Effect of ageing on precipitation evolution and corrosion behaviour of Al–Zn–Mg

Abstract

The Al–Zn–Mg–Cu alloy was subjected to 4 passes equal channel angular pressing (ECAP) at 400°C, followed by different solutions and ageing treatments. The size, type and distribution of precipitates in aluminium matrix are different in different ageing treatments. In single-stage ageing, a large number of η′ precipitates and Guinier-Preston-II (GP-II) zones are generated in the aluminium matrix. The precipitates are mainly MgZn2 (η) precipitates and η′ precipitates during the double-stage ageing. The size and type of precipitates in three-stage ageing are similar to those in two-stage ageing. Simultaneously, the single-stage ageing sample has poor corrosion resistance and good hardness. Two-stage ageing and three-stage ageing samples have better corrosion resistance, but their hardness is lower than that of single-stage ageing.

Keywords

Al–Zn–Mg–Cu alloy ageing treatment precipitates corrosion behavior

Introduction

In the aerospace lightweight materials, the Al–Zn–Mg–Cu alloy with good comprehensive properties occupies an important position [1,2]. The Al–Zn–Mg–Cu alloy is formed by adding other metal elements on the basis of Al–Zn alloys, which have the characteristics of low density, good processing performance, and excellent corrosion resistance [3 -5]. However, the common Al–Zn–Mg–Cu alloy cannot meet the requirements of corrosion resistance in some important aerospace parts. Therefore, the distribution and size of precipitates can be adjusted by heat treatment and severe plastic deformation (SPD) to improve the mechanical properties and corrosion resistance of the alloy [3,6 -8]. During the solution-ageing heat treatment of 7xxx series aluminium alloys, the precipitation sequence of precipitates is generally solid solution – GP zone – metastable η′ – stable η [9].

ECAP, as a common method of SPD, improves its mechanical properties by producing fine grains under large strain [7,10 -16]. In order to further improve the properties, we also need to carry out appropriate heat treatment on the deformed alloy [15,17 -20]. Hou et al. [21] carried out solution and ageing treatment on 7A99 aluminium alloy after hot extrusion and described the mechanical properties and stress corrosion resistance of different ageing samples. At the same time, the fracture morphology was analyzed, and the mechanism relationship between microstructure evolution, tensile properties and SCC resistance of 7A99Al was explained. At the same time, pitting corrosion, intergranular corrosion and electrochemical corrosion of aluminium alloy are also worthy of our attention [22 -28]. Li et al. [23] studied hot extrusion and warm cross rolling of AA7075 alloy and then annealed at different temperatures. Annealing treatment greatly improves the corrosion resistance by consuming distortion energy, reducing the number of crystal defects and increasing the fraction of low-∑ coincident site lattice grain boundaries. After annealing, the corrosion resistance of rolled AA7075 sheet is better than that of the extruded sheet. Chen et al. [27] comprehensively described the corrosion behaviour of selective laser melted aluminium alloys. Appropriate heat treatment can eliminate the residual stress of the SLM aluminium alloys, but with the increase of heat treatment temperature, the corrosion resistance decreases. Goodarzy et al. [19] carried out different ageing treatments on 2024 aluminium alloy after ECAP and described the microstructure evolution and tensile properties. The dislocation density is calculated by XRD, and it shows that the increase of dislocation density improves the microhardness and yield strength of the deformed alloy. Chen et al. [7] studied the evolution of precipitates and the change of microhardness of 7A85 alloy during ageing at 434 K. The precipitation sequence of the alloy is clearly described, and the relationship between precipitates and microhardness is discussed. However, there are few studies on different ageing treatments of aluminium alloy after high-temperature ECAP, especially on single-stage ageing, two-stage ageing and three-stage ageing. At the same time, the mechanism of single-stage ageing, double-stage ageing and three-stage ageing on the precipitates evolution and corrosion resistance in alloy after high-temperature ECAP is not clear.

In this paper, the Al–Zn–Mg–Cu alloy was carried out ECAP at 400°C, followed by different ageing treatments. After different ageing treatment, the size, type and number density of the precipitates have changed significantly. The corrosion resistance of the single-stage ageing sample is poor, but the hardness is significantly increased. Compared with single-stage ageing, the corrosion resistance of double-stage ageing and three-stage ageing samples is better, and the hardness decreases.

Experimental materials and procedures

The Al–Zn–Mg–Cu alloy (87.1% Al + 1.9% Mg + 8.0% Zn + 2.3% Cu + 0.04% Cr, in wt%) with the specification of φ40 mm * 300 mm was used as raw material. In this experiment, the Al–Zn–Mg–Cu alloy was subjected to 4 passes ECAP at 400°C. Then the alloy after ECAP was solution treated at 450°C for 2 h, and then quenched immediately. Subsequently, the samples after solution treatment were subjected to corresponding ageing treatment. The specific heat treatment scheme is shown in Table 1.

