Research On Stress Corrosion Property Of Aluminum Alloys Friction Stir Welds

Friction stir welding (FSW) is a new solid state joint technique which was invented by The Welding Institute in the UK in 1991. Nowadays, aluminum alloy, magnesium alloy, copper alloy, titanium alloy, carbon steel, stainless steel, composite, plastics and dissimilar metal material have been successfully joined by FSW. With the broadening of its application space, the stress corrosion cracking (SCC) behavior and mechanism of friction stir weld have become a public concern issue.In this paper,2A12-T4 aluminum alloy was FSW, and the microstructure and mechanism property of friction stir weld were analyzed by OM, TEM, SEM and EDS. The SCC behavior and mechanism were studied using slow strain rate tensile, four-point loaded test and electrochemical technology. The effect of welding parameter, strain rate, polarization potential and loading regime on the SCC property of weld was obtained, and the relationship between localized pitting and SCC behavior was discussed. The mainly results are as follows:The friction stir weld of 2A12-T4 aluminum consists of four distinct microstructural zones:the nugget (NZ), the thermo mechanically affected zone (TMAZ), the heat affected zone (HAZ) and the base metal (BM). The NZ is characterized by reasonably fine and equiaxed recrystallized grains with 2-3um. The size of the strengthening precipitates increases with the increase of the rotation speed, while it decreases and distributes uniformly with the decrease of the welding speed in the NZ/TMAZ region. The microhardness profile of the cross-section of friction stir weld is "W" type, indicating the loss of the microhardness. The optimized FSW parameter is at a rotation speed of 950 rpm and welding speed of 37.5 mm/min in the experimental region. The SCC susceptibility of the welds is lower at a strain rate of 1.33×10-5 s-1, while it is higher at a strain rate of 1.26×10-6 s-1. The SCC susceptibility of the welds increases with the increase of the rotation speed and welding speed. The four-point loaded specimens do not fracture. NZ/TMAZ region becomes the SCC susceptible region. The SCC property of the lower surface of friction stir weld is low. The anodic polarization and strong cathodic polarization increase the SCC susceptibility of the welds while weak cathodic polarization decreases the SCC susceptibility. The stress corrosion mechanism of the FSWed 2A12 alloy is mainly controlled by the metal anodic dissolution and the pitting accelerates the nucleation and growth of the stress corrosion cracks. The hydrogen reaction takes place because the localized galvanic cells cause acidification at the bottom of the stress corrosion crack, which accelerates metal anodic dissolution.