Microstructure, mechanical behavior and corrosion properties of friction stir welded aluminum alloys used in the aerospace industry

Kurzfassung

Friction stir welding (FSW) has been identified as “key” technology for the production of primary aerospace structures, being able to substitute conventional riveted airframes. FSW is a solid state welding process that avoids any problems caused by the solidification of the melted weld pool. Besides the production of high quality similar joints from high strength aluminum alloys, it allows for joining materials of different metallurgical characteristics. However, problems concerning the corrosion behavior of dissimilar joints may arise, since aircrafts operate in corrosive media. For the safe implementation of dissimilar joints, a good understanding of the relevant corrosion mechanisms is required. In this study, the correlation between microstructure, local strength and corrosion behavior of dissimilar FSW joints produced from 2024A and 6056 alloys in two different tempering conditions was investigated. The heat affected zone of 6056 in 2024A-T3/6056-T6 as-welded joints could easily be identified by a marked hardness drop. Coarsening and dissolution of strengthening precipitates in that region brought about the weakest link during static tensile tests. Because of the low strength of as-welded dissimilar joints, welds were also produced from 6056 in the naturally aged T4 condition. After FSW, these joints were aged at 190°C for 10h resulting in a 2024A-T8/6056-T7X tempering condition. Due to a more homogeneous precipitation of hardening particles, the post-weld heat treatment improved hardness and static strength remarkably. The corrosion performance of the dissimilar joints observed in various immersion tests was governed by galvanic coupling between both aluminum alloys. For as-welded joints. The more active 6056-T6 alloy provided cathodic protection to 2024-T3 when exposed to aqueous chloride solutions, particularly under permanent immersion conditions. Strips of alloy 6056 being present in the nugget region corroded preferentially. A significant improvement of the corrosion behavior was observed in the case of the post-weld heat treated 2024A-T8/6056- T7X joints. Because of the higher degree of precipitation, the post-weld heat treatment would degrade corrosion behavior of similar 2024 joints. For dissimilar joints, however, the difference in corrosion potential of both alloys became distinctly lower. Therefore, the postweld heat treatment equalized the corrosive attack and suppressed preferential dissolution of 6056 alloy.