The influence of the ageing temperature on different properties of the EN AW-7075 aluminium alloy

a. The differential thermal analysis (DTA)

From the Fig. 1, it can be seen that one wide peak appears at around 200 °C. Based on the literary data (Panigrahi and Jayaganthan, 2011Panigrahi, S.K., Jayaganthan, R. (2011). Effect of Annealing on Thermal Stability, Precipitate Evolution, and Mechanical Properties of Cryorolled Al 7075 Alloy. Metall. Mater. Trans. A. 42 (10), 3208-3217. https://doi.org/10.1007/s11661-011-0723-y.; Ku et al., 2018Ku, M.H., Hung, F.Y., Lui, T.S., Lai, J.J. (2018). Enhanced Formability and Accelerated Precipitation Behavior of 7075 Al Alloy Extruded Rod by High Temperature Aging. Metals. 8 (8), 648. https://doi.org/10.3390/met8080648.; Hebbar et al., 2020Hebbar, S., Kertsch, L., Butz, A. (2020). Optimizing Heat Treatment Parameters for the W-Temper Forming of 7xxx Series Aluminum Alloys. Metals 10 (10), 1361. https://doi.org/10.3390/met10101361.) and the appearance of one wide DTA peak at around 200 °C, it can be concluded that this peak represents the precipitation range of the metastable η’ phase. Due to the wideness of the peak, it can be assumed that some other phases will precipitate in that temperature range, i.e., GP zones (a temperatures lower than 200 °C). Based on the DTA curve, the temperature range of the ageing regime was defined (110 °C - 250 °C).

b. Investigation of the hardness

Figure 2 shows the change in hardness values as a function of the ageing temperature during the isochronal ageing for 30 min The hardness gradually increases with the ageing temperature, reaching the maximum value of 158 ± 5 HV₁₅ after ageing for 30 min at 150 °C. The hardness values obtained for all aged samples are higher in comparison with the quenched sample (98 ± 3 HV₁₅). Similar results were reported by other authors (Fan et al., 2021Fan, Y., Tang, X., Wang, S., Chen, B. (2021). Comparisons of Age Hardening and Precipitation Behavior in 7075 Alloy Under Single and Double-Stage Aging Treatments. Met. Mater. Int. 27, 4204-4215. https://doi.org/10.1007/s12540-020-00875-7.). These results confirm that the EN AW-7075 aluminium alloy has a very good response to this type of ageing treatment. The hardness peak obtained appears in the temperature range where the DTA peak appeared (Fig. 1). However, the tip of the DTA peak is shifted to a higher temperature compared to the hardness peak due to the factors such as: low calorimetric effect, sample size, heating rate and factors concerning measuring equipment (furnace atmosphere, type of sample holders, sensitivity of the recording system etc.). It should be borne in mind that diffusionally activated phase transformations, like the precipitation reactions, significantly affect the heating rate. Somewhat similar results were obtained by other authors (Panigrahi and Jayaganthan, 2011Panigrahi, S.K., Jayaganthan, R. (2011). Effect of Annealing on Thermal Stability, Precipitate Evolution, and Mechanical Properties of Cryorolled Al 7075 Alloy. Metall. Mater. Trans. A. 42 (10), 3208-3217. https://doi.org/10.1007/s11661-011-0723-y.; Ku et al., 2018Ku, M.H., Hung, F.Y., Lui, T.S., Lai, J.J. (2018). Enhanced Formability and Accelerated Precipitation Behavior of 7075 Al Alloy Extruded Rod by High Temperature Aging. Metals. 8 (8), 648. https://doi.org/10.3390/met8080648.; Hebbar et al., 2020Hebbar, S., Kertsch, L., Butz, A. (2020). Optimizing Heat Treatment Parameters for the W-Temper Forming of 7xxx Series Aluminum Alloys. Metals 10 (10), 1361. https://doi.org/10.3390/met10101361.; Liu et al., 2020Liu, Y., Zhu, B., Wang, Y., Li, S., Zhang, Y. (2020). Fast solution heat treatment of high strength aluminum alloy sheets in radiant heating furnace during hot stamping. Int. J. Lightweight Mater. 3 (1), 20-25. https://doi.org/10.1016/j.ijlmm.2019.11.004.).

