(Invited) Localized Electrochemical Techniques for Characterizing Welded Areas of Dissimilar Al Alloys Joined By FSW
Authors: Abreu Caio Palumbo , de Melo Hercilio G. , Turmine Mireille , Pebere Nadine , Costa Isolda , Vivier Vincent 
Published: February 2020

Advances in aerospace industry are mainly based in the research for weight reduction. Welding of aluminum alloys can replace overlapping plates and rivets, which could reduce up to 15% of the total weight of an aircraft.1,2 The assembly of aluminum alloys by fusion processes leads to the degradation of their mechanical properties (modification of the alloy microstructure), and cannot be used for high strength alloys used in the aerospace industry. Friction stir welding (FSW), developed in the 1990’s, allows the welding of components at temperatures below their melting temperatures. However, FSW also affects the alloy microstructure generating four different zones: the base metal (BM), a heat affected zone (HAZ), a thermo-mechanically affected zone (TMAZ) and the nugget zone. Consequently, galvanic cells can be generated due to potential differences between these zones, mainly at their interfaces3.

For similar aluminum alloys, many investigations showed the relation between microstructure, hardness and the anodic and the cathodic behavior according to process parameters4,5. Jariyaboon et al.
6 determined the open circuit potential of a FSW welded AA2024-T3 alloy at the different zones and found that the nugget area was more anodic than the BM. For the same material, Bousquet et al.
4 demonstrated that the interface between the HAZ and TMAZ was preferably corroded comparatively to the other zones whereas Lumsden et al.
7 working with the AA7050-T7651 alloy showed that the interface between the nugget and the TMAZ was more susceptible to corrosion. These results suggest that the microstructural differences between the dissimilar zones created by the FSW process are the main driving force for localized corrosion initiation.

In the present study, the corrosion behavior of AA2024-T3 and AA7475-T761 alloys joined by FSW was investigated in 0.1 M Na2SO4 + 1mM NaCl solution by means of both electrochemical impedance spectroscopy (EIS) and local electrochemical impedance spectroscopy (LEIS) since the latter has been successfully used for investigating the galvanic corrosion associated to microstructural heterogeneities8,9,10. Interestingly, LEIS was performed using a bi-microelectrode or in a confined environment by using a micro droplet.

OCP value after 2 h immersion was much lower for the AA7475-T761 (-0.60/Ag/AgCl) than for the AA2024-T3 (-0.2 V/Ag/AgCl), whereas, as expected, the FSW joined alloys stabilized at an intermediate potential (-0.5 V/Ag/AgCl). These results clearly indicate the existence of a galvanic coupling when the alloys are welded, with a cathodic OCP shift of the AA2024-T3 BM, whereas the AA7475-T761 BM is anodically polarized.

Global EIS diagrams showed that the impedance modulus of AA7475-T761 is higher than that of AA2024-T3, whereas for the FSW joined alloys the modulus is much lower. In addition, the impedance diagrams obtained with the micro droplet cell showed similar results: the impedance modulus for AA7475-T761 was higher than for AA2024-T3, whereas smaller impedances for AA7475-T761 and higher impedances for AA2024-T3 were found in the different welding zones.

Interestingly, when LEIS investigations were performed with the bi-microelectrode at the different position above the welding zones, the high frequency region of the diagrams show the onset of an inductive response, which has been associated with galvanic coupling in model alloys by other authors 11. This feature is more relevant the closest the probe is placed near the weld joint.

These results and data interpretation will be deeply discussed in the presentation.

References

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2) DURSUN, T.; SOUTIS, C; Materials and Design, 56 (2014) 962-871.

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