Corrosion Behavior of Top and Bottom Surfaces for Single-Side and Double-Side Friction Stir Welded 7085-T7651 Aluminum A
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TRODUCTION
7XXX series aluminum alloys have low cost, low density, fairly high strength, fracture toughness, excellent resistance to stress corrosion cracking and excellent workability, which are very attractive engineering materials used in aerospace and other engineering applications.[1,2] As a new solid-state joining technique, friction stir welding (FSW), invented at The Welding Institute (TWI) of the United Kingdom in 1991, was initially applied to aluminum alloys. Nowadays, FSW is widely used for the 2XXX and 7XXX-series aluminum alloys, both of which are known as difficult-to-weld metals using the conventional fusion welding methods, porosities tend to appear at the joints of these materials and deteriorates the mechanical properties.[2–4] So there are many researches on the joining of aluminum alloy, such as microstructure evolution,[3–5] tensile properties,[6–10] fatigue properties[11,12] and corrosion behavior.[13–15] The effect of FSW on the material combines frictional heat flow and mechanical stirring plastic strain. These thermo-mechanical conditions vary through the joint, introducing a corresponding variation of microstructures and mechanical properties. For the aluminum alloy thick plate, the materials along the thickness direction experienced more nonuniform
WEIFENG XU, WEI ZHANG, and XIAOLI WU are with the State Key Laboratory of Solidification Processing, Shaanxi Key Laboratory of Friction Welding Technologies, Northwestern Polytechnical University, Xi’an 710072, Shaanxi, PR China. Contact e-mail: [email protected] Manuscript submitted September 2, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
thermal-mechanical coupling using the larger size welding tool during single-side friction stir welding (SS-FSW). Thus, a significant difference on the grain size and second-phase particles was presented along the weld thickness direction.[6–10] Recently, a new FSW approach named double-side friction stir welding (DS-FSW) was developed using the relatively small size of welding tool. With the help of temperature gradient decrease and the improvement of uneven plastic deformation along the thickness direction, a significant microstructure homogeneity could be achieved in the weld zone.[4,5] With the increasing of the FSW application in severe corrosion environment, there is every need to understand the microstructure and corrosion behavior of FSW joints. Up to now, many investigations have been conducted on the corrosion behavior of FSW joints.[13–20] The corrosion results of dissimilar friction stir lap welded AA7050-T7651 and AA2024-T3 Al-alloys show that initial pitting nucleation at open circuit potential (OCP) conditions was associated to precipitates attack of the 7050-weld nugget zone (WNZ). AA2024 heat-affected zone (HAZ) was the most susceptible intergranular corrosion (IC) region. However, IC-attack was less intense than that on parent AA2024-T3 alloy using scanning vibrating electrode technique (SVET) in 0.1 M NaCl solution.[13] Deng et al.[14] reported that the thermo-mechanically affected zone (TMA
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