Up to the temperature of 130 °C, the increase in hardness values is significant in comparison with the quenched sample. At these ageing temperatures, the formation of the GP zones occurs and the interaction with the moving dislocations starts. GP zones are coherent with the aluminium matrix; therefore, dislocations either cut them or lock onto them. During this process the surface energy increases and the kinetic energy of the dislocations is spent, causing the hardness of the investigated alloy to increase as it is seen in Fig. 2. Siddiqui et al. (2000)Siddiqui, R.A., Abdullah, H.A., Al-Belushi, K.R. (2000). Influence of aging parameters on the mechanical properties of 6063 aluminium alloy. J. Mater. Process. Technol. 102 (1-3), 234-240. https://doi.org/10.1016/S0924-0136%2899%2900476-8. confirmed that the higher the ageing temperature and time, the faster the formation kinetics of the GP zones and the larger their density were. As the ageing temperature increases, the GP zones start to transform into the metastable η’ phase. It can be assumed that there is a temperature range where GP zones and metastable η’ phase coexist. The dissolution of GP zones and nucleation of the metastable η’ phase is a diffusion driven process that needs higher temperatures and longer times to complete. Therefore, the increase in the hardness values and the achievement of the peak hardness after ageing at 150 °C for 30 min is due to the existence of these hardening phases, i.e., GP zones and the metastable η’ phase. The metastable η’ phase is semi-coherent with the aluminium matrix and, therefore, a small lattice distortion is expected. With this distortion, several slip systems begin to appear that cause the accumulation of a larger number of dislocations. Dislocations are inhibited by the newly formed η’ precipitates and they accumulate at grain boundaries, leading to the highest hardness values (Sha and Cerezo, 2004Sha, G., Cerezo, A. (2004). Early-stage precipitation in Al-Zn-Mg-Cu alloy (7050). Acta Mater. 52 (15), 4503-4516. https://doi.org/10.1016/j.actamat.2004.06.025.; Yang et al., 2014Yang, X., Liu, J., Chen, J., Wan, C., Feng, L., Liu, P., Wu, C. (2014). Relationship Between the Strengthening Effect and the Morphology of Precipitates in Al-7.4Zn-1.7Mg-2.0Cu Alloy. Acta Metall. Sin. 27, 1070-1077. http://doi.org/10.1007/s40195-014-0122-7.; Kacar and Guleryuz, 2015Kacar, R., Guleryuz, K. (2015). Effect of Quenching Rate and Pre-strain on the Strain Ageing Behaviors of 7075 Aluminum Alloys. Mater. Res. 18 (2), 328-333. https://doi.org/10.1590/1516-1439.307414.; Ozer and Karaaslan, 2017Ozer, G., Karaaslan, A. (2017). Properties of AA7075 aluminum alloy in aging and retrogression and reaging process. Trans. Nonferrous. Met. Soc. China 27 (11), 2357-2362. https://doi.org/10.1016/S1003-6326(17)60261-9.; Cai et al., 2020Cai, S.W., He, Y., Song, R.G. (2020). Study on the Strengthening Mechanism of Two-Stage Double-Peaks Aging in 7075 Aluminum Alloy. Trans. Indian Iinst. Met. 73 (1), 109-117. https://doi.org/10.1007/s12666-019-01809-7.). Dislocation accumulation and inhibition at grain boundaries can be viewed in two ways: 1) grain boundaries are obstacles to dislocation movement; if the dislocation does not surpass the grain boundaries, it will accumulate at them (Pan et al., 2021Pan, H., Yue, H., Zhang, X. (2021). Interactions between Dislocations and Boundaries during Deformation. Materials 14 (4), 1012. https://doi.org/10.3390/ma14041012.); 2) grain boundaries are an excellent heterogenous site for the nucleation of phases, so it can be assumed that the η’ and other precipitates nucleate at the grain boundaries, also causing the hindering of dislocation movement (Goswami et al., 2013Goswami, R., Lynch, S., Holroyd N.J.H., Knight, S.P., Holtz R.L. (2013). Evolution of Grain Boundary Precipitates in Al 7075 Upon Aging and Correlation with Stress Corrosion Cracking Behavior. Metall. Mater. Trans. A. 44, 1268-1278. https://doi.org/10.1007/s11661-012-1413-0.). With the increase of the ageing temperature above 160 °C, the hardness decreases due to the gradual disappearance of GP zones and the coarsening of the η’ precipitates. In addition, upon reaching the ageing temperature of 230 °C, it can be assumed that the formation of the stable η phase (MgZn₂) starts. This stable phase is incoherent with the Al matrix, but in the early stages of formation, it can be assumed that the precipitates are fine and evenly dispersed so the increase in hardness values up to 230 °C can be explained in this way. Afterwards, the hardness values decrease, reaching the values obtained for the quenched samples due to over-ageing and the coarsening of the stable η precipitates (Mukhopadhyay et al., 1994Mukhopadhyay, A.K., Yang, Q.B., Singh, S.R. (1994). The influence of zirconium on the early stages of aging of a ternary Al-Zn-Mg alloy. Acta Metall. Mater. 42 (9), 3083-3091. https://doi.org/10.1016/0956-7151(94)90406-5.; Kacar and Guleryuz, 2015Kacar, R., Guleryuz, K. (2015). Effect of Quenching Rate and Pre-strain on the Strain Ageing Behaviors of 7075 Aluminum Alloys. Mater. Res. 18 (2), 328-333. https://doi.org/10.1590/1516-1439.307414.; Kilic et al., 2019Kilic, S., Kacar, I., Sahin, M., Ozturk, F., Erdem, O. (2019). Effects of Aging Temperature, Time, and Pre-Strain on Mechanical Properties of AA7075. Mater. Res. 22 (5), e20190006. http://doi.org/10.1590/1980-5373-mr-2019-0006.). These processes cause the relaxation of the Al matrix, the easier movement of dislocations and, thus, lower values of hardness.

