**2.2 Aluminium production**

Bauxite production has increased 50% in the past decade to an all time high of over 200 million tonnes worldwide; with Australia the largest producer, followed by China and Brazil. Four tonnes of bauxite are used to produce two tonnes of alumina, which can then produce one tonne of aluminium. Recycling of aluminium requires 95% less energy than for primary aluminium production. In order to meet the mechanical and corrosion performance requirements for many alloys as required under performance specifications, much of the recycled metal must be blended or diluted with primary metal to reduce impurity levels. The result is that, in many cases, recycled metal tends to be used primarily for lower grade casting alloys and products (Polmear 2004), however with ~35% of Al being produced from recycled material, the future ramifications for corrosion will need addressing.

### **2.3 Physical metallurgy of aluminium alloys**

The functional properties of aluminium alloys (mechanical, physical, and chemical) depend on alloy composition and alloy microstructure as determined by casting conditions and

their roles in corrosion of alloys. The physico-chemical studies have led to a better characterisation of the composition of intermetallic particles, and, importantly of their spatial distributions. The convergence of the electrochemical and physicochemical approaches, in combination with modelling, is leading to a statistical basis for understanding the influence of the intermetallic particles on corrosion of aluminium alloys. This chapter therefore aims to summarise our current understanding of the microstructure of high strength aluminium alloys, particularly the more microstructurally complex alloys such as the 2xxx, and 7xxx series alloys. It then looks at how different components of the microstructure contribute to corrosion processes and finishes by examining the principles of protection of aluminium alloys using traditional and newer techniques used to assess the

While aluminium alloy microstructure, for some specific alloys, is relatively well known, the microstructures for some high strength aluminium alloys, particularly the older AA2*xxx* alloys, is not well described or understood in the scientific literature, particularly the corrosion literature. This is partially due to manufacturing processing conditions which do not realise the intended microstructure and partially due to quasi or non-equilibrium microstructure existing in real alloys because of the difficulty of obtaining full thermodynamic equilibrium. Typical examples of common high strength alloys used in aircraft manufacture, for example, include AA2024-T3, AA7075-T6 and AA6061-T6. This section, therefore, provides a general

Processing can significantly alter the bulk microstructure, resulting in microstructural gradients and zones with different characteristics. A good example of these changes can be found in wrought alloy sheet product. First there is a gradient in grain size and constituent particle size across the sheet. Second, shear deformation, resulting from rolling, creates a surface layer called a near surface deformed layer (NSDL) with a very fine microstructure which may have a different degree of precipitation compared to the bulk depending on the

Bauxite production has increased 50% in the past decade to an all time high of over 200 million tonnes worldwide; with Australia the largest producer, followed by China and Brazil. Four tonnes of bauxite are used to produce two tonnes of alumina, which can then produce one tonne of aluminium. Recycling of aluminium requires 95% less energy than for primary aluminium production. In order to meet the mechanical and corrosion performance requirements for many alloys as required under performance specifications, much of the recycled metal must be blended or diluted with primary metal to reduce impurity levels. The result is that, in many cases, recycled metal tends to be used primarily for lower grade casting alloys and products (Polmear 2004), however with ~35% of Al being produced from

The functional properties of aluminium alloys (mechanical, physical, and chemical) depend on alloy composition and alloy microstructure as determined by casting conditions and

recycled material, the future ramifications for corrosion will need addressing.

overview of the relationship between processing and microstructure.

degree of protection.

**2. Microstructure** 

heat treatments.

