Corrosion-assisted fatigue crack growth behaviour of AA 7075-T651 friction stir welded joints: Influence of extended seawater exposure on microstructure and fracture characteristics

Abstract

This study investigates the corrosion-assisted fatigue crack growth rate (FCGR) of 16 mm thick AA 7075-T651 friction stir welded (FSW) joints. Compact tension (CT) specimens were extracted from both the base material and FSW joints to evaluate FCGR under varying corrosion exposure durations (0, 7, 30, 60, and 90 days) at a constant stress ratio of 0.5. Microstructural analysis of the welds was conducted using optical and transmission electron microscopy (TEM). Results indicate that the critical stress intensity factor range (ΔK_cr) of FSW joints is lower than that of the base material, primarily due to precipitate dissolution in the weld zone during the FSW process, as confirmed by TEM analysis. The fatigue life of FSW joints was significantly lower than that of the base material, but with prolonged exposure to seawater corrosion, the gap in fatigue life narrowed. Specimens exposed to seawater for more than 60 days exhibited minimal differences in fatigue life between the base material and the FSW joints. This was attributed to the higher corrosion rate of the base material compared to the weld nugget, resulting in the formation of deeper pits that facilitated crack initiation and accelerated fatigue failure. The findings conclude that extended corrosion exposure leads to similar fatigue life and crack growth behaviour in both the base material and FSW joints. SEM and EDX analysis of AA7075-T651 revealed corrosion pits and rust products in initiation zones, ductile striations in growth regions, and secondary cracks with micro voids in fracture zones. FSW joints exhibited ultra-fine grains, smooth ductile fracture in initiation and growth regions, and brittle fracture in the fracture zones under both corroded and uncorroded conditions.