c. Investigation of the electrical conductivity

Essentially, there are two processes caused by precipitation: 1) the depletion of the matrix which causes relaxation and easier movement of electrons, thus increasing the electrical conductivity; 2) the appearance of metastable precipitates (GP zones, η’ precipitates) that distort the matrix causing the scattering of electrons, thus decreasing the electrical conductivity. Which of these two processes is more active, directly determines the behaviour of electrical conductivity at a given heat treatment conditions.

In the temperature range of 110 °C - 170 °C aged samples have a lower electrical conductivity values in comparison with the quenched sample. The minimum electrical conductivity value was detected after ageing at 140 °C. Lower values of the electrical conductivity in this temperature range are due to the formation of the GP zones and early stage η’ precipitates formation. The existence of these precipitates causes the scattering of electrons (process number 2 dominates). Scattering then causes the rise of the electrical resistance and therefore, a decrease in electrical conductivity. Coherency and semi-coherency of GP zones and early stage η’ precipitates formation, respectively, are causing the appearance of stress fields in the crystal lattice, which leads to an increase in the electrical resistance (Ozer and Karaaslan, 2017Ozer, G., Karaaslan, A. (2017). Properties of AA7075 aluminum alloy in aging and retrogression and reaging process. Trans. Nonferrous. Met. Soc. China 27 (11), 2357-2362. https://doi.org/10.1016/S1003-6326(17)60261-9.). After the initial decrease, the electrical conductivity gradually increases as the ageing continues, and the matrix gets less saturated (loses the quenched-in elements). Salazar-Guapuriche et al. (2006)Salazar-Guapuriche, M.A., Zhao, Y.Y., Pitman, A., Greene, A. (2006). Correlation of Strength with Hardness and Electrical Conductivity for Aluminium Alloy 7010. Mater. Sci. Forum. 519-521, 853-858. https://doi.org/10.4028/www.scientific.net/MSF.519-521.853. suggested that almost all alloying elements increase the electrical resistance when they are in solid solution (as-quenched condition after the solution treatment). A lower saturation of the matrix in alloying elements gives higher values of the electrical conductivity, as it is seen in Fig. 3, after ageing above 170 °C. After ageing at the temperatures above 170 °C it can be concluded that there is a gradual disappearance of GP zones and the coarsening of the η’ precipitates, causing the process number 1 to dominate. In addition, the highest values of the electrical conductivity are obtained due to over-ageing of the investigated alloy similar to the works of Pankade et al. (2018)Pankade, S.B., Khedekar, D.S., Gogte, C.L. (2018). The influence of heat treatments on electrical conductivity and corrosion performance of AA 7075-T6 aluminium alloy. Procedia Manuf. 20, 53-58. https://doi.org/10.1016/j.promfg.2018.02.007. and Simsek (2019)Simsek, I. (2019). Investigation of the effect of second phase precipitates on the corrosion and electrical conductivity of 7075 aluminum alloys. Anti-Corros. Method. M. 66 (5), 683-688. https://doi.org/10.1108/ACMM-02-2019-2082.. As the over ageing leads to the decrease in the hardness values, in this case it leads to an increase in the electrical conductivity, as concluded by Ozer and Karaaslan, (2017)Ozer, G., Karaaslan, A. (2017). Properties of AA7075 aluminum alloy in aging and retrogression and reaging process. Trans. Nonferrous. Met. Soc. China 27 (11), 2357-2362. https://doi.org/10.1016/S1003-6326(17)60261-9.. Khangholi et al. (2021Khangholi, S.N., Javidani, M., Maltais, A., Chen, X. (2021). Effects of natural aging and pre-aging on the strength and electrical conductivity in Al-Mg-Si AA6201 conductor alloys. Mater. Sci. Eng. A. 820, 141538. https://doi.org/10.1016/j.msea.2021.141538.) explained in his paper that the electrical conductivity decreases due to the sum of different contributions of various microstructural defects that cause electron scattering. It can be assumed that, due to over-ageing, a stable η phase is present in the microstructure. This phase is incoherent with the matrix so the electron scattering effect is low and is dominated by process number 1 (strong precipitation from the matrix). This can be seen on Fig. 2 and Fig. 3 after ageing at 230 °C for 30 min At that temperature, even though the η phase is coherent with the matrix, some distortion is surely present in the matrix, causing hardness values to increase and electrical conductivity to decrease.

