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J. Mater. Sci. Technol.  2018, Vol. 34 Issue (1): 228-236    DOI: 10.1016/j.jmst.2017.11.039
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Inhomogeneity of microstructure and mechanical properties in the nugget of friction stir welded thick 7075 aluminum alloy joints
Yuqing Mao*(), Liming Ke, Yuhua Chen, Fencheng Liu, Li Xing
National Defence Key Discipline Laboratory of Light Alloy Processing Science and Technology, Nanchang Hangkong University, Nanchang 330063, China
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Abstract  

In this study, 20 mm thick AA7075-T6 alloy plates were joined by friction stir welding. The microstructure and mechanical properties of the nugget zone along the thickness direction from the top to the bottom was investigated. The results showed that the microstructure including the grain size, the degree of dynamic recrystallization, the misorientation angle distribution and the precipitation phase containing its size, type and content exhibited a gradient distribution along the thickness direction. The testing results of mechanical properties of the slices showed that the nugget was gradually weakened along the depth from the top to the bottom. The maximum ultimate tensile strength, yield strength and elongation of the slice in the nugget top-middle are obtained, which are 415 MPa, 255 MPa and 8.1%, respectively.

Key words:  AA7075 thick plate      Friction stir welding      Nugget      Heterogeneity      Microstructure      Mechanical properties     
Received:  10 April 2017     
Corresponding Authors:  Mao Yuqing     E-mail:  maoyuqing-8888@163.com

Cite this article: 

Yuqing Mao, Liming Ke, Yuhua Chen, Fencheng Liu, Li Xing. Inhomogeneity of microstructure and mechanical properties in the nugget of friction stir welded thick 7075 aluminum alloy joints. J. Mater. Sci. Technol., 2018, 34(1): 228-236.

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https://www.jmst.org/EN/10.1016/j.jmst.2017.11.039     OR     https://www.jmst.org/EN/Y2018/V34/I1/228

Zn Mg Cu Si Fe Mn Cr Ti Al
5.7-6.0 2.6-2.85 1.6-2.1 0.35 0.5 0.3 0.25 0.2 Bal.
Table 1  Chemical composition and mechanical properties of 7075-T6 aluminum alloy.
Fig. 1.  Optical micrograph of cross section of FSW joint.
Fig. 2.  Schematic diagram of tensile specimens: (a) cut position; (b) detailed size.
Fig. 3.  EBSD maps of microstructure at various depths in the WNZ: (a) 1 mm, (b) 3 mm, (c) 8 mm, (d) 13 mm, (e) 18 mm inside the nugget, respectively; and (f) the BM.
Fig. 4.  Average grain size at different depths in the WNZ from top surface.
Fig. 5.  Fraction of dynamic recrystallization in the WNZ from top surface.
Fig. 6.  Misorientation angle distribution at various depths in the WNZ: (a) 1 mm, (b) 3 mm, (c) 8 mm, (d) 13 mm, (e) 18 mm from top surface, respectively; and (f) the BM.
Fig. 7.  TEM bright-field micrographs of plane view at various depths in the WNZ: (a) 1 mm, (b) 3 mm, (c) 8 mm, (d) 13 mm, (e) 18 mm from top surface, respectively, and (f) the BM; (g) SAED-1, (h) SAED-2, (i) SAED-3.
Fig. 8.  XRD patterns of the WNZ at (a) 1 mm, (b) 3 mm, (c) 8 mm, (d) 13 mm, (e) 18 mm from top surface, respectively; and (f) the BM.
Fig. 9.  Microhardness of the BM and the WNZ at different depths along the thickness direction.
Fig. 10.  Tensile properties of all slices in the WNZ and the BM: (a) stress-strain curves; (b) strength-elongation.
Fig. 11.  SEM micrographs of tensile fracture surfaces for (a) top, (b) top-middle, (c) middle, (d) middle-root, (e) root, respectively; and (f) the BM.
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