Characterization of corrosion behavior in friction stir weld Al-Li-Cu AF/C458 alloy

The corrosion behavior of a friction stir welded Al-2.05Li-2.70Cu-0.6Mg-0.3Zn alloy AF/C458 has been characterized using several corrosion techniques. Vicker's hardness profiles collected transverse to the weld direction and optical microscopy helped distinguish different zones within the weld. Zones identified were the parent material, advancing heat affected zone (HAS), trailing heat affected zone (HTS), the thermo-mechanically affected sides on both sides of the weld (TMAZ) and nugget. The nugget and parent material were found to have Vicker's hardness values of 118 ± 4 HV and 170 ± 4 HV, respectively. The environmentally assisted cracking (EAC) and the corrosion behavior were investigated using constant extension rate testing (CERT), intergranular corrosion immersion test, and potentiodynamic scans. The CERT experiments were completed in air and 10mM NaCl solution on round tensile bar samples cut from within the weld micro-zones parallel to the weld and also transverse (perpendicular) to the weld direction. In samples from each of the micro-zones cut parallel to the weld, the ductility ratio (D.R.) was greatest for parent material (0.88), and lowest for the HTS (0.45), indicating higher corrosion susceptibility in this zone. The HAS had a slightly higher D.R. (0.72) than the nugget (0.62). From the metallography and fractography data of the weld transverse samples in air and 10mM NaCl solution, it appeared that the heat-affected zones were the most susceptible to intergranular and intersub-granular corrosion. Immersion testing conducted according to ASTM G110-92 standard revealed equiaxed pitting over the entire surface of the weld. Under high magnification, pitting, discontinuous sub-grain attack and borderline intergranular attack were observed in the HTS, HAS, TMAZ of the trailing and advancing sides as well as the parent material. The nugget displayed borderline intergranular attack and pitting only. Polarization scans showed that the pitting potentials in each micro-zone increased in the following manner: parent material (-506 ± 27 mV) > advancing and trailing HAZs (-522 ± 15 mV and -519 ± 19 mV, respectively) > nugget (-532 ± 5 mV). The repassivation and corrosion potentials did not show such a trend and were approximately the same for each weld zone.