d. Investigation of the thermal properties

Figures 4 and 5 show the influence of ageing temperature on the thermal diffusivity and thermal conductivity values of the investigated alloy, respectively, during the isochronal ageing for 30 min

Since the thermal properties, as well as the electrical ones, depend on the electron flow, the presented diagrams in Figs. 4 and 5 are somewhat similar to the one given for the electrical conductivity (Fig. 3). Observing the presented diagrams, it can be concluded that by ageing the alloy in the temperature range from 110 °C to 180 °C, the values of thermal diffusivity and thermal conductivity are below the value of the quenched sample. In this temperature range, GP zones and other type of precipitates are active and are responsible for the scattering of electrons, but also serve as obstacles to the movement of heat (Edwards et al., 1998Edwards, G.A., Stiller, K., Dunlop, G.L., Couper, M.J. (1998). The precipitation sequence in Al-Mg-Si alloys. Acta Mater. 46 (11), 3893-3904. https://doi.org/10.1016/S1359-6454(98)00059-7.; Lumley et al., 2013Lumley, R.G., Deeva, N., Larsen, R., Gemberovic, J., Freeman J. (2013). The Role of Alloy Composition and T7 Heat Treatment in Enhancing Thermal Conductivity of Aluminum High Pressure Diecastings. Metall. Mater. Trans. A. 44 (2), 1074-1086. https://doi.org/10.1007/s11661-012-1443-7.; Cui et al., 2014Cui, L., Liu, Z., Zhao, Xi., Tang, J., Liu, K., Liu, X., Qian, C. (2014). Precipitation of metastable phases and its effect on electrical resistivity of Al-0.96Mg₂Si alloy during aging. Trans. Nonferrous. Met. Soc. China 24 (7), 2266-2274. https://doi.org/10.1016/S1003-6326(14)63343-4.).

With an increase in ageing temperature (190 °C-250 °C), the values of thermal diffusivity and thermal conductivity of the aged samples are higher compared to the quenched sample. The reason for this can be found in the activation and easier movement of electrons. The increase in the ageing temperature leads to more intensive precipitation, transformation and decomposition of GP zones, and thus the formation of a metastable semi-coherent η’ phase. All this, together with the constant impoverishment of the solid solution, clears the way for the movement of electrons, increasing the values of thermal properties. Padap et al. (2020)Padap, A.K., Yadav, A.P., Kumar, P., Kumar, N. (2020). Effect of aging heat treatment and uniaxial compression on thermal behavior of 7075 aluminum alloy. Mater. Today: Proc. 33 (8), 5442-5447. https://doi.org/10.1016/j.matpr.2020.03.196. also obtained higher values of thermal conductivity after ageing in comparison with the quenched state. Greb et al. (2019)Greb, T., Mittler, T., Schmid, S., Volk, W., Chen, H., Khalifa, N.B. (2019). Thermal analysis and production of As-Cast Al 7075/6060 Bilayer billets. Int. J. Metalcast. 13 (4), 817-829. https://doi.org/10.1007/s40962-018-0282-8. had an increase in thermal conductivity and diffusivity values around 200 °C after continuous heating the EN AW-7075 alloy.

