Background
The heat-treatment process of age-hardenable aluminium alloys called ageing is essential to achieve the high mechanical properties required for aerostructures. Without the ageing process, the heat-treatable alloys would not have the properties needed for highly loaded aircraft components. As mentioned, the ageing process is only effective in the 2000, 4000 (containing Mg), 6000, 7000 and 8000 alloys; the ageing of the other alloy series provides no significant improvement to their mechanical properties.
The heat-treatment process consists of three operations which are performed in the following sequence: solution treatment, quenching, artificial (or thermal) ageing. The process of age-hardening is described in and the explanation provided here is specific to aluminium. Solution treatment involves heating the aluminium to dissolve casting precipitates and disperse the alloying elements through the aluminium matrix. Quenching involves rapid cooling of the hot aluminium to a low temperature (usually room temperature) to avoid the formation of large, brittle precipitates. After quenching the aluminium matrix is supersaturated with solute (alloying) elements. The final operation of ageing involves reheating the alloy to a moderately high temperature (usually 150–200 °C) to allow the alloying elements to precipitate small particles that strengthen the alloy. The heat-treatment process can improve virtually every mechanical property that is important to an aircraft structure (except Young’s modulus that remains unchanged). Properties that are improved include yield strength, ultimate strength, fracture toughness, fatigue endurance and hardness. The heat-treatment process is performed in a foundry using specialist furnaces and ovens, and then the alloy is delivered in the final condition to the aircraft manufacturer. Some large aerospace companies have their own foundry because of the large amount of aluminium alloys used in the production of their aircraft.
In this section, we examine the heat treatment of age-hardenable aluminium alloys. Special attention is given to the age-hardening of 2000, 7000 and 8000 alloys because of their use in aircraft. We focus on the effect of the heat treatment process on the chemical and microstructural changes to aluminium, and the affect of these changes on the mechanical properties. It is worth noting that changes caused by heat treatment are subtlely different between alloy types, and the description provided here is a general overview of the ageing process.
Solution treatment of aluminium
Solution treatment is the first stage in the heat-treatment process, and is performed to dissolve any large precipitates present in the metal after casting. These precipitates can seriously reduce the strength, fracture toughness and fatigue life of aluminium, and therefore it is essential they are removed before the metal is processed into an aircraft structure. The precipitates are formed during the casting process. As the metal cools inside the casting mould, the alloying elements react with the aluminium to form intermetallic precipitates. Depending on the cooling rate and alloy content, the precipitates may develop into coarse brittle particles. The particles can crack at a small plastic strain that effectively lowers the fracture toughness. The purpose of the solution treatment process is to dissolve the large precipitates, and thereby minimise the risk of fracture.
The solution treatment process involves heating the aluminium to a sufficiently high temperature to dissolve the precipitates without melting the metal. The rate at which the precipitates dissolve and the solubility of the alloying elements in solid aluminium both increase with temperature, and therefore it is desirable to solution treat the metal at the highest possible temperature that does not cause melting. The solution treatment temperature is determined by the alloy composition, and allowances are made for unintended temperature variations of the furnace. Control of the temperature during solution treatment is essential to ensure good mechanical properties. When the temperature is too low, the precipitates do not completely dissolve, and this may cause a loss in ductility and toughness. When the temperature is too high, local (or eutectic) melting can occur that also lowers ductility and other mechanical properties. The treatment temperature for most aluminium alloys is within the range of 450–600 °C. The alloy is held at the treatment temperature for a sufficient period, known as the ‘soak time’, to completely dissolve the precipitates and allow the alloying elements to disperse evenly through the aluminium matrix. The soak time may vary from a few minutes to one day, depending on the size and chemical composition of the part. After the alloy has been solution treated it is ready to be quenched.
Quenching of solution-treated aluminium
Quenching involves rapid cooling from the solution-treatment temperature to room temperature to suppress the reformation of coarse intermetallic precipitates and to freeze-in the alloying elements as a supersaturated solid solution in the aluminium matrix. Quenching is performed by immersing the hot aluminium in cold water or spraying the metal with water, and this cools thin sections in less than a few seconds. However, with aluminium components with a complex shape it is often necessary to quench at a slower rate to avoid distortion and internal (residual) stress. Slow quenching is done using hot water or some other fluid (e.g. oil, brine). Ideally, the aluminium alloy should be in a supersaturated solid solution condition with the alloying elements uniformly spread through the aluminium matrix after quenching. However, when slow cooling rates are used some precipitation can occur, and this reduces the ability to strengthen the alloy by thermal ageing. Figure 8.7 shows the effect of quenching rate on the final yield strength of 2024 Al and 7075 Al, which are alloys used in aircraft structures. The final yield strength is determined after the alloys have been quenched and thermally aged. It is seen that increasing the quenching rate results in greater final yield strength. Therefore, it is important to quench at an optimum cooling rate that maximises the concentration of alloying elements dissolved into solid solution whilst minimising distortion and residual stress.
