd generation alloys is more similar to that of conventional alloys. Nevertheless, the 3rd generation alloys tend to have better FCG properties than equivalent conventional alloys; and these and other improvements have already led to many aircraft applications.
Airworthiness regulatory bodies are authorised and responsible for verifying and ensuring the safety and reliability of aircraft. There are many civil and military aviation organisations and regulatory bodies. The functions and responsibilities of several of these organizations are summarised in this chapter. Owing to the importance of aircraft structural fatigue, a survey of fatigue design philosophies is also given. This is followed by (i) a discussion of the airworthiness certification methodology for materials and structures, starting with the initial mill products and proceeding via incremental levels to the finished aircraft; and (ii) an example of material certification for an aluminium–lithium (Al-Li) alloy that is a candidate for use in the airframes of light combat aircraft (LCA).
Mechanical working of Al–Li alloys is primarily concerned with aerospace alloy rolled products (sheet and plate), extrusions, and to a lesser extent forgings. These products are fabricated by hot working with intermittent and final heat treatments. This thermomechanical processing (TMP) can be rather complex for the modern 3rd generation Al-Li alloys, but is necessary to obtain optimum combinations of properties. This Chapter is in two parts. Part 1 discusses the ‘workability’ of metals and alloys and the hot deformation characteristics of Al–Li alloys, leading to the concept of Process Maps. A comprehensive Process Map for a binary Al–Li alloy illustrates the usefulness of these Maps for defining temperature–strain rate regions for safe and unsafe hot working, recrystallization and recovery, and superplastic behaviour Part 2 provides some general considerations about processing Al–Li alloy products, followed by a review and discussion of the currently available information for 3rd generation alloys. It is concluded that their complex TMP schedules may make it difficult to obtain optimum combinations of properties for thicker products.