Galvanic Corrosion Behavior of Al Alloy A6061 / High-Tensile Strength Steel Couples Jointed By Friction Stir Lap Welding in an NaCl Solution
Authors: Noguchi Kotaro , Tada Eiji , Ooi Azusa , Nishikata Atsushi 
Published: December 2020

Recently, improvements in fuel efficiency of transportation systems have been called for, as part of measures to reduce carbon dioxide emissions. Reducing the body weight is quite effective in improving fuel efficiency. Therefore, part of heavier materials like steel is replaced by lighter materials such as aluminum alloys, carbon fiber reinforced plastics, and so on. However, when dissimilar materials are jointed, appropriate jointing methods are needed in terms of assembly, bonding performance, and costs. Friction stir welding (FSW) has attracted much attention as a promising jointing method in various engineering applications. FSW is a solid-state joining technique that has shown significant advantages over conventional fusion welding in terms of higher strength and quality of the joint produced. For this reason, FSW has attracted considerable attention for industrial applications, and further development of the technology is expected. However, during bonding of metallic materials by FSW, microstructure changes are produced that can affect their corrosion degradation. In this study, galvanic corrosion behavior of couples of A6061 and high-strength steel (SPCC) jointed by FSW in aqueous NaCl solutions is investigated.
Friction stir lap welding was performed on A6061 and SPCC plate materials, and the corrosion behavior of the cross section was investigated by microstructure observation, hardness test, immersion test, and electrochemical measurement. The microstructure observation was performed using an optical microscope and a scanning electron microscope. On the A6061 side, after mirror polishing, further polishing was performed using a cross section polisher to observe grain boundaries and inclusions. Hardness testing was done to examine the change in hardness on A6061 with a micro Vickers hardness meter. The immersion test was conducted in an aqueous NaCl solution. In addition, micro-electrochemical polarization tests were performed on A6061 to investigate pitting potentials of each microstructure of A6061.
It was found from the results of the surface observation that grain size and precipitate distributions were varied depending on the distance from the jointed part on A6061. The microstructures on A6061 after FSW were classified into 4 main regions: nugget zone (NZ), heat affected zone (HAZ), thermo-mechanically affected zone (TMAZ), and base metal (BM). It was found from the results of the hardness tests that BM has the highest hardness and that TMAZ, HAZ and NZ were softened. In the immersion tests, localized corrosion was observed mainly on BM region of A6061 at early stage of the galvanic corrosion. Micro-electrochemical polarization tests for A6061 revealed that pitting potential (E
p) of NZ region was higher than BM. This suggests that pitting corrosion is less likely to occur on the regions of NZ and TMAZ rather than BM due to the change in the microstructure of A6061 caused by FSW.

Acknowledgments

This paper is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).