8.7 Effect of average quenching rate on the final yield strength of aerospace alloys 2024 Al and 7075 Al.
After quenching, the aluminium is soft and ductile, and this is the best condition to press, draw and shape the metal into the final product form. For example, when manufacturing aircraft parts using age-hardenable alloys it is easiest to plastically form the components when in the quenched condition. After forming, the aluminium is ready for ageing.
Thermal ageing of aluminium
Ageing is the process that transforms the supersaturated solid solution to precipitate particles that can greatly enhance the strength properties. It is the formation of precipitates that provide aluminium alloys with the mechanical properties required for aerospace structures. Ageing can occur at room temperature, which is known as natural ageing, or at elevated temperature, which is called artificial ageing. Natural ageing is a slow process in most types of age-hardenable alloys, and the effects of the ageing process may only become significant after many months or years. Figure 8.8 shows the increase in yield strength of 2024 Al and 7075 Al alloys when naturally aged at room temperature for more than one year. The strength of 2024 Al alloy rises rapidly during the first few days following quenching, and then reaches a relatively stable condition. The strength of 7075 Al alloy, on the other hand, continues to rise over the entire period. Natural ageing can occur, albeit very slowly, at temperatures as low as − 20 °C. For this reason, it is sometimes necessary to chill aluminium below this temperature immediately after quenching to suppress or delay the ageing process. It is sometimes necessary to postpone ageing when manufacturing aircraft components and, therefore, the metal must be refrigerated immediately after quenching. For example, it is common practice to refrigerate 2024 Al rivets until they are ready to be driven into aircraft panels to maintain their softness which allows them to deform more easily in the rivet hole. More often, however, the alloy is artificially aged immediately or shortly after quenching.
8.8 Effect of natural ageing time on the yield strength of 2024 Al and 7075 Al.
The artificial ageing process is performed at one or more elevated temperatures, which are usually in the range of 150 to 200 °C. The alloy is heated for times between several minutes and many hours, depending on the part size and the desired amount of hardening. During ageing, the alloy undergoes a series of chemical and microstructural transformations that have a profound impact on the mechanical and corrosion properties. The order of occurence of the transformations is:
supersaturated solid solution (αSS);
solute atom clusters (GP1 and GP2 zones);
intermediate (coherent) precipitates; and
equilibrium (incoherent) precipitates.
A summary of the transformations that occur to 2000, 7000 and 8000 aerospace alloys are provided in Table 8.7. It is seen that all the alloys undergo the transformation sequence: supersaturated solid solution → GP zones → intermediate precipitates → equilibrium precipitates. However, the changes that occur depend on the types and concentration of the alloying elements.
Table 8.7
Ageing transformations of 2000, 7000 and 8000 alloys
2000 Alloys
αss → GP zones → Coherent θ″ (CuAl2) → Semicoherent θ′ (CuAl2) → Incoherent θ (CuAl2)
αss → GP zones → Coherent S′ (Al2CuMg) → Incoherent S (Al2CuMg) – high Mg content
7000 Alloys
αss → GP zones → Coherent θ″ (CuAl2) → Semicoherent θ′ (CuAl2) → Incoherent θ (CuAl2)
αss → GP zones → Semicoherent η′ (MgZn2) → Incoherent η (MgZn2)
αss → GP zones → Semicoherent T′ [Al32(Mg,Zn)49] → Coherent T [Al32(Mg,Zn)49]
8000 Alloys
Al–Li: αss → Semicoherent δ′ (Al3Li) → Incoherent δ (AlLi)
Al–Li–Mg: αss → Semicoherent δ′ (Al3Li) → Incoherent Al2MgLi
Al–Li–Cu (low Li:Cu): αss → GP zones → T1 (Al2CuLi) → Coherent θ″ (CuAl2) → Semicoherent θ′ (CuAl2) → Incoherent θ (CuAl2)
Al–Li (high Li:Cu): αss → GP zones → Incoherent T1 (Al2CuLi)
Al–Li–Cu–Mg: αss → GP zones → Semicoherent S′ (Al2CuMg) → Incoherent S (Al2CuMg)
Figure 8.9 shows the different stages of the ageing process with time and the growth in the average size of the solute cluster and precipitate with time. The sizes shown are approximate and depend on the alloy composition and ageing temperature, although the trend shown is similar for most aged metals. We now examine each of the transformations in greater detail.
