Stress Corrosion Cracking Susceptibility of Friction Stir Welded AA7075 – AA6056 Dissimilar Joint

Abstract

Two aluminium alloys, AA7075 and AA6056, were friction stir welded, with the AA7075 alloy placed in the advancing side of the welding tool. Microstructural observations revealed the development of a recrystallised fine-grained weld nugget, with two different grain sizes, resulting from the two different base materials. Slow strain rate tensile (SSRT) tests in air have shown that the weld nugget is marginally overmatched in the weldment, and the fracture occurred in the relatively weaker thermo-mechanically affected zone/heat affected zone (TMAZ/HAZ) of the AA6056 alloy. SSRT tests in 3.5% NaCl solution at a strain rate of 10-6 s-1 have shown that this dissimilar weldment is not susceptible to stress corrosion cracking (SCC) under these test conditions; and in this case, too, fracture was observed in the TMAZ/HAZ of the AA6056 alloy, with a behaviour very similar to that observed in the tests in air. However, at a still lower strain rate, 10-7 s-1, the TMAZ/HAZ region of AA7075 alloy was found to be susceptible to SCC, exhibiting intergranular fracture. As a whole, based on this investigation, it is concluded that though the weld nugget is resistant to SCC, the TMAZ/HAZ region of AA7075 in the weldment is highly susceptible to SCC at strain rates of 10-7 s-1 and below in 3.5% chloride solutions.