The Study of Corrosion Properties of Al6061-T6 containing Friction Stir Welding and Shot Peening

Abstract

Aluminum is a useful material to solve low carbon and recycling problems. This is because aluminum can be used to save fuel through weight reduction and can be recycled when scrapped or dismantled. If weldability and fatigue properties are improved through friction stir welding (FSW) and shot peening (SP), it would be effective to apply aluminum to structural materials subjected to variable stress. However, the problem of maintaining corrosion resistance remains. This is because the welded part is generally vulnerable to corrosion, and during shot peening, the peening ball hits the aluminum passivation film. As a result of most studies, although shot peening has been helpful in improving the corrosion properties of aluminum, most of them are short-term electrochemical experiments, and the effect varies depending on the conditions of FSW and SP. Therefore, in this study, the effects of FSW and SP on the corrosion properties of Al6061-T6 were investigated. In addition, the mechanical properties according to the heat input of FSW were confirmed. Micro Vickers hardness, tensile strength and yield stress all decreased in FSW specimens compared to BM specimens. As the FSW heat input increased, the NZ area widened, and the difference in hardness for each FSW area was shown. Mechanical properties were not proportional to heat input, and TMAZ on the AS side was the weakest in all conditions. This is because the material flow is faster on the AS side, where the welding progress direction and the tool rotation direction are the same, forming a clear boundary with mechanical deformation. SP damages the oxide film and regenerates it thinly and activates corrosion by stagnating ions in the SP indentation. Therefore, more corrosion products Al(OH)3 were observed on the surface of the SP specimen. In addition, the Fe elements remained on the surface of the SP specimen under the influence of the subject (SP ball), affecting corrosion resistance. Differences in the form of pitting corrosion could be confirmed through comparison of SAM images according to depth. In addition, by comparing the area roughness, it was confirmed that peening reduced pitting corrosion. The pitting corrosion area of the cross section appeared in the order: FSW > SPFSW > SPBM ≃ BM. Also, more corrosion was identified in AS, compared to NZ. This is caused by tensile residual stress resulting from the thermal and mechanical deformation of FSW, and the corrosion resistance of the SPFSW specimen that was converted to compressive residual stress through SP was effectively improved. In general, overall corrosion is dominant in SP specimens compared to Non-SP specimens in which pitting corrosion is dominant. Therefore, SP reduces pitting corrosion, which is a problem in structures, and reduces differences between FSW areas, helping to weld aluminum structures. However, since different corrosion reactions, such as red rust, may occur depending on the material of the subject, additional research is needed.