8.9 Effect of time on the ageing of an aluminium alloy.
When aluminium in the supersaturated solid solution condition is aged, the first significant change is the formation of solute atom clusters, known as Guinier–Preston (GP) zones. GP zones develop by the solute (alloying) atoms moving over relatively short distances to cluster into solute-rich regions. When the zones first develop, the atoms of the alloying elements are randomly arranged relative to the lattice structure of the aluminium matrix, and these are called GP1 zones. The composition of the GP zone is dependent on the alloy content. For example, GP zones formed in 2024 Al alloy are rich in copper and in 7075 Al are rich in copper and zinc. Minor alloying elements (e.g. Mn) can also be present in the zones.
With further ageing the solute atoms become arranged into an ordered pattern that is coherent with the aluminium lattice matrix, and these are known as GP2 zones. The number of GP zones is dependent on the temperature, time and alloy content, and their density can reach 1023 to 1024 m−3. However, GP zones are very small, typically one or two atom planes in thickness and several tens of atom planes in length. Despite their small size, GP zones generate elastic strains in the surrounding matrix that raise the yield strength and hardness. The GP zones grow in size and decrease in number with ageing time until eventually they transform into intermediate precipitates.
During ageing the GP2 zones transform into metastable intermediate precipitates. These precipitates are coherent or semicoherent with the lattice structure of the aluminium matrix. The precipitates are often plate- or needle-shaped and grow along the crystal planes of the matrix. The precipitates nucleate at the sites of GP2 zones and this is known as homogeneous nucleation. Precipitates also grow in regions rich in solute atoms, such as dislocations and grain boundaries, and this is called heterogenous nucleation. Both homogeneous and heterogenous processes are important in the nucleation of precipitates. Following nucleation, the precipitates grow with ageing time as they scavenge solute atoms from the surrounding matrix, and they can reach 0.1 μm or more in size.
In many aluminium alloys, the intermediate precipitates undergo a number of transformations before developing into the final stable condition. For example, in aluminium containing copper, a number of intermediate CuAl2 precipitates (θ′, θ″) having different degrees of coherency with the matrix lattice develop before the final formation of stable CuAl2 (θ) precipitates. During the nucleation and growth of intermediate particles many of the mechanical properties, such as yield strength, fatigue endurance and hardness, are improved. Eventually, the intermediate precipitates transform into stable, equilibrium particles and, at this point, the mechanical properties are maximised.
Equilibrium of the precipitates occurs when the particles reach a final chemical composition and crystal structure that does not change with further ageing. The type of equilibrium precipitates produced by ageing is determined by the composition of the aluminium alloy. The main equilibrium precipitates found in aerospace 2000, 7000 and 8000 alloys are given in Table 8.7. The single most important precipitate in 2000 alloys (Al–Cu) is θ (CuAl2). Various precipitates occur in 7000 alloys (Al–Cu–Zn) including θ, η (MgZn2) and T [Al32(Mg,Zn)49]. Many types of precipitates are also found in 8000 alloys (Al–Li). In Al–Li–Mg alloys, the other main precipitate is Al2MgLi; in Al–Li–Cu alloys, the other precipitates are θ (CuAl2) and T1 (Al2CuLi); and in Al–Li–Cu–Mg alloys the other precipitate is S (Al2CuMg). Examples of precipitates are shown in Fig. 8.10.
8.10 Precipitates in an age-hardenable aluminium alloy (from I. J. Polmear, Light alloys, Butterworth–Heinemann, 1995).
The mechanical properties reach their highest value at the stage when the precipitates transform from coherent to incoherent particles. Continued ageing at too high a temperature for too long a time degrades properties such as strength and hardness as the equilibrium particles grow in size. The largest precipitates continue to grow whereas the smaller particles disappear, resulting in an increase in the average particle size and a reduction in the number of particles. The softening of an alloy as a result of particle coarsening is called over-ageing, and it must be avoided if optimum properties are required.
