**2.5 Processing of aluminium alloys**

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

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 under substantial pressure using a hydraulic ram.

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

High Strength Al-Alloys: Microstructure, Corrosion and Principles of Protection 229

rolling can have a profound effect on corrosion properties like stress corrosion cracking and exfoliation corrosion. For example, the exfoliation corrosion of 7xxx alloys was shown to be due to manganese segregation to interfaces (grain boundaries and the external surface)

Typically, a NSDL is characterised by ultra-fine, equiaxed grains, with grain boundaries decorated by nano-sized oxide particles (Fishkis and Lin 1997; Leth-Olsen, Nordlien et al. 1998; Plassart 2000; Scamans 2000; Afseth, Nordlien et al. 2001; Zhou, Thompson et al. 2003; Liu, Frolish et al. 2010; Scamans, Frolish et al. 2010; Thompson 2010; Zhou 2011). It is associated with the susceptibility of several aluminium alloys to filiform corrosion (Zhou, Thompson et al. 2003; Liu, Zhou et al. 2007; Liu, Laurino et al. 2010).The depth of the modified surface region ranges from a few nanometers (after polishing) to 8 µm (during rolling). In the latter case the thickness varies with each rolling pass (Fishkis and Lin 1997). Further, a transition region, characterized by microbands that consist of elongated grains aligned parallel to the working surface, may be sandwiched between the surface regions and the bulk alloy. The deformed layers are stable at ambient temperature, associated with the local presence of a large fraction of high angle grain boundaries. The structure is also stabilized through pinning of the grain boundaries by oxide particles and precipitates (Figure 2). NSDLs with grain boundaries decorated by oxide can survive typical annealing and solution heat treatment processes. As a result, metal finishing and surface treatments are

Fig. 2. Transmission electron micrographs of ultramicrotomed sections displaying the surface/near-surface regions of an AA5754 H19 aluminium alloy: (a) as cold rolled,

during DC casting (Evans 1971).

transverse to rolling direction

to realise that corrosion performance of an alloy is dictated by each processing step, starting with the solidification of the molten alloy during DC casting.

DC casting starts by pouring molten metal into a water-cooled aluminium or copper mould. Accumulation of alloying elements at the surface can occur through segregation processes where mobile elements diffuse from the bulk and from grain boundaries. In general, the surface enriched elements have a high negative free energy for oxide formation and high diffusion coefficient through aluminium metal. Such elements include lithium, magnesium and silicon (Carney, Tsakiropoulos et al. 1990). The segregation occurs during forming and heat treatments and has been demonstrated to influence the corrosion and wear properties of the alloys (Nisancioglu 2004).

Thermodynamic considerations often fail to correctly predict the phase and solid solution content of an as-cast microstructure because of the non-equilibrium nature of solidification during DC casting. This is important as alloy corrosion properties are controlled by solid solution levels and intermetallic phase crystallography and morphology, which depend on complex kinetic competitions during nucleation and growth. Most importantly, the constituent particles do not appreciably dissolve during subsequent solution heat treatment, and will thus persist into the final product.

### **2.6 Surface microstructures**

Fabrication processes, including rolling, machining and mechanical grinding, produce aluminium products of the required gauge thickness and shape for various applications. Rolling blocks or slabs that are up to many tonnes, requires heating to temperatures up to 500°C and passing through a breakdown mill using heavy reductions per pass to reduce the slab gauge from as large as 5000 mm down to 15 to 35 mm. The slab surface undergoes intense shear deformation during this process and a NSDL develops. The shearing process also influences IM particles below the surface resulting in a larger number of IM particles in the vicinity of the surface than in the body of the material. This is not as a result of precipitation processes but is due to fragmentation of brittle particles during rolling. Hence the particle number density at the surface is higher, but the percentage of surface area is the same indicating particle fracture rather than new particle formation (Hughes, Boag et al. 2006). The characteristics of the surface of sheet AA2024-T3 with respect to the body of the material (obtained by polishing) are compared in Table 1 (Hughes, Boag et al. 2006). The slab from the breakdown mill is then typically hot rolled on a multistand tandem mill down to a gauge of 2.5 to 8 mm. Hot rolling deforms the original cast structure with the grains being elongated in the rolling direction. The elongated microstructure developed during hot


Table 1. IM particle size (area) for polished (body) and as-rolled (surface) AA 2024

to realise that corrosion performance of an alloy is dictated by each processing step, starting

DC casting starts by pouring molten metal into a water-cooled aluminium or copper mould. Accumulation of alloying elements at the surface can occur through segregation processes where mobile elements diffuse from the bulk and from grain boundaries. In general, the surface enriched elements have a high negative free energy for oxide formation and high diffusion coefficient through aluminium metal. Such elements include lithium, magnesium and silicon (Carney, Tsakiropoulos et al. 1990). The segregation occurs during forming and heat treatments and has been demonstrated to influence the corrosion and wear properties

