Corrosion and stress corrosion cracking behaviour of 2050 aluminium-lithium alloy joined by friction stir welding (FSW)

摘要

To reduce in weight the aircraft metallic structures, the industry has considered the combined use of 3rd generation aluminium-lithium alloys and the friction stir welding process. Actually a typical friction stir weld consists of the unaffected base material (BM), a heat affected zone (HAZ), a thermo-mechanically affected zone (TMAZ) and a dynamically recrystallized zone (nugget). The present work deals with the influence of an external mechanical stress on the corrosion kinetics of the BM and the nugget first taken individually then considered in the whole welded joint. In the present work, a butt weld was performed on two rolled plates of 2050 T351 Al-Cu-Li alloy to obtain the welded structures. The welded joint was then submitted to an additional T8 post-welding heat treatment. Some samples were removed from the BM and the nugget of the welded joint in order to study the corrosion and stress corrosion cracking behaviour of the BM and the nugget individually. Immersion tests without an external mechanical stress were performed in a 0.7M NaCI solution to study the corrosion behaviour of the two zones. Stress corrosion cracking (SCC) experiments were performed in the same solution for samples removed from each zone of the welded joint as well as for the whole welded joint. For SCC tests, the experimental device consisted in an electrochemical cell related to a potentiostat combined with a tensile machine which allowed the monitoring of the Open Circuit Potential (OCP) of the sample while the sample displacement was maintained constant. For both kinds of experiments, the corrosion damage was determined by measuring the average depth and the maximal depth of the corrosion defects. The results showed that the two zones presented different corrosion morphologies. The BM exhibited susceptibility to intragranular corrosion while the nugget presented both intragranular and intergranular corrosion. For both samples, the external mechanical stress did not change neither the corrosion morphologies nor the open circuit potential. Nevertheless, considering the experimental conditions, the external mechanical stress increased the average and maximum depth of corrosion damage for the nugget whereas there was no influence for the BM. Considering the whole welded joint, the corrosion morphologies of the BM and the nugget were the same than those observed for each zone considered individually. A previous work [1] has shown that galvanic coupling phenomena existed between the different parts of the welded joint and the BM acted as a sacrificial anode. As the external stress did not change the OCP of the different parts, no modification of the galvanic coupling phenomena was expected and the BM was found to be more damaged than the other zones. This work was financially supported by the ANR MatetPro program (ANR-08-MAPR-0020-05). The authors thank C Henon who provided the material (Constellium).