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J. Mater. Sci. Technol.  2018, Vol. 34 Issue (1): 173-184    DOI: 10.1016/j.jmst.2017.05.015
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Comparative study on local and global mechanical properties of bobbin tool and conventional friction stir welded 7085-T7452 aluminum thick plate
Weifeng Xua*(), Yuxuan Luoa, Wei Zhanga, Mingwang Fub
a State Key Laboratory of Solidification Processing, Shaanxi Key Laboratory of Friction Welding Technologies, Northwestern Polytechnical University, Xi'an 710072, Shaanxi, China
b Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China
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Abstract  

7085-T7452 plates with a thickness of 12 mm were welded by conventional single side and bobbin tool friction stir welding (SS-FSW and BB-FSW, respectively) at different welding parameters. The temperature distribution, microstructure evolution and mechanical properties of joints along the thickness direction were investigated, and digital image correlation (DIC) was utilized to evaluate quantitatively the deformation of different zones during tensile tests. The results indicated that heat-affected zone (HAZ), the local softening region, was responsible for the early plastic deformation and also the fracture location for SS-FSW samples, while a rapid fracture was observed in weld nugget zone (WNZ) before yield behavior for all BB-FSW specimens. The ultimate tensile strength (UTS) of SS-FSW joints presented the highest value of 410 MPa, 82% of the base material, at a rotational speed of 300 rpm and welding speed of 60 mm/min, much higher than that of BB-FSW joints, with a joint efficiency of only 47%. This should be attributed to the Lazy S defect produced by a larger extent of heat input during the BB-FSW process. The whole joint exhibited a much higher elongation than the slices. Scanning electron microscopic (SEM) analysis of the fracture morphologies showed that joints failed through ductile fracture for SS-FSW and brittle fracture for BB-FSW.

Key words:  Aluminum alloy      Friction stir welding      Bobbin tool      Temperature distribution      Microstructure      Mechanical properties     
Received:  20 February 2017     
Corresponding Authors:  Xu Weifeng     E-mail:  xwf1982@nwpu.edu.cn

Cite this article: 

Weifeng Xu, Yuxuan Luo, Wei Zhang, Mingwang Fu. Comparative study on local and global mechanical properties of bobbin tool and conventional friction stir welded 7085-T7452 aluminum thick plate. J. Mater. Sci. Technol., 2018, 34(1): 173-184.

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https://www.jmst.org/EN/10.1016/j.jmst.2017.05.015     OR     https://www.jmst.org/EN/Y2018/V34/I1/173

Chemical compositions (wt%)
Al Zn Mg Cu Fe Si Zr
Bal. 7.0-8.0 1.2-1.8 1.3-2.0 0.08 0.06 0.08-0.15
Table 1  Chemical compositions of 7085-T7452 aluminum alloys.
Fig. 1.  Tools of single side (a), bobbin tool (b) during FSW process.
Welding technology Rotational speed (rpm) Welding speed (mm/min)
SS-FSW 300 60
600 60
BB-FSW 150 150
200 150
Table 2  Parameters for SS-FSW and BB-FSW.
Fig. 2.  Arrangement of blind holes for temperature measurement, (a) SS-FSW, (b) BB-FSW.
Fig. 3.  Temperature curves at different positions of SS-FSW (a) and BB-FSW (b) joints under different parameters.
Fig. 4.  Peak temperature of different thermocouple points at SS-FSW and BB-FSW joints.
Fig. 5.  Macrographs of FSW welds on cross section at different process conditions (a) and (b) SS-FSW, (c) and (d) BB-FSW.
Fig. 6.  Grain morphologies of BM (a), WNZ (b), TMAZ in the AS (d) and HAZ (f) of SS-FSW, WNZ (c), TMAZ in the AS (e) and HAZ (g) of BB-FSW.
Fig. 7.  Grains characteristics of the upper, middle and lower layers of WNZs along the thickness direction of (a-c) SS-FSW and (d-f) BB-FSW joints.
Fig. 8.  Grain sizes in WNZ along the thickness direction under different welding conditions.
Fig. 9.  Maps of microhardness distribution along the transverse cross-section of (a) BM and (b-e) FSW joints.
Fig. 10.  Tensile test results of the whole and different slices of FSW joints, (a) stress-strain, (b) YS, (c) UTS, and (d) elongation.
Fig. 11.  DIC results for strain distribution of (a) the whole and different slices (b-top, c-middle, d-bottom) of a SS-FSW joint (300 rpm, 60 mm/min).
Fig. 12.  Distribution of YS along the transverse section of SS-FSW joint (300 rpm, 60 mm/min).
Fig. 13.  DIC results for strain distribution of (a) the whole and different slices ((b)-top, (c)-middle, (d)-bottom) of a BB-FSW joint (150 rpm, 150 mm/min).
Fig. 14.  Typical SEM images showing the fracture surfaces of (a and b) SS-FSW and (c and d) BB-FSW samples (a and c: secondary electron images, b and d: back-scattered electron images).
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