Table 1

The ageing treatments processes of Al–Zn–Mg–Cu alloy by 4 passes ECAP.

Number	Ageing treatments
Single-stage ageing	120°C/24 h
Double-stage ageing	120°C/8 h+150°C/16 h
Three-stage ageing	120°C/24 h+190°C/1.5 h+120°C/24 h

In order to test the corrosion resistance of samples under different ageing treatments, the intergranular corrosion test and immersion corrosion test were carried out on the samples. The samples of intergranular corrosion test were first washed with acetone and then washed with 10% NaOH and 30% HNO3 until the surface was clean. Configure the corrosion solution of 57 g NaCl + 10 mL H₂O₂(30%) + 1L deionised water. The solution was placed in a water bath at (35 ± 0.5)°C and then the cleaned sample was suspended in the corrosive solution. After the corrosion test, the specimen was cleaned and the longitudinal section was polished to measure the corrosion depth. The samples were polished and then immersed in 3.5 wt-% NaCl solution for immersed corrosion test and immersed at room temperature for 72 h. After the test, 5% dilute HNO3 was used to clean the corrosion products and then deionised water and C₂H₅OH were used to rinse the samples carefully. The hardness test was carried out on the microhardness tester of HVS-1000, and the average value of 5 points was calculated for each sample.

The X-ray diffraction (XRD) test was carried out under the conditions of the scanning speed of 2°/min and diffusion angle of 10° to 90°. The texture was examined by electron backscatter diffraction (EBSD), in which the samples were electro-polished in 15% HNO₃ + 85% C₂H₅OH at 16 V. Finally, the microstructure and precipitation behaviour of the samples were characterised by transmission electron microscopy (TEM).

Results

XRD analysis

The XRD profiles of 7055 aluminium alloy after different states are shown in Figure 1(a). The Al–Zn–Mg–Cu alloy after 4 passes ECAP includes not only α-Al matrix but also η precipitates [29]. However, the alloy, after different ageing treatments, only includes α-Al matrix. These phenomena indicate that η precipitates have been dissolved into the aluminium matrix during solution treatment of the samples after ECAP. In different ageing processes, the supersaturated solid solution will re-precipitate from aluminium matrix and form fine η precipitates. However, it is difficult to detect the fine η precipitates in the samples by XRD after different ageing treatments, resulting in the disappearance of the η precipitates [30,31]. In addition, the (111) diffraction peak of samples after different heat treatments is significantly lower than that of ECAP samples. In Figure 1(b), it can be clearly seen that there are some coarse grains in the deformation specimen of 4 passes-ECAP, and many fine grains are distributed around and inside the coarse grains [29].

Texture evolution

Under the influence of external conditions such as plastic deformation or heat treatment, the texture of the alloy will change significantly. Some typical textures appear in aluminium alloy during plastic deformation and heat treatment, such as copper texture, brass texture and cube texture. In order to study the texture evolution after different ageing treatments, the textures of samples under three different conditions were obtained by EBSD. Figure 2 shows the sectional views of constant φ₂ (45°and 90°)-ODF of samples after different ageing treatments. In single-stage ageing process, there are mainly strong Cube 001 <100>texture, Goss 011 <100>texture and Brass 110 <112> texture. Among them, Cube texture is stronger. Compared with single-stage ageing, the texture intensity of Goss texture and Brass texture decreased in double-stage ageing. In three-stage ageing, there are some Goss textures and Cube textures. However, the texture intensity changes little during ageing, indicating that ageing has no significant effect on texture intensity [27,32,33].

Precipitation evolution

Figure 3 shows the typical bright-field TEM images of Al–Zn–Mg–Cu alloy under different ageing treatments. The corresponding selected area electron diffractions (SAED) patterns in <110>Al orientation is shown in Figure 4. It can be obviously found from Figure 3 that the distribution and size of precipitates of Al–Zn–Mg–Cu alloy change greatly under different ageing treatments. As shown in Figure 3(a), the precipitates in the grains are small and homogeneous, and the average size of the precipitates is ∼9.5 nm in single-stage ageing. During the double-stage ageing process, the size of precipitates increased, the number density of precipitates decreased significantly, and the average size of precipitates was ∼16.7 nm. Compared with double-stage ageing, the size of precipitates in three-stage ageing did not change significantly. Due to the long retrogression time in three-stage ageing, the effect of re-ageing is not good.