**2.2 Aluminium production** 

**2.3 Physical metallurgy of aluminium alloys** 

**2.1 Aluminium alloys in general** 

thermomechanical processing. Only a small number of metals have sufficient solubility to serve as major alloying elements (Das 2006) and alloys derived from these few form the basis of the present classes of commodity Al-alloys. Magnesium, zinc, copper and silicon have significant solubility, whilst additional elements (of <1% solubility) are also used to confer improvements to alloy properties, namely grain refinement, and such elements include manganese, chromium, zirconium, titanium and less commonly (due to cost) scandium (Hatch 1984; Polmear 2004). Alloying of Manganese with Fe-containing intermetallic particles reduces the electrochemical activity of these Fe-containing particles thus improving the corrosion resistance of the alloy (Polmear 1995)

The low strength of pure aluminium (~10 MPa) mandates alloying. The simplest strengthening technique is solution hardening, whereby alloying additions have appreciable solid solubility over a wide range of temperatures and remain in solution after many thermal cycles.

The most significant increase in strength for aluminium alloys is derived from age hardening (often called precipiation hardening) which can result in strengths as high as 800 MPa. The principal of age hardening requires that the solid solubility of alloying elements decreases with temperature. The age hardening process can be summarised by the following stages:


The fundamental aspects of decomposition of a supersaturated solid solution are complex (Raviprasad, Hutchinson et al. 2003; Kovarik, Miller et al. 2006; Winkelman, Raviprasad et al. 2007). Typically however, Guinier-Preston (GP) zones and intermediate phases are formed as precursors to the equilibrium precipitate phase (Hatch 1984) (Figure 1 reveals a

Fig. 1. Dark field scanning transmission electron micrograph of coarse Al2CuMg precipitate particles in an AA2xxx (Al-Cu-Mg) alloy - imaged down <100> zone axis

High Strength Al-Alloys: Microstructure, Corrosion and Principles of Protection 227

(Hatch 1984)). The temper designation system adopted by the Aluminium Association is

Copper is one of the most common alloying additions, since it has appreciable solubility and a significant strengthening effect by its promotion of an age hardening response. These alloys were the foundation of the modern aerospace construction industry and, for example AA2024 (Al-4.4Cu-1.5Mg-0.8Mn), can achieve strengths of up to 520MPa depending on temper. The microstructure of this series is considered further below. Cu, however, is one of the nobler alloying elements and therefore supports a high rate of oxygen reduction which drives one half of the galvanic reaction. The cell is completed by the dissolution of any element less noble, particularly Al thereby facilitating the onset and propagation of

The Al-Zn-Mg alloy system provides a range of commercial compositions, primarily where strength is the key requirement. Al-Zn-Mg-Cu alloys have traditionally offered the greatest potential for age hardening and as early as 1917 a tensile strength of 580MPa was achieved, however, such alloys were not suitable for commercial use until their high susceptibility to stress corrosion cracking could be moderated. Military and commercial aerospace needs led to the introduction of a range of high strength aerospace alloys of which AA7075 (Al-5.6Zn-2.5Mg-1.6Cu-0.4Si-0.5Fe-0.3Mn-0.2Cr-0.2Ti) is perhaps the most well known, and which is now essentially wholly superseded by AA7150 (or the 7x50 family). The high strength 7xxx series alloys derive their strength from the precipitation of η-phase (MgZn2) and its precursor forms. The heat treatment of the 7xxx series alloys is complex, involving a range of heat treatments that have been developed to balance strength and stress corrosion cracking performance, comprising secondary (or more) heat treatments that can include

The surface layers of aluminium alloys can be altered during processing and storage environments, which adds complexity to the surface finishing and corrosion performance (Fishkis and Lin 1997). These effects include the formation of near surface deformed layers (NSDL) during mechanical processing, the elongation of crystalline structure during rolling and extrusion, breakup of brittle intermetallic particles, differences in surface roughness and

Casting from the melt is the first processing step. The three most commonly used processes are sand casting, permanent mould casting and die casting. Sand moulds are gravity fed whereas the metal moulds used in permanent mould casting are either gravity fed or by using air or gas pressure to force metal into the mould. In high pressure die castings, parts up to approximately 5 kg are made by injecting molten aluminium alloy into a metal mould

For large production scale, direct chill (DC) casting is a semi-continuous process used for the production of rectangular ingots or slab for rolling to plate, sheet, foil and cylindrical ingots or billet for extruded rods, bars, shapes, hollow sections, tube and wire. DC casting is the first step in the production of Al alloys prior to the thermomechanical treatments, and whilst it may appear to be a topic not requiring discussion in such a chapter, it is important

porosity, and the segregation of specific alloying elements to the surface.

similar for both wrought and cast aluminium alloys.

retrogression and re-aging (Sprowls 1978).

under substantial pressure using a hydraulic ram.

