PSI - Issue 68

Louka Eleftheria-Sotiria et al. / Procedia Structural Integrity 68 (2025) 894–900 Louka Eleftheria-Sotiria et al. / Structural Integrity Procedia 00 (2025) 000–000

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1. Introduction The advanced mechanical performance, e.g. , (Prasad, Gokhale, & Rao, 2003) and (Moreto, et al., 2012), as well as the enhanced damage tolerance and corrosion resistance, e.g. , (Dursun & Soutis, 2014) and (Rioja & Liu, 2012), of third generation wrought Al-Cu-Li alloys increased the interest of aviation industry for their wider exploitation against the conventional 2xxx series alloys, such as aluminium alloy (AA) 2024. Their improved hardness and mechanical strength properties are ascribed to their complex precipitation hardening system, including δ (Al 3 Li), θ (Al 2 Cu), T 1 (Al 2 CuLi) and S (Al 2 CuMg) precipitation phases. The major strengthening phase of these alloys is the T 1 which is preferably nucleated on dislocations, grain and sub-grain boundaries, GP zones, dispersoids, as well as inside certain grains. Nevertheless, 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 the different microstructural characteristics of the complex precipitates and the Al – matrix and their different electrochemical potential as was referred by (Ma, et al., 2015) and (Zhang, et al., 2017). Localized corrosion in the first stages of attack develops without obvious / distinguishable damage and can lead to catastrophic failure of the components. Thus, it is of major importance for the aviation industries to be able to predict the corrosion – induced damage and subsequent degradation of these alloys. 2. Experimental procedure 2.1. Materials Material used in the present work was wrought Al-Cu-Li AA2198 in two different commercial ageing tempers, named T3 and T8. The material was 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% Mn, 0.25-0.8% Mg, 0.04 0.18% Zr, ≤ 0.08% Si, 0.1-0.5% Ag, ≤ 0.01% Fe and Al rem., according to sheet manufacturer. T3 temper includes solution heat-treatment at 495 °C, quenching in water at 0 °C, and natural ageing in room temperature (25 °C) to a substantially stable condition. The T8 temper corresponds to stretch-forming, and subsequent artificial ageing heat treatment. The respective artificial ageing heat treatment of T8 temper refers to under-ageing for AA2198 since it exhibits moderate tensile strength properties values, which are significantly lower than the respective values at the peak-ageing condition, according to (Alexopoulos, et al., 2016). Rectangular specimens of 10 mm × 20 mm × 3.2 mm were machined from the longitudinal (L) rolling direction for metallographic investigations, while typical tensile specimens with flattened ends were machined according to the specification ASTM E8. The tensile test specimen had geometrical dimensions of 155 mm total length and 57 mm × 12.5 mm × 3.2 mm at the reduced cross section. 2.2. Corrosion exposure Prior to corrosion exposure, specimens were ground with SiC papers up to 1200 grit, then rinsed with deionized water and acetone, and eventually dried with cool flowing air. Further preparation including covering of specific areas of the tensile specimens’ surfaces was performed. Masking with appropriate insulating PVC tape was performed in the areas attached to machine’s grips in order to be exposed only at the reduced cross-section, as well as in the specimens’ side-surfaces (through thickness). Thus, only one flat upper surface of the tensile specimen was exposed to corrosion solution since it better simulates the real-life exposure conditions. The laboratory exfoliation corrosion environment (EXCO) was selected for this case, prepared according to the ASTM G34 standard (ASTM International , 2001), due to time savings because it is an aggressive solution and better simulates the exfoliation corrosion on 2xxx and 7xxx aeronautical aluminium alloys. The solution volume was calculated per exposure area of the specimens and was 20 ml/cm 2 for all cases, while the temperature was kept at 25 ± 1 °C. The specimens were then immersed in beakers and exposed to electrolyte for different periods of time. The immersion times were selected according to previous articles from the authors and the open literature, e.g. , (Alexopoulos, Siskou, Charalampidou, & Kourkoulis, 2019), (Alexopoulos, Velonaki, Stergiou, & Kourkoulis, 2016) and (Alexopoulos, et al., 2016). After completion of the immersion test, the tensile specimens were directly rinsed with acetone,