Corrosion Behavior in MIG-Welded 7xxx Al Alloys

Details

Supervisors

Simar, Aude

Faculty

Ecole polytechnique de Louvain

Degree label

Master [120] : ingénieur civil en chimie et science des matériaux, à finalité spécialisée

Abstract

enOwing to their high strength-to-weight ratios, 7xxx series Al alloys are gaining increased attention in the automotive industry. A cost-effective and reliable industrial joining process for metal parts is MIG welding. Previous work revealed localized corrosive attack in the proximity of the weld seam in MIG-welded 7003.52-T6 alloy sheets with 5183 Al alloy filler material. The present thesis focused on relating the microstructure of the welded specimen to the observed localized corrosion. First, the 7003.52-T6 base material (BM) was characterized in terms of its microstructure and electrochemical behaviour. SEM revealed the presence of Fe-rich IMPs in the BM, which were identified to initiate pitting corrosion through microgalvanic coupling with the less noble Al matrix. Further, OCP measurements indicated that predominantly metastable pitting occurs on the BM’s surface. Following the BM characterisation, the individual zones in the weld joint region were accordingly analysed in terms of their microstructure and electrochemical behaviour. The results were compared to each other and to the BM to identify the differences responsible for localized corrosion. The previous work further reported different Vickers hardness values for the areas around the weld joint. Based on these findings, the individual zones WZ (Weld Zone), HAZ1 (Heat Affected Zone 1), and HAZ2 (Heat Affected Zone 2) were identified. In the present thesis, polarized light microscopy revealed comparable grain morphologies within the HAZ1, the HAZ2, and the BM. Based on this, the deviations in mechanical hardness were associated with different precipitation states in the respective zones, arising from the thermal cycle during MIG welding. SEM/EDS acquisition indicated that the precipitates tend to predominantly dissolve in the HAZ1, while they dissolve and coarsen in the HAZ2. Furthermore, different grain boundary states were observed. While the BM exhibits finer and more continuously distributed grain boundary precipitates than the HAZ2, no precipitate network could be observed at the grain boundaries of the HAZ1. Nevertheless, those findings alone did not allow to draw a comprehensive conclusion about the intergranular corrosion susceptibility in the respective zones. As coarsening and dissolution of precipitates alter the elemental composition of the zones’ Al matrices, a decrease in corrosion potential was measured in the following order: BM > HAZ2 > HAZ1. While some studies suggest galvanic coupling between the individual zones, resulting from the potential differences, as the reason for localized corrosion, the zero-resistance ammeter measurements demonstrated that a potential difference alone is not sufficient to predict significant galvanic corrosion. Overall, the results of this master’s thesis do not allow the precise identification of the underlying mechanism of the localized corrosion in the MIG-welded 7003.52- T6 alloy. However, the findings provide a basis for subsequent studies of the observed localized corrosion behaviour.