PSI - Issue 37

Dimitris Georgoulis et al. / Procedia Structural Integrity 37 (2022) 941–947 Georgoulis et al. / Structural Integrity Procedia 00 (2021) 000 – 000

942

2

induced advantages that make them suitable for replacing the conventional Al alloys used in aircraft structures, e.g. Prasad et al. (2003) and Moreto et al. (2012). These materials have shown improvements in mechanical properties, damage tolerance as well as corrosion resistance, e.g. Dursun et al. (2014). However, Al-Cu-Li alloys were found to be highly susceptible to localized corrosion, especially to selective attack of certain grains or grain boundaries, due to their complex precipitation hardening system, e.g. Ma et al. (2015) and Zhang et al. (2016). AA2198 studied in this work is one of the most advanced alloys among the commercially available from the third generation Al – Cu – Li alloys and it has been chosen as the material for the fuselage skin sheet material of the A350 Airbus, e.g. Abd et al. (2018). Cu, Mg and Li are added in Al-Cu-Li alloys as alloying elements to improve their mechanical properties as referred to Li et al. (2003). The addition of these elements introduces precipitation hardening due to the formation of several micrometric and nanometric intermetallic particles (IMCs). The main strengthening nanometric particles in Al-Cu-Li alloys include δ ΄ (Al 3 Li), θ ΄ (Al 2 Cu) and T 1 (Al 2 CuLi). Other precipitates such as β (Al 3 Zr) and Al 20 Cu 2 Mn 3 are formed by the addition of other alloying elements that control the recrystallization and grain refinement of the metal, e.g. Proton et al. (2014). T 1 is considered as the major strengthening phase of these alloys, according to Blankenship et al. (1992), Nie et al. (1996) and Araullo et al. (2014) that nucleate preferably in dislocations, sub-grain and grain boundaries. These phases are formed by applying appropriate heat treatments, such as artificial ageing, and are significantly affected by thermomechanical treatments (tempers), which influence the mechanical and corrosion behaviour of the alloys, e.g. Deschamps et al. (2012). The 2xxx aluminium alloys were found to have higher corrosion resistance when artificially aged at certain tempers, such as T6 or T8, than that at T3 condition. According to Kim et al. (2016) enhanced mechanical properties were revealed for AA2195, since the artificial ageing heat treatment induced the precipitation of θ ΄ (Al 2 Cu), β (Al 3 Zr) and T 1 (Al 2 CuLi) phases. Artificial ageing changed the corrosion morphology of the Al-Cu-Li alloy 2050 from intergranular to intragranular and decreased the corrosion potential of the alloy due to T 1 precipitation inside the grains with increasing ageing time, e.g. Proton et al. (2014). Ma et al. (2015) investigated the effects of microstructure on the corrosion resistance of AA2099-T83 and showed that the corrosion attack caused by the constituent particles was superficial when compared with that caused by T 1 phase dissolution, referred as severe localised corrosion (SLC). Additionally, Li et al. (2008) proposed another corrosion mechanism associated with the T 1 phase, where they suggested that during exposure to corrosive environments, the anodic T 1 phase initially exhibits selective dissolution of Li and Al and afterwards becomes cathodic with respect to the matrix due to Cu-enrichment that leads to the attack of the surrounding matrix. According to Zou et al. (2018) the corrosion resistance of AA2198-T83 was found to gradually decrease from the solution-anneal to peak-ageing condition due to artificial ageing-induced microstructural transformations, but after this stage, the corrosion resistance increased due to T 1 phase precipitation. However, many research works, e.g. Ma et al. (2015), Araujo et al. (2018), Donatus et al. (2018) etc. have showed that precipitation of T 1 phase leads to low corrosion resistance in Al – Cu – Li alloys. Thus, it is generally accepted that thermomechanical conditions (tempers) significantly affect the corrosion resistance of the Al – Cu – Li alloys and, consequently, the corrosion behaviour and mechanisms related to each of the thermomechanical treatment used needs proper investigation. In the present work, an investigation of corrosion behaviour of AA2198 alloy in the T3 and T8 temper conditions is performed and proposes the prevalent corrosion mechanisms that are correlated with the microstructure of the alloy. 2. Materials and experimental procedure 2.1. Materials Materials used in this work were wrought AA2198-T3 and AA2198-T8 which were both received in sheet form of 3.2 mm nominal thickness. The weight percentage chemical composition of AA2198 is 2.9-3.5 % Cu, 0.8-1.1 % Li, ≤ 0.35 % Zn, ≤ 0.5 0 % Mn, 0.25-0.80 % Mg, 0.04-0.18 % Zr, ≤ 0. 08 % Si, 0.1-0.5 % Ag, ≤ 0.01 % Fe and Al rem. according to sheet manufacturer. The T3 condition included solution heat-treated to 495 ◦ C, cold worked, and naturally aged in room temperature (25 ◦ C) to a substantially stable condition. The T8 temper corresponds to the under-ageing condition for AA2198 since it does not exhibit extremely high tensile yield stress when compared to the respective of the T3 temper. Rectangular specimens with dimension of 10 mm x 20 mm x 3.2 mm were machined from the longitudinal (L) rolling direction. Prior to the measurements, specimens were ground with SiC papers up to 1200 grit, then rinsed with deionized water and acetone, and eventually dried with cool flowing air.