Corrosion resistance of Friction Stir Welded joints of Airware® AA2074 T8 sheet compared to conventional AA2024 T3 for fuselage applications

High performance metallic aerospace structures require advanced and cost effective assembly technologies such as Friction Stir Welding (FSW). An optimized durability of such welded joints is strongly influenced by an appropriate choice of aluminum alloy. In this study, the corrosion resistance of a FSW jointed Airware^® AA2074 T8 (Al-Cu-Li alloy) is compared with a similar joint executed on a reference AA2024 T3 with or without a 8h/120°C heat treatment simulating any possible adhesive curing operation on the same part. Several combined testing methods were used to characterize corrosion mechanisms on these configurations: (1) Full welded joints were first investigated using an accelerated InterGranular Corrosion (IGC) test according to ASTM G110. (2) In order to understand how galvanic coupling unfolds in such a continuous immersion test, a methodology to determine the electrochemical potential profile through the different zones of the welded joints was set up. (3) A “Mastmaasis” exposure including dry and wet cycles was also performed in a climatic chamber according to ASTM G85 A2. The different testing methods reveal the IGC sensitivity of the AA2024 both in the Base Metal (BM) and to a larger extent in the Heat Affected Zone (HAZ) as well as the negative impact of the curing treatment simulation. Lower electrochemical potentials measured in the HAZ compared to the BM explain why the corrosion is further accelerated in the nugget and in the HAZ for the AA2024. The opposite is observed for the AA2074, i.e. an absence of IGC attack both in the BM and in the HAZ after ASTM G110 exposure maximizing galvanic coupling effects. Higher electrochemical potentials measured in the HAZ compared to BM offers extra sacrificial protection of the welded joint.