Electrochemical and environmental assisted behavior of AA0224-T3 and AA7075-T6 welded by means of FSW

The paper deals with the stress corrosion behavior of butt joints obtained by friction stir welding (FSW). The experimental study was performed on prismatic specimens obtained from FSWed joints of AA7075-T6 and AA2024-T3 alloy sheets having a thickness equal to 4 mm. The tests were executed on the same alloy (AA7075-AA7075 and AA2024-AA2024) and mixed joints (AA7075-AA2024). Tensile tests and four-point bending tests were carried out to evaluate the corrosion behavior and stress corrosion cracking susceptibility of FSW joints. During the tests, the electrochemical behavior of both the loaded (80% of the yield strength) and unloaded specimens in 3.5% NaCl solution was monitored by means of corrosion potential measurement, and electrochemical impedance spectroscopy (EIS). In the first hours of dipping, the EIS spectra of the loaded specimens of AA2024-T3 were different from the unloaded ones. After 24 hours, the electrochemical response became the same for all the specimens. This behavior was attributed to the formation of a thick layer of corrosion products that hindered the electrochemical couple within the specimens. After 1500 hours of immersion, the loaded specimens showed an intense intergranular attack which was not observed on the unloaded specimens. This stress enhanced intergranular attack was observed in the nugget of the welding, close to the continuous copper-rich precipitates at the recrystallized grain boundaries, induced by the thermo-mechanical action of the welding tool. These precipitates were formed of the coalescence of sub-micrometric precipitates present in the naturally aged base metal. Intergranular attacks were not observed in the heat-affected zone and on the base metals – where grain recrystallization did not occur. In these zones, the presence of sporadic shallow pits was only observed in correspondence of the large copper rich precipitates. The loaded specimens of AA7075-T6 showed EIS spectra different from the unloaded ones, in particular, the un-stressed specimens had three phase constants, whereas the stressed only two. The third phase constant at high frequencies disappears as the time of dipping increased, and the EIS spectra of all the specimens became similar. At the end of the tests, all the AA7075-T6 specimens – stressed and un-stressed - showed a very intense exfoliating attack started from the heat affected zone and propagated along the rolling direction. On the four-point bending specimens it was not possible to evidence the effect of the load on the morphology of the attacks. Finally, in the first hours of immersion, the EIS spectra of the AA7075-AA2024 welds showed EIS spectra similar to those of the AA2024-T3, increasing the dipping time, the EIS spectra became similar to the AA7075-T6 ones. Independently from the stress applied on these specimens, the corrosion morphology, was located only on the AA7075-T6 side. The complex behavior of the considered welds was also evidenced by means of local open circuit potential (LOCP) measurements: in fact, it was possible to observe the more active areas located in correspondence of heat affected zone. The differences in the LOCP can be attributed to the changing in the microstructure owing to the thermal mechanical action of the welding. The hardness along the welds showed a similar profile to the LOCP