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Acta Metall Sin  2016, Vol. 52 Issue (11): 1395-1402    DOI: 10.11900/0412.1961.2016.00026
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Yanxin QIAO1,Yang ZHOU1,Shujin CHEN1(),Qining SONG2
1 School of Materials Science and Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, China 2 School of Mechanical and Electrical Engineering, Hohai University, Changzhou 213022, China
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Friction stir welding (FSW) is a new solid-state joining method which offers several advantages compared with conventional welding methods, including better mechanical properties, lower residual stress and reduced occurrence of defects. It has already been used for joining Al alloys in the aerospace and automotive industries. In spite of the advantages, FSW also has drawbacks, such as the risk of root flaws in single-side welds. Using a bobbin tool instead is a promising way to solve this problem since the root region is avoided. Compared with standard (single-side) FSW techniques, the bobbin tool FSW has an extra shoulder attached to the tip of the probe, namely the lower shoulder. This setup makes BTFSW capable of joining closed profiles like hollow extrusions. Furthermore, root flaws, such as lack of penetration, which occasionally occurred in standard FSWtechiques, can be completely avoided. In this work, 6061-T6 aluminum alloy was welded by using bobbin tool friction stir weld (BTFSW). The influence of BTFSW on the microstructure development and hardness distribution in the weldment has been investigated. The corrosion behaviors of the base metal and weld nugget in 3.5%NaCl (mass fraction) solution were investigated using SEM, XRD and electrochemical measurements. The results showed that the weld surface of 6061-T6 welded by BTFSW is of good quality. No welding defect was detected in the joints. Three microstructural zones, i.e., nugget zone, thermo-mechanically affected zone, and heat affected zone were discernible. The microstructural analysis indicates that the weld nugget region exhibited fine and equiaxed grain structure with an average grain size of ~8 μm, indicating the occurrence of dynamic recrystallization due to severe plastic deformation and thermal exposure. The thermo-mechanically affected zone underwent plastic deformation and recrystallization occured in this zone due to deformation strain and thermal input. The low hardness zone, determined by constructing the hardness distribution profile on cross-section of joint, located at thermo-mechanically affected zone of advancing side. Although 6061-T6 alloys are readily weldable, they suffered from severe softening in the heat affected zone because of the dissolution of Mg2Si precipitates during the weld thermal cycle. BTFSW can improve the corrosion resistance of 6061-T6 aluminum alloy in 3.5%NaCl solution. The corrosion behavior results showed that both anodic dissolution and pitting were observed after the immersion test due to the inhomogeneous microstructure of 6061-T6 aluminum alloy. The corrosion products mainly composed of Al(OH)3 and Al2O3. Furthermore, the corrosion process and mechanism were also discussed.

Key words:  corrosion,      aluminum      alloy,      bobbin      tool      friction      stir      welding,      microstructure     
Received:  13 January 2016     
Fund: Supported by National Natural Science Foundation of China (Nos.51401092, 51575252 and 51205175)

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Fig.1  BTFSW tools (BTFSW—bobbin tool friction stir welding)

(a) top and bottom shoulders (b) stirring pin (c) welding tool

Fig.2  Schematic of BTFSW 6061-T6 joints for corrosion and hardness test across the weld[25] (unit: mm)
Fig.3  Morphologies of top (a) and bottom (b) surfaces of 6061-T6 aluminum alloy after BTFSW
Fig.4  Cross-sectional macrostructure of BTFSW 6061-T6 joints (BM—base metal, HAZ—heat affected zone, TMAZ—thermo-mechanically affected zone, NZ—nugget zone)
Fig.5  Microstructures on the top surface of the BTFSW 6061-T6 joint (a) BM (b)NZ (c)TMAZ of advancing side (d) TMAZ of retreat side (e) HAZ of advancing side (f) HAZ of retreat side
Fig.6  Cross-section microhardness of the BTFSW 6061-T6 joint
Fig.7  Polarization curves of BM and NZ in 3.5%NaCl solution (E—corrosion potential, i—corrosion current density)
Fig.8  Nyquist plots of BM and NZ of 6061-T6 aluminum alloy in 3.5%NaCl solution
Point Mass fraction / % Atomic fraction / %
O Mg Al Si Cl O Mg Al Si Cl
1 32.30 0.47 66.60 0.37 0.26 44.59 0.43 54.52 0.29 0.16
2 61.06 - 38.27 0.33 0.34 72.61 - 26.98 0.22 0.18
3 31.33 2.61 73.74 1.31 1.01 43.48 2.39 52.46 1.03 0.63
4 66.68 - 32.23 0.47 0.62 77.23 - 22.13 0.31 0.32
Table 1  EDS results of corrosion products formed on BM and WN of 6061-T6 aluminum alloy pointing in Fig.9
Fig.9  SEM images of corrosion products formed on BM (a) and NZ (b) of 6061-T6 aluminum alloy after immersion in 3.5% NaCl solution for 480 h (1, 3—passive film; 2, 4—“furuncle” like corrosion product)
Fig.10  XRD spectra of corrosion products formed on BM and NZ of 6061-T6 aluminum alloy after immersion in 3.5%NaCl solution for 480 h
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