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J. Mater. Sci. Technol.  2018, Vol. 34 Issue (1): 185-191    DOI: 10.1016/j.jmst.2017.11.014
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Local melting mechanism and its effects on mechanical properties of friction spot welded joint for Al-Zn-Mg-Cu alloy
Yunqiang Zhao*(), Chungui Wang, Jizhong Li, Jinhong Tan, Chunlin Dong
Guangdong Welding Institute (China-Ukraine E.O.Paton Institute of Welding), Guangzhou, 510651, China
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Abstract  

Local melting and the eutectic film and liquation crack formation mechanisms during friction spot welding (FSpW) of Al-Zn-Mg-Cu alloy were studied by both experiment and finite element simulation. Their effects on mechanical properties of the joint were examined. When the welding heat input was high, the peak temperature in the stir zone was higher than the incipient melting temperature of the Al-Zn-Mg-Cu alloy. This resulted in local melting along the grain boundaries in this zone. In the retreating stage of the welding process, the formed liquid phase was driven by the flowing plastic material and redistributed as a “U-shaped” line in the stir zone. In the following cooling stage, this liquid phase transformed into eutectic films and liquation cracks. As a result, a new characteristic of “U” line that consisted of eutectic films and liquation cracks is formed in the FSpW join. This “U” line was located in the high stress region when the FSpW joint was loaded, thus it was adverse to the mechanical properties of the FSpW joint. During tensile shear tests, the “U” line became a preferred crack propagation path, resulting in the occurrence of brittle fracture.

Key words:  Friction spot welding      Numerical simulation      Local melting      Liquation crack      Mechanical property     
Received:  28 March 2017     
Corresponding Authors:  Zhao Yunqiang     E-mail:  zhaoyq@gwi.gd.cn

Cite this article: 

Yunqiang Zhao, Chungui Wang, Jizhong Li, Jinhong Tan, Chunlin Dong. Local melting mechanism and its effects on mechanical properties of friction spot welded joint for Al-Zn-Mg-Cu alloy. J. Mater. Sci. Technol., 2018, 34(1): 185-191.

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https://www.jmst.org/EN/10.1016/j.jmst.2017.11.014     OR     https://www.jmst.org/EN/Y2018/V34/I1/185

Fig. 1.  Schematic of FSpW process and thermocouple positions.
Zn Mg Cu Mn Fe Cr Si Ni Ti Al
5.86 2.51 1.62 0.34 0.18 0.15 0.07 0.05 0.03 Bal.
Table 1  Chemical composition of 7B04-T74 aluminum alloy (wt%).
Fig. 2.  Assembly of numerical model.
Temperature (°C) Thermal conductivity (N/sec °C) Thermal capacity (N/mm2 °C)
20 155 2.38
100 186 2.54
200 197 2.66
300 194 2.7
400 196 2.83
500 196 2.97
532 193 2.99
628 85 2.83
700 84 2.8
Table 2  Thermal properties of 7B04-T74 aluminum alloy at different temperatures.
Yong’s modulus (GPa) Poisson’s ration Thermal expansion (°C-1) Density (kg m-3)
68.9 0.3 2.2 × 10-5 2800
Table 3  Invariable thermal properties of 7B04-T74 aluminum alloy.
Fig. 3.  Comparisons between experiment and simulation results of welding thermal cycles at different monitor positions at tool rotation speeds of (a) 1250 and (b) 2500 rpm.
Fig. 4.  Simulation results of (a) instantaneous maximum temperatures as a function of welding time obtained at different tool rotation speeds and (b) temperature distribution on a half weld at the welding time of 3.5 s.
Fig. 5.  Distributions of material flow velocity on the cross-section of a half weld at the welding time of (a) 1.7 s and (b) 3.7 s at the tool rotation speed of 2500 rpm.
Fig. 6.  Simulated effective strain distribution on the cross-section of the as welded FSpW joint welded at the tool rotation speed of 2500 rpm.
Fig. 7.  Cross-sections of FSpW joints welded at tool rotation speeds of (a) 1250 and (b) 2500 rpm.
Fig. 8.  SEM images of “U” line in regions (a) A, (c) B, (d) C, (e) D as marked in Fig. 7b and (b) the energy dispersive spectrum result at the marked point.
Fig. 9.  Simulated Von Mises stress distribution on the cross-section of the FSpW joint in the tensile shear test.
Fig. 10.  Cross-sections of the failed FSpW joint welded at (a) 1250, (b) 2500 rpm and (c) displacement-load curves.
Fig. 11.  Fracture surfaces of the failed FSpW joints welded at tool rotation speeds of (a) 1250 and (b) 2500 rpm.
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