Investigation of stress corrosion cracking behavior of friction stir welded thick al 6061-t6 alloy plate

Abstract

In this study, the stress corrosion cracking (SCC) behavior of friction stir welded (FSWed) Al 6061-T6 alloy 10 mm thick plate is investigated. The general corrosion resistance of the base alloy and welded joint is evaluated using a potentiodynamic polarization test, electrochemical impedance spectroscopy (EIS), and immersion test in NaCl solution. SCC susceptibility is evaluated in 3.5% NaCl and 7.5% NaCl aqueous solution using a slow strain rate tensile (SSRT) test. Potentiodynamic polarization test results show that the FSWed joint experienced a higher corrosion rate (9.658 × 10⁻⁶ mmpy) compared to base metal (0.734 × 10⁻⁶ mmpy). EIS results show that the charge transfer resistance (R_ct) of the joint was higher (118 Ω) compared to base metal (242 Ω) which further confirms that the joint has a higher susceptibility to pitting corrosion. SEM results also show that the welded joint immersed for 168 h in a 3.5% chloride environment experienced higher pitting corrosion than the base metal, while EDX results confirm that the pitting occurred due to the anodic dissolution of Mg-Si particles within the Al matrix. SCC results show that FSWed joints tested in air, 3.5% and 7.5% NaCl environment, show maximum load vs. displacement of 23.5 kN vs. 23.5 mm, 20.9 kN vs. 20.9 mm, and 16.8 kN vs. 16.8 mm, respectively indicating a weakened SCC resistance to increasing chloride environment. Samples tested in a chloride environment during the SSRT test showed quasi-cleavage fracture, while samples tested in an air environment showed ductile-type fracture. The higher general corrosion rate and SCC susceptibility of FSWed joints are attributed to the inhomogeneous microstructure developed during welding and the anodic dissolution of intermetallic compounds.