**2.5 Processing of aluminium alloys** 

**2xxx** 

corrosion.

**7xxx** 

typical micrograph showing precipitate particles). GP zones are formed when solute atoms (e.g. Cu, Zn and Mg) accumulate along preferred crystal directions in the Al lattice and form a strengthening phase.

Properties can be enhanced further by careful thermo-mechanical processing that may include heat treatments like duplex aging and retrogression and re-aging. Maximum hardening in commercial alloys is often achieved when the alloy is cold worked by stretching after quenching and before aging, increasing dislocation density and providing more heterogeneous nucleation sites for precipitation. Whilst only moderate increases in strength can be obtained in Al-alloys by exploiting the Hall-Petch1 relationship (Polmear 1995), refinement is important for a range of properties including fracture and toughness. Grain refinement in aluminium alloys is achieved by additions of small amounts of low solubility elements such as Ti and B to provide grain nuclei, and by recrystallisation control using precipitates called dispersoids (typically 40 x 200 nm) which are formed from aluminium and alloying additions such as Cu, Cr, Zr or Mn to promote insoluble particles which subsequently can restrict or pin grain growth.

The microstructures developed in aluminium alloys are complex and incorporate a combination of equilibrium and non-equilibrium phases. Non-equilibrium phases exist in essentially all high-strength alloys, and as such, their properties are very temperature dependent.

Typical commercial alloys can have a chemical composition incorporating as many as ten alloying additions (with a number of these additions being unavoidable impurities). As such, from a corrosion perspective, one must understand the role of impurity elements on microstructure. Whilst not of major significance to alloy designers, impurity elements such as Fe, Mn and Si can form insoluble compounds called constituent particles. These are comparatively large and irregularly shaped with characteristic dimensions ranging from 1 to ~ 50 μm. These particles are formed during alloy solidification and are not appreciably dissolved during subsequent thermo-mechanical processing. Rolling and extrusion tend to break-up and align constituent particles within the alloy. Often constituents are found in clusters made up of several different intermetallic compound types. Because these particles are rich in alloying elements, their electrochemical behaviour can be significantly different to the surrounding matrix phase. In most alloys pitting is associated with specific constituent particles present in the alloy (Buchheit 1995; Liao, Olive et al. 1998; Wei, Liao et al. 1998; Guillaumin and Mankowski 1999; Park, Paik et al. 1999; Ilevbare, Schneider et al. 2004; Schneider, Ilevbare et al. 2004; Lacroix, Ressier et al. 2008; Lacroix, Ressier et al. 2008; Boag, Taylor et al. 2010. These are discussed below.

### **2.4 Alloy classification**

The International Alloy Designation System (IADS) gives each wrought alloy a four-digit number of which the first digit is assigned on the basis of the major alloying element(s) (Polmear 1995; Winkelman, Raviprasad et al. 2007)). The main alloying element for AA2xxx is Cu and for AA7xxx is Zn, with Mg playing a important role is both classes of alloys.

For cast aluminium alloys, alloy designations principally adopt the notation of the Aluminium Association System. The casting compositions are described by a four-digit system that incorporates three digits followed by a decimal (described in more detail in

<sup>1</sup> The Hall-Petch relationship states that the yield strength is proportional to the inverse square root of the grain size.

(Hatch 1984)). The temper designation system adopted by the Aluminium Association is similar for both wrought and cast aluminium alloys.
