Corrosion effects on fatigue behavior of dissimilar friction stir welding of high-strength aluminum alloys

This paper examines the effects of corrosion defects on the low-cycle fatigue performance of dissimilar friction stir welded AA6061-to-AA7050 aluminum alloys. Corrosion defects were produced on the crown surface of the weld by static immersion in 3.5% NaCl for various exposure times. Results revealed localized corrosion damage in the thermo-mechanically affected and heat-affected zones. The corrosion damage featured general pitting, pit clustering, and exfoliation that revealed increasing depth with increasing exposure time. The higher corrosion attack was measured in the AA7050 compared to the AA6061. Results demonstrated a decrease in the fatigue life with evidence of crack initiation at the corrosion defects; however, the fatigue life was nearly independent of the exposure time. This can be attributed to total fatigue life dominated by incubation time. Furthermore, two types of failure were observed: crack propagation in the AA6061 side at high-strain amplitudes (> 0.3%) and crack propagation in the AA7050 side at low-strain amplitudes (< 0.2%). This can be attributed to the cyclic strain hardening evolution and the localized high-stress field at the tip of the corrosion defect. Lastly, a microstructure-sensitive fatigue model was employed to capture the effect of corrosion defects for the life prediction of the dissimilar friction stir welded AA6061-to-AA7050.