The optimum ageing condition is achieved by heat treating the aluminium alloy in a foundry at the correct temperature and time. The optimum heat-treatment condition is governed by the composition of the alloy and geometry of the part. However, it is possible that natural ageing of the alloy occurs after the part has been put into service on an aircraft, which may cause over-ageing. Although this is not a significant issue for subsonic aircraft, it may be problem with supersonic aircraft when frictional heating of the aluminium skins at high flight speeds may cause over-ageing. Surface temperatures in excess of 150 °C occur at the leading edges of aircraft during supersonic flight, and this has the potential to weaken the skins. However, structural failures of aluminium alloys on supersonic aircraft caused by over-ageing do not occur because of the design safety margins.
Properties of age-hardened aluminium
The mechanical properties of age-hardenable alloys are dependent on the temperature and time of the ageing operation. Figure 8.11 shows the typical effect of ageing temperature on the tensile strength of an aluminium alloy. The strength increases as the metal undergoes the transformations from a supersaturated solid solution to GP zones to intermediate (coherent) precipitates. At the stage when the precipitates transform from coherent to incoherent particles the maximum strength is reached. Over-ageing causes a deterioration in strength owing to coarsening of the incoherent particles.
8.11 Effect of ageing temperature on the tensile strength of an aluminium alloy.
Aluminium alloys are strengthened by a combination of mechanisms involving solid solution hardening, work hardening and grain boundary hardening, although the dominant mechanism is precipitation hardening. Without the extra strength provided by the precipitates many alloys would not have sufficient strength and toughness for use in lightweight aircraft structures. The initial improvement in strength shown in Fig. 8.11 results from GP zones resisting the movement of dislocations. GP zones generate an elastic strain in the surrounding matrix lattice that resists dislocation slip. Each GP zone provides only a small amount of resistance, but the very high density of GP zones (up to 1023 to 1024 m−3) generates a sufficiently high internal strain to impede dislocation movement. It is the restriction of dislocation movement that causes the yield strength of aluminium to increase during the early stage of ageing. However, GP zones cannot completely stop the movement of dislocations. Dislocations can cut through the zones and continue to move through the aluminium matrix. The coherent precipitates cause a further improvement in strength because they generate a higher internal strain in the matrix lattice than GP zones. As the coherent particles grow in size they provide greater resistance to dislocation slip. As with GP zones, when a dislocation reaches a coherent precipitate it cuts through and then continues to move through the matrix. Maximum strength is achieved at the stage when the precipitates transform from coherent to incoherent particles. Dislocations are unable to cut through incoherent particles, and instead must move around them by the Orowan mechanism. The Orowan strengthening mechanism is described. This mechanism is very resistant to dislocation movement and, thereby, is extremely effective in raising the yield strength. High resistance to dislocation slip occurs when the precipitates are small and closely spaced, that is the situation when the particles are initially transformed into incoherent particles. Over-ageing beyond this stage causes the incoherent precipitates to coarsen and become more widely spaced, and this reduces the efficacy of the strengthening process.
The maximum strength that can be achieved by ageing is dependent on the temperature. Figure 8.12 shows the maximum strength and the heat-treatment time taken to reach the maximum strength for a range of ageing temperatures. The peak strength decreases with increasing temperature, although the time required to reach maximum strength increases rapidly with decreasing temperature. It is often not practical to heat treat a metal product over many days or weeks. A temperature should be selected that provides a compromise between high strength and short ageing time. In the production of aluminium aircraft structures, the ageing temperature is usually in the range of 150–200 °C, which provides a good balance between strength and process time.
18.12 Effect of ageing temperature on (a) maximum tensile strength and (b) time to reach maximum strength of an age-hardenable aluminium alloy.
Although ageing improves mechanical properties such as strength and fatigue resistance, the ageing process may degrade some other properties. Ageing lowers the ductility of aluminium, although the elongation-to-failure of many fully-aged alloys is above 5–10%. The resistance of aluminium alloys to stress corrosion cracking (SCC) may also be affected by age-hardening. The involves the growth of cracks under the combined effects of tension loads and corrosive fluids that lowers the fracture stress of the material. Figure 8.13 shows the effect of ageing time on the SCC resistance of an aluminium alloy, and it reaches a minimum level when the alloy is fully hardened. It is therefore necessary to protect age-hardened alloys against SCC when used in aircraft by using corrosion-resistant protective coatings. Several types of coatings are used for aircraft, including cladding and anodised films.
8.13 Effect of ageing time on the resistance of aluminium to stress corrosion cracking (SCC).
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