Achieving Superior Corrosion Resistance in Friction Stir Processed AA6061-T6 Aluminum Alloy Joints

AA6061-T6 aluminum alloy (Al–Mg–Si alloy) has gathered widespread acceptance in the fabrication of light weight constructions necessitating high strength-to-weight ratio and good corrosion resistance. In this study, ultra-fine grained (UFG) structure (~0.65 µm) was produced in the stir zone (SZ) of AA6061-T6 alloy joints using friction stir processing (FSP) cooled by liquid nitrogen (N₂). A new experimental set-up was designed and used to simultaneously quench the lower and upper surfaces of weldments during the processing. The temperature changes were monitored via attaching thermocouples (K-type) to samples. In addition, a conventional solid state welding technique, friction stir welding (FSW), was used to fabricate reference joints. Micro- and sub-structural studies were carried out to using high resolution EBSD. The corrosion properties of the cooled joints were investigated by potentiodynamic polarization and NaCl-induced atmospheric corrosion tests. The atmospheric corrosion exposures were carried out at a relative humidity (RH) of 95%, 400 ppm CO₂ and at 22°C. Crystalline corrosion products formed on the surface of the samples were identified by X-ray diffraction (XRD). The rapidly cooled joints exhibited exceptional corrosion resistance in both corrosion experiments as well as better mechanical properties compared to those of the FSWed joints.