Aluminum alloys of 2000 and 7000 series containing cooper and zinc, respectively, as the main alloying element are extensively used in aircraft structures. However, these classes of aluminum alloys are difficult to join by conventional fusion welding techniques because the dendritic structure formed in the fusion zone can seriously compromise the mechanical and corrosion resistance properties of the joint. Friction Stir Welding (FSW) is attractive for joining high strength aluminum alloys since there is far lower heat input during the process compared with conventional welding methods. FSW is produced by rotating and plunging a specially-designed cylindrical, shouldered tool with a small diameter pin into the joint line between two butted plates. Frictional heat causes the metal to soften and allows the tool to traverse along the joint line [1]. The FSW process generates three distinct microstructural zones: the nugget, the thermomechanically affected zone (TMAZ) and the heat-affected zone (HAZ). These microstructural modifications generally interfere in the performance of the corrosion resistance of welded alloys. In the present study, FSW was used to join two dissimilar aluminum alloys, AA2024-T3 (Al-Cu-Mg) and AA7475-T651 (Al-Cu-Mg-Zn) and the effect of this process on the corrosion resistance of the welded joints and on the microstructure of the alloys was evaluated. Microstructural characterization was carried out by optical microscopy, scanning electron microscopy and atomic force microscopy. The weld zone regions showed significant microstructural changes. Electrochemical impedance spectroscopy (EIS) and Electrochemical noise measurements (ENM) were used to evaluate the corrosion resistance of the different zones of the weld in NaCl solution. Local electrochemical measurements using an electrochemical microcell were carried out across the transverse cross section of the welds. The cell containing the solution, counter and reference electrodes is attached to the carousel of an optical microscope. The electrochemical results showed that the corrosion behavior of the AA2024-T3 / AA7475-T651 alloy is dominated by the electrochemical activity within the intermetallics (IMs). The weld zone regions containing copper IMs have a more noble character than the regions containing Zn and Mg IMs. Therefore, the results showed an increased susceptibility of welded joints to these forms of corrosion in comparison to the unaffected base metal by a more severe attack related to AA7475-T651 alloy.

Reference

[1] M. Jariyaboon et al. Corrosion Science 49 (2007) 877–909.

[2] J.-Q. Su et al. Acta Materialia 51 (2003) 713–729

Acknowledgement: Fapesp 2013/132356