e. Microstructural investigation

A more detailed study of the microstructural changes during the isochronal ageing was performed using SEM-EDS analyses. The peak aged sample with the highest hardness value (aged at 150 °C for 30 min) was selected for this analysis. Figure 6 shows different sites of interests in the peak aged sample, at different magnifications. Precipitated particles and their chemical compositions were investigated and the results of the investigation are given in Fig. 6 and Tables 2-4.

Spectrum S1 in Fig. 6 a (Table 2) and spectrum S3 in Fig. 6 b (Table 3) indicate the presence of Al₂Cu phase in the aged samples. Dos Santos et al. (2021)Dos Santos, S.L., Toloczko, F.R., Merij, A.C., Saito, N.H., Da Silva, D.M. (2021). Investigation and Nanomechanical Behavior of the Microconstituents of Al-Si-Cu alloy After Solution and Ageing Heat Treatments. Mater. Res. 24 (2), e20200329. https://doi.org/10.1590/1980-5373-MR-2020-0329. stated that this phase appears in a white colour after etching the sample with Dix-Keller solution as in this investigation. Similar results were obtained by Chen et al. (2015)Chen, G., Chen, Q., Wang, B., Du, Z.-M. (2015). Microstructure evolution and tensile mechanical properties of thixoformed high performance Al-Zn-Mg-Cu alloy. Met. Mater. Int. 21 (5), 897-906. https://doi.org/10.1007/s12540-015-5139-6. and Ku et al. (2018)Ku, M.H., Hung, F.Y., Lui, T.S., Lai, J.J. (2018). Enhanced Formability and Accelerated Precipitation Behavior of 7075 Al Alloy Extruded Rod by High Temperature Aging. Metals. 8 (8), 648. https://doi.org/10.3390/met8080648.. The S2 spectrum in Fig. 6 a (Table 2) may indicate the presence of the Al₇Cu₂Fe phase, which remained undissolved in the solid solution by previous annealing. The spectrum S3 in Fig. 6 a (Table 2) and spectrum S2 in Fig. 6 b (Table 3) probably indicates the existence of the Al₂CuMg phase (sometimes denoted in literature as S-phase). The presence of this phase is also mentioned by other authors. Woznicki et al. (2021)Woznicki, A., Leszczynska-Madej, B., Wloch, G., Grzyb, J., Madura, J., Lesniak, D. (2021). Homogenization of 7075 and 7049 Aluminium Alloys Intended for Extrusion Welding. Metals 11 (2), 338. https://doi.org/10.3390/met11020338. mentioned the existence of this phase after the homogenization of the cast EN AW-7075 aluminium alloy, while MacKenzie (2000)MacKenzie, D.S. (2000). Quench rate and ageing effects in aluminum-zinc-magnesium-copper aluminum alloys. PhD Thesis, Faculty of the Graduate School of the University of Missouri-Rolla, Missouri, USA. https://www.researchgate.net/publication/241318106. obtained this phase after quenching and obtaining the super saturated solid solution. From Fig. 6 a-b and Table 2-3, it can be seen that particles containing Cu sometimes precipitate on grain boundaries. An analysis of the large scanning area given by the spectrum S4 in Fig. 6 a (Table 2) shows the presence of Al, Mg and Zn, which can indicate that the whole scanning area is covered with finely distributed metastable η’ phase (MgZn₂). The scanning area, like the one represented with spectrum S4 in Fig. 6 a, was magnified to the magnification of x10000 and analysed (Fig. 6 c, Table 4). From the presented Fig. 6 c it can be seen that the whole area is covered by small precipitates. Some precipitates have triangular and some plate-like shapes. EDS analysis was performed on both types of precipitates (spectrums S1-S3, Fig. 6 c). The obtained results cannot confirm the exact proportion of the magnesium and zinc in this phase, but from the analysis of the obtained spectrums and Table 4, it can be assumed that those particles represent the η’ phase (MgZn₂). Ultra-fine particles of this phase are very difficult to detect at these magnifications; however, hardness results obtained for the sample aged at 150 °C for 30 min indirectly anticipate their presence.