Figure 3(b,d,f) shows the precipitate-free zones (PFZs) changes of Al–Zn–Mg–Cu alloy with different ageing treatments. It can be seen from Figure 3(b) that grain boundary precipitates (GBPs) show slightly discontinuous distribution, and a narrow PFZs with a width of 20 ± 3 nm can be observed near the grain boundary. During the double-stage ageing, the precipitates on the grain boundary gradually coarsened and distributed independently along the grain boundary. Meanwhile, the width of PFZs increased to 45 ± 5 nm, as shown in Figure 3(d). Compared with the double-stage ageing, the number and size of precipitates in the grain boundary during the three-stage ageing have no obvious change, and the width of PFZs is 47 ± 5 nm.

Figure 4 shows the SAED patterns in <110>Al orientation of Al–Zn–Mg–Cu alloy under different ageing treatments. Furthermore, the SAED pattern corresponds to the bright field TEM images in Figure 3. In Figure 4(a), it can be clearly observed that η′ precipitates and GP-II zones are included in single-stage ageing, as shown by the red arrow. In double-stage ageing, η and η′ precipitates can be observed from Figure 4(b), indicating that η and η′ precipitates are the main precipitates in this stage. As shown in Figure 4(c), the diffraction characteristics of η and η′ precipitates can be obviously observed in <110>Al orientation during three-stage ageing, indicating that the η′ phase is formed in re-ageing stage. We describe the precipitation reflection relative to the Al matrix in the incidence direction of [110]Al, as shown in Figure 4(d).

Figure 5 shows the high-resolution transmission electron microscope (HRTEM) images, local amplification HRTEM maps and corresponding the fast Fourier transformation (FFT) pattern of Al–Zn–Mg–Cu alloy under different ageing treatments. Simultaneously, the HRTEM images corresponds to the SAED patterns in <110>Al orientation in Figure 4. According to the coherent relationship between η precipitates and aluminium matrix, the state of η precipitates can be further determined [34 -39]. At the same time, the HRTEM can be used to verify each other with the above methods. In Figure 5(a,d,e), based on the results of HRTEM and FFT diffraction patterns of precipitates in single stage ageing, GPII zones and η′ precipitates exist in the Al matrix. Figure 5(b,f) shows the HRTEM images and corresponding FFT patterns with double-stage ageing in <110>Al orientation. It can be seen from Figure 5(f) that there are η′ precipitates in the alloy, as shown in the red box. The HRTEM image of three-stage ageing is shown in Figure 5(g), where the coarse η precipitates can be obviously observed.

Properties test

Intergranular corrosion is one of the most common corrosion in aluminium alloys, and it is easy to form microcircuits in corrosive solution [40 -43]. Figure 6 shows the intergranular corrosion cross-section depth of 4 passes-ECAP deformed samples after different ageing treatments. Intergranular corrosion appeared on the cross-section of the samples with different ageing treatments, and the corrosion morphology and maximum corrosion depth changed significantly. In Figure 6(a,b), the maximum corrosion depth of the single-stage ageing sample is ∼78.5 µm. Moreover, the local corrosion is serious, and gradually extends to the interior of the alloy. In two-stage ageing and three-stage ageing, the corrosion depth of the sample is ∼30.2 and ∼38.6 µm, respectively, and the corrosion morphology of the samples is similar. These phenomena show that the corrosion resistance of single-stage ageing is the worst, and the corrosion resistance of double-stage ageing and three-stage ageing is similar.

In order to further study the corrosion behaviour of the alloy, the samples with different ageing treatments were immersed in 3.5 wt-% NaCl solution, and the pitting corrosion on the surface of the samples was observed, as shown in Figure 7. It can be clearly observed from Figure 7 that there are a large number of corrosion pits in the samples with different ageing treatments. In single-stage ageing, there are not only a large number of small corrosion pits but also some small corrosion pits gather to form large corrosion pits. In two-stage ageing and three-stage ageing, the number and size of corrosion pits of the samples are very similar, indicating that their resistance to pitting corrosion is also similar. By observing the number and size of the corrosion pits of the samples, we can determine that the resistance of pitting corrosion of the samples with single-stage ageing is poor and that of the samples with double-stage ageing and three-stage ageing is similar.

Figure 8 shows the average hardness changes of samples under different conditions. Compared with ECAPed sample, the hardness of the samples after different ageing treatments was significantly improved. In single-stage ageing process, the hardness of the sample increases obviously, and the hardness is ∼195 HV. The hardness of two-stage ageing and three-stage ageing also increased, and the hardness was ∼150 and ∼160 HV, respectively. Due to the long retrogression time in three-stage ageing, the increase of hardness is lower than that in single-stage ageing. However, compared with two-stage ageing, the hardness of three-stage ageing is also improved. These phenomena indicate shows that three different ageing treatments can improve the hardness of the samples, and the single-stage ageing is the best. The TEM analysis shows that the precipitates of single-stage ageing are fine and homogeneous, which is very beneficial to the improvement of hardness.

