Local melting mechanism and its effects on mechanical properties of friction spot welded joint for Al-Zn-Mg-Cu alloy

doi:10.1016/j.jmst.2017.11.014

Abstract

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

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]. J. Mater. Sci. Technol., 2018, 34(1): 185-191.

Figures/Tables 14

Fig. 1. Schematic of FSpW process and thermocouple positions.

Table 1 Chemical composition of 7B04-T74 aluminum alloy (wt%).

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.

Fig. 2. Assembly of numerical model.

Table 2 Thermal properties of 7B04-T74 aluminum alloy at different temperatures.

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 3 Invariable thermal properties of 7B04-T74 aluminum alloy.

Yong’s modulus (GPa)	Poisson’s ration	Thermal expansion (°C^-1)	Density (kg m^-3)
68.9	0.3	2.2 × 10^-5	2800

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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