Stress corrosion cracking susceptibility of friction stir welded AA7075-AA6056 dissimilar joint

Two aluminium alloys, AA7075 and AA6056, were friction stir welded, with the AA7075 alloy placed on 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 nominal 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. Also in this case 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 nominal 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 it is concluded that though the weld nugget is resistant to SCC, the TMAZ/HAZ region of AA7075 in the weldment is prone to SCC in 3.5% chloride solutions at nominal strain rate levels in the order of 10−7 s−1.