Discussion

Precipitation mechanism

Al–Zn–Mg–Cu alloy is heat-treated strengthening aluminium alloys, which can regulate the type and number density of precipitates through heat treatment to improve mechanical properties and corrosion resistance of aluminium alloys. There are a large number of precipitation behaviours in Al–Zn–Mg–Cu alloy during ageing heat treatment. The ageing temperature and time affect the integrity and precipitation degree of the precipitation process, and these factors also affect the size, type, distribution of the precipitates and the formation of PFZs [30,44 -47]. Three different ageing treatments were used to study the changes of size, distribution and types of precipitates in this paper. In this section, TEM and HRTEM are used to analyze the relevant mechanisms of precipitates after different ageing treatments.

The as-cast Al–Zn–Mg–Cu alloy was subjected to 4 passes ECAP at 400°C, followed by different ageing treatments. After the original sample through SPD, the dislocation density inside the grain increases, as shown in Figure 9. Then, the ECAP samples containing a large number of dislocation densities were subjected to solution-ageing heat treatment. Supersaturated solid solution is obtained by solution treatment, which makes it easier to form fine and uniform precipitates in the ageing process. In Figure 3, the size and distribution of precipitates with different ageing treatments can be clearly seen. Finally, the size and distribution mechanism of precipitates in the microstructure were analyzed, as shown in Figure 10.

In single-stage ageing, a large number of fine and homogeneous precipitates are produced in the aluminium matrix. The strengthening phases are η′ precipitates and GP-II zones, and continuous precipitates are also formed on the grain boundaries. Compared with single-stage ageing, the size of precipitates increases, the number density of precipitates decreases and the GBPs coarsens in double-stage ageing. At the same time, the main types of precipitates are η and η′ precipitates. In two-stage ageing, some GP-II zones are transformed into η′ precipitates, and part η′ precipitates coarsened. Compared with the double-stage ageing, the size and number density of the precipitates did not change significantly, and the types of the precipitates are still η and η′ precipitates in three-stage ageing.

Corrosion mechanism

The traditional Al–Zn–Mg–Cu alloy is prone to local corrosion, such as intergranular corrosion, stress corrosion [40,48,49]. At the same time, the corrosion resistance of the alloy is closely related to the distribution of GBPs and the width of PFZ [50,51]. In the intergranular corrosion process of the alloy, the η precipitates, PFZ and Al matrix have different potentials, which are −0.86, −0.57 and −0.68 V, respectively [52]. Therefore, there is a large potential difference between the Al matrix and the η precipitates. The η precipitates form a corrosion circuit with the Al matrix, and resulting in the anodic dissolution of η precipitates. Discontinuous GBPs on grain boundaries can reduce the anodic dissolution rate, improving the corrosion resistance of the alloy. The wider PFZ can weaken the corrosion potential difference between GBPs and Al matrix and improve the corrosion resistance of the alloy [53,54]. Figure 11 shows the corrosion mechanism diagram of different ageing treatments. In the single-stage ageing process, the distribution of GBPs is roughly continuous, and the width of PFZ is small, resulting in poor corrosion resistance. In two-stage ageing and three-stage ageing, the discontinuous GBPs and larger PFZ width improve the corrosion resistance of the samples.

Conclusions

In this paper, the microstructure, corrosion properties and microhardness of samples under different ageing treatments were studied. The type and size of precipitates and the width of PFZs were described by TEM, and the properties of different samples were tested by corrosion and microhardness. The main conclusions can be summarised as follows:

During the single-stage ageing process, a large number of η′ precipitates and GP-II zones are produced in the aluminium matrix. Compared with single-stage ageing, the precipitates coarsening and the type of precipitates changes in double-stage ageing. In three-stage ageing, the size of the precipitates has no obvious change, and the main types are η and η′ precipitates.

Compared with single-stage ageing, the corrosion resistance of double-stage ageing and three-stage ageing samples is better. The main reason is that large and discontinuous GBPs on grain boundaries and wide PFZ can improve the corrosion resistance of the alloy.

The microhardness of samples with different ageing treatments increased, while the hardness of samples with the single-stage ageing increased significantly. The main reason is that fine and uniform precipitates in single-stage ageing are beneficial to improve the microhardness.

Footnotes

Disclosure statement

All participating authors declare that they have no conflict of interest in this work. We once again solemnly declare that there is no conflict of interest with the submitted work, such as commercial interest or ancillary interest.

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Effect of ageing on precipitation evolution and corrosion behaviour of Al–Zn–Mg–Cu alloy

Abstract

Keywords

Introduction

Experimental materials and procedures

Results

XRD analysis

Texture evolution

Precipitation evolution

Properties test

Discussion

Precipitation mechanism

Corrosion mechanism

Conclusions

Footnotes

References