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J. Mater. Sci. Technol.  2018, Vol. 34 Issue (1): 219-227    DOI: 10.1016/j.jmst.2017.11.033
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Effects of welding parameters and post-heat treatment on mechanical properties of friction stir welded AA2195-T8 Al-Li alloy
J. Zhang*(), X.S. Feng*(), J.S. Gao, H. Huang, Z.Q. Ma, L.J. Guo
Technical Center, Shanghai Aerospace Equipments Manufacturer, Shanghai 200245, China
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Abstract  

In this study, the effects of main welding parameters (rotation speed (ω) and welding speed (v)) on the microstructure, micro-hardness distribution and tensile properties of friction stir welded (FSW) 2195-T8 Al-Li alloy were investigated. The effects of T6 post-treatments at different solution and aging conditions on the mechanical properties and microstructure characteristics of the FSW joints were also investigated. The results show that with increasing ω and v, both strength and elongation of the joints increase first, and then decrease with further increase of ω and v. All the joints under varied welding parameters show significant strength loss, and the strength reaches only 65% of the base metal. The effect of T6 post-heat treatment on the mechanical properties of the joints depends on the solution and aging conditions. Two heat treatment processes (480 °C × 0.5 h quenching + 180 °C × 12 h, 520 °C × 0.5 h quenching + 180 