Thermodynamic considerations often fail to correctly predict the phase and solid solution content of an as-cast microstructure because of the non-equilibrium nature of solidification during DC casting. This is important as alloy corrosion properties are controlled by solid solution levels and intermetallic phase crystallography and morphology, which depend on complex kinetic competitions during nucleation and growth. Most importantly, the constituent particles do not appreciably dissolve during subsequent solution heat treatment,

Fabrication processes, including rolling, machining and mechanical grinding, produce aluminium products of the required gauge thickness and shape for various applications. Rolling blocks or slabs that are up to many tonnes, requires heating to temperatures up to 500°C and passing through a breakdown mill using heavy reductions per pass to reduce the slab gauge from as large as 5000 mm down to 15 to 35 mm. The slab surface undergoes intense shear deformation during this process and a NSDL develops. The shearing process also influences IM particles below the surface resulting in a larger number of IM particles in the vicinity of the surface than in the body of the material. This is not as a result of precipitation processes but is due to fragmentation of brittle particles during rolling. Hence the particle number density at the surface is higher, but the percentage of surface area is the same indicating particle fracture rather than new particle formation (Hughes, Boag et al. 2006). The characteristics of the surface of sheet AA2024-T3 with respect to the body of the material (obtained by polishing) are compared in Table 1 (Hughes, Boag et al. 2006). The slab from the breakdown mill is then typically hot rolled on a multistand tandem mill down to a gauge of 2.5 to 8 mm. Hot rolling deforms the original cast structure with the grains being elongated in the rolling direction. The elongated microstructure developed during hot

**IM Particle Characteristic Body Surface**  Number Density: 5.3x105cm-2 11.7x105cm-2 Average Particle Size: 6.66μm2 1.98μm2 Median Particle Size: 1.6μm2 1.2μm2 %Surface Area: 2.89% 2.82% Total Particles per 1mm2: 5300 11690 Minimum Particle Size: 0.40μm2 0.34μm2 Maximum Particle Size: 327μm2 114μm2

Table 1. IM particle size (area) for polished (body) and as-rolled (surface) AA 2024

with the solidification of the molten alloy during DC casting.

of the alloys (Nisancioglu 2004).

**2.6 Surface microstructures** 

and will thus persist into the final product.

rolling can have a profound effect on corrosion properties like stress corrosion cracking and exfoliation corrosion. For example, the exfoliation corrosion of 7xxx alloys was shown to be due to manganese segregation to interfaces (grain boundaries and the external surface) during DC casting (Evans 1971).

Typically, a NSDL is characterised by ultra-fine, equiaxed grains, with grain boundaries decorated by nano-sized oxide particles (Fishkis and Lin 1997; Leth-Olsen, Nordlien et al. 1998; Plassart 2000; Scamans 2000; Afseth, Nordlien et al. 2001; Zhou, Thompson et al. 2003; Liu, Frolish et al. 2010; Scamans, Frolish et al. 2010; Thompson 2010; Zhou 2011). It is associated with the susceptibility of several aluminium alloys to filiform corrosion (Zhou, Thompson et al. 2003; Liu, Zhou et al. 2007; Liu, Laurino et al. 2010).The depth of the modified surface region ranges from a few nanometers (after polishing) to 8 µm (during rolling). In the latter case the thickness varies with each rolling pass (Fishkis and Lin 1997). Further, a transition region, characterized by microbands that consist of elongated grains aligned parallel to the working surface, may be sandwiched between the surface regions and the bulk alloy. The deformed layers are stable at ambient temperature, associated with the local presence of a large fraction of high angle grain boundaries. The structure is also stabilized through pinning of the grain boundaries by oxide particles and precipitates (Figure 2). NSDLs with grain boundaries decorated by oxide can survive typical annealing and solution heat treatment processes. As a result, metal finishing and surface treatments are

Fig. 2. Transmission electron micrographs of ultramicrotomed sections displaying the surface/near-surface regions of an AA5754 H19 aluminium alloy: (a) as cold rolled, transverse to rolling direction

High Strength Al-Alloys: Microstructure, Corrosion and Principles of Protection 231

strength. Defects are generally undesirable since some types of defects give rise to poorer creep resistance. As will be seen below they appear to enhance corrosion in the form of grain

IM particles such as constituent and impurity particles exist at larger scales with minimum sizes generally between 0.5 to 1.0 μm. Some types of these particles can achieve local thermodynamic equilibrium during ingot formation. However, processing anomalies may mean that an equilibrium structure is not achieved in practice. Other constituents such as compositions in the Al-Cu-Mg ternary subphase field for 2xxx alloys and Al-Mg-Zn for 7xxx

