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Acta Metall Sin  2017, Vol. 53 Issue (12): 1651-1658    DOI: 10.11900/0412.1961.2017.00025
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Microstructures and Mechanical Properties of Thin Plate Aluminium Alloy Joint Prepared by High Rotational Speed Friction Stir Welding
Fenjun LIU1,2, Li FU1,3,4(), Haiyan CHEN1,3,4
1 School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an 710072, China
2 College of Energy Engineering, Yulin University, Yulin 719000, China
3 State Key Laboratory of Solidification, Northwestern Polytechnical University, Xi'an 710072, China
4 Shaanxi Key Laboratory of Friction Welding Technologies, Northwestern Polytechnical University, Xi'an 710072, China
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Abstract  

Aluminium alloys were widely applied in rail transit, ships and aerospace owing to their unique properties, such as low density, high strength and stiffness, outstanding corrosion resistance and low temperature performance. As a type of structure material, aluminium alloy joining was inevitable. However, these alloys were often considered very difficult to weld using traditional fusion welding technique since the welding seams were often accompanied with metallurgical defects, large deformation and stress. Friction stir welding (FSW), an innovative solid-state welding technology invented at the welding institute (TWI), was seen by designers as an effective joining methods in welding aluminium alloys due to low heat input, small stress-strain and environment friendly. In this work, 0.8 mm thick plate of 6061-T6 aluminium alloy was successfully welded by use of high rotational speed fiction stir welding technology. The microstructure and mechanical property of the butt joints prepared by high rotational speed friction stir welding were analysed in detail. The results show that the well surface topography and excellent bonding interface existed in the nugget zone (NZ) were observed. Both of the microhardness of the weld seam was lower than that of the substrate. The lowest microhardness of the butt joints located between the thermo-mechanically affected zone (TMAZ) and heat affected zone (HAZ). Compared with the conventional rotational speed, the number of β-Mg2Si, Al2CuMg and Al8Fe2Si precipitated phases existed in the NZ was more, which made the microhardness in the NZ improved significantly. The rod-shaped precipitates (Mg2Si) have the greatest influence on the microhardness. The excellent mechanical properties were obtained at the rotational speed of 8000 r/min and welding speed of 1500 mm/min. The maximum tensile strength was 301.8 MPa, which was 85.8% of the as-received 6061-T6 (351.7 MPa). And the toughness-brittleness fracture mode appeared.

Key words:  thin plate 6061-T6 aluminium alloy      high rotational speed      friction stir welding (FSW)      microstructure      tensile property     
Received:  19 January 2017     
ZTFLH:  TG146.2  
Fund: Supported by National Natural Science Foundation of China (No.51575450), Key Areas of Innovation Team in Shaanxi Province (No.2014KCT-12), Natural Science Foundation of Shaanxi Province (No.S2016-YFJZ0164), Research Fund of the State Key Laboratory of Solidification (NWPU) (No.127-QP-2015)

Cite this article: 

Fenjun LIU, Li FU, Haiyan CHEN. Microstructures and Mechanical Properties of Thin Plate Aluminium Alloy Joint Prepared by High Rotational Speed Friction Stir Welding. Acta Metall Sin, 2017, 53(12): 1651-1658.

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https://www.ams.org.cn/EN/10.11900/0412.1961.2017.00025     OR     https://www.ams.org.cn/EN/Y2017/V53/I12/1651

Fig.1  Schematic of 6061-T6 friction stir welding (FSW) device (a), pin tool (b) and tensile specimen (c) (RS—retreating side, AS—advancing side, unit: mm)
Fig.2  Surface morphologies of 6061-T6 FSW joints obtained at rotational speeds of 2000 r/min (a) and 8000 r/min (b)
Fig.3  Morphologies of 6061-T6 FSW joints obtained at rotational speeds of 2000 r/min (a) and 8000 r/min (b) (BM—base material, HAZ—heat affected zone, TMAZ—thermo-mechanical affected zone, NZ—nugget zone)
Fig.4  Microstructures of 6061-T6 FSW joints corresponding to areas of HAZ (a, d), TMAZ (b, e), and NZ (c, f) at 2000 r/min (a~c) and 8000 r/min (d~f)
Fig.5  Bright field TEM images of precipitated phases morphology and distribution of HAZ (a, c) and NZ (b, d) obtained at 2000 r/min (a, b) and 8000 r/min (c, d), respectively
Fig.6  Grain morphology maps of 6061-T6 FSW joints corresponding to Fig.3b(a) region 1 (NZ) (b) region 2 (TMAZ) (c) region 3 (HAZ)
Fig.7  Fraction of different grain type in 6061-T6 FSW joints corresponding to Fig.6
Fig.8  Microhardness distributions of 6061-T6 FSW joints along the transverse to the weld center
Fig.9  EBSD grain-boundary maps showing the grain structure in the NZ of 6061-T6 FSW joint obtained at 8000 r/min
ω / (rmin-1) v / (mmmin-1) σUTS / MPa σYS / MPa δ / %
0 0 351.7 296.8 21.50
2000 300 239.0 179.2 4.80
7000 1500 289.4 207.8 4.88
8000 1500 301.8 216.6 5.39
9000 1500 300.8 213.0 5.44
10000 1500 292.6 205.7 5.31
11000 1500 292.2 201.8 5.49
Table 1  Tensile properties of 6061-T6 and FSW butt joints with different rotational speeds
Fig.10  Fractographies of 6061-T6 FSW joint obtained at 8000 r/min after tensile test (a) and magnified morphologies of area A in Fig.10a (b) (Inset shows the fracture position)
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