PSI - Issue 28

Dimitris Georgoulis et al. / Procedia Structural Integrity 28 (2020) 2297–2303 Author name / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction The increasing interest in Al-Cu-Li-Mg based alloys arises from the lithium addition-induced advantages such as the decrease in density (almost 3 % for 1 wt.% lithium added), increase in elastic modulus (approximately 6 % for 1wt. % lithium added), the high strength-to-weight ratio as well as the high damage tolerance and corrosion resistance (Dursun et al. 2014, Rioja et al. 2012). Third generation Al-Cu-Li alloys were developed to replace the conventional alloys such as AA2024-T3 in aircraft structures, where damage tolerance is the critical design factor, due to their good combination of mechanical properties, e.g. Prasad et al. (2003) and Moreto et al. (2012). 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 (Ma et al. 2015, Zhang et al. 2016). The addition of Li introduces precipitation hardening due to the formation of several strengthening precipitates, including δ΄ (Al 3 Li), θ΄ (Al 2 Cu) and T1 (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). Initially, the δ΄΄ phase is formed that is coherent with the matrix (e.g. at T3 condition). By applying appropriate heat treatment conditions, heat-treatable metastable phases such as θ΄ and T1 can be detected that contribute to the mechanical behaviour of the alloy, as was mentioned by Liu et al. (2017) and Zhang et al. (2014). T1 is considered as the major strengthening phase of these alloys, according to Blankenship et al. (1992), Blankenship et al. (1993) and Nie et al. (1996), that nucleate preferably in dislocations, sub-grain and grain boundaries (Araullo et al. 2014). Commercial wrought aluminium alloys from 2xxx, 6xxx, and 7xxx alloy series are heat-treatable, that corresponds to microstructural and tensile mechanical properties changes with the appropriate artificial ageing heat treatment (Korb et al. 1992). 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 T1 (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 T1 precipitation inside the grains with increasing ageing time (Proton et al. 2014). Corrosion susceptibility of aluminium alloys is strongly influenced by the strengthening intermetallic phases due to microgalvanic processes between the intermetallic phases and the matrix (Boag et.al. 2011, Guillaumin et al. 1999, Blanc et al. 1997). In the case of Al-Cu-Li alloys, the T1 precipitates contribute to localized corrosion due to the anodic behaviour with respect to the adjacent Cu depleted zone and the matrix (Kertz et al. 2001). Kai (1990) found that in the presence of aggressive chloride solutions, aluminium alloys are subjected to localized corrosion by pitting, intergranular attack, or exfoliation corrosion that severely shorten the aircrafts service life due to stress concentration in the area next to these defects. According to Zou et al. (2018) the corrosion resistance of AA2198 was found to decrease from the solution-anneal to peak-ageing condition due to artificial ageing-induced microstructural transformations. Investigation on the corrosion behaviour of Al-Cu-Li AA2198 alloy is of major importance. Identifying the corrosion-induced degradation mechanism is of crucial interest in the aircraft maintenance protocols. In the present work, the above mechanisms are identified for the AA2198 at the T8 temper via accelerated corrosion tests on tensile coupons directly compared against the well-established AA2024 in order to report experimental evidence regarding the corrosion potential of this alloy and 2. Materials and Methods 2.1. Materials Materials studied in this work were wrought AA2198-T8 which was received in sheet form of 3.2 mm nominal thickness and was provided by Constellium and AA2024-T3 of 3.2 mm nominal thickness provided by Hellenic Aerospace Industry (HAI). The weight percentage chemical composition of the 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. The T8 temper corresponds to the under-ageing condition for AA2198 since it does not possess