> Point Defects <1Å Line defect – tens of nm long

Dislocations

Table 2. Microstructural features in high strength aluminium alloys

The AA2xxx series of alloys are among the most complicated to analyse. While there have been several reports of the compositions of different phases within this group, most have focused on the legacy alloy AA2024-T3, which, unfortunately, is one of the most complex of the 2xxx series of alloys. Perhaps one of the better known of these works was published in 1950 by Phragmen (Phragmen 1950). He examined all the binary, ternary, quaternary quinternary and senary compositions in order to under stand the IM particles in AA2024. Unfortunately the classification of these particles relied heavily on metallographic techniques (etches) and optical microscopy meaning that assignment of IM particles was, in many cases made on appearance and not on composition. These types of studies, however,

tens of nm wide (including the zone of influence such as depleted zones)

**Size Associated Corrosion** 

energy.

20 nm x 200 nm Can facilitate intergranular

Generally 0.5 μm to 50 μm Localised attack of particle if

50 nm x 400 nm Under some conditions

50 μm to 500 μm Associated with pitting

attack.

matrix

the surface.

evidence from

Grain etchout associated with higher grain stored

Intergranular attack. Some

generally only facilitated by second phase precipitates.

undergo preferential attack

anodic wrt the matrix and trenching in surrounding matrix if cathodic to the

attack that propagates into

misorientation angle for preferred corrosion,

etchout and have an influence on intergranular attack.

can redissolve during subsequent heat and other treatments.

**Microstructural Feature and when formed** 

Atomic Defects (At any time during

Grain Boundaries (At any time during

Hardening Precipitates (Ageing after Solution

Dispersoids (Ageing after

Constituent Particles and Impurity Particles

(Primary Ingot Production)

Solution treating)

Clusters of particles (Ingot Working)

processing)

processing)

treating)

**AA2xxx** 

required to remove these electrochemically active layers (Leth-Olsen, Nordlien et al. 1997; Mol, Hughes et al. 2004; Hughes, Mol et al. 2005). However, the presence of fine grains alone in the deformed layer, with grain boundaries free of oxide particles, is insufficient to hinder grain coarsening during typical annealing treatments (Zhou 2011).

Importantly, the NSDL has significant influence on properties such as the electrochemical and corrosion behaviour as well mechanical properties, material joining and optical properties. The high population of grain boundaries and severe deformation in the deformed layer promote precipitation of intermetallic particles during subsequent heat treatment (Liu, Zhou et al. 2007). For example, a near-surface deformed layer on AA6111 automotive closure sheet alloy can be generated by mechanical grinding during rectification, as shown in Figure 3 (left). Subsequent paint baking, i.e. thermal exposure at 180C for 30 minutes, promotes the precipitation of Q phase (with various compositions: Al5Cu2Mg8Si6 (Pan, Morral et al. 2010), Al4CuMgSi4 (Hahn and Rosenfield 1975)) particles, ~20 nm diameter, at preferred grain boundaries within the deformed layer (Figure 3 centre), but with no precipitates being formed in the underlying bulk alloy. The presence of Q phase precipitation in the near-surface deformed layer increases dramatically the susceptibility of the alloy to cosmetic corrosion that propagates intergranularly, with micro-galvanic coupling between the Q phase precipitates and the adjacent aluminium matrix providing the driving force (Figure 3(right)).

Fig. 3. Transmission electron micrographs of ultramicrotomed section of AA6111 aluminium alloy after SHT, mechanical grinding and 30 minutes at 180C: (a) bright field image, revealing a near-surface deformed layer and (b) dark field image at increased magnification, revealing grain boundary precipitates. (c) transmission electron micrograph showing intergranular corrosion

### **2.7 High strength aluminium alloys AA2xxx and AA7xxx**

Microstructural variation in the high strength Al-alloys exists over a range of scales as reported in Table 2. At the atomic and nanoscopic scale the microstructure is related to the mechanical properties of the alloy. This microstructure involves defect structures, hardening precipitates and dispersoid particles. The high strength of the 2xxx and 7xxx series alloys is due to the hardening precipitates with dispersoids playing a secondary role. Dispersoids can pin grain growth limiting grain size thus making a small contribution to increased strength. Defects are generally undesirable since some types of defects give rise to poorer creep resistance. As will be seen below they appear to enhance corrosion in the form of grain etchout and have an influence on intergranular attack.

IM particles such as constituent and impurity particles exist at larger scales with minimum sizes generally between 0.5 to 1.0 μm. Some types of these particles can achieve local thermodynamic equilibrium during ingot formation. However, processing anomalies may mean that an equilibrium structure is not achieved in practice. Other constituents such as compositions in the Al-Cu-Mg ternary subphase field for 2xxx alloys and Al-Mg-Zn for 7xxx can redissolve during subsequent heat and other treatments.


Table 2. Microstructural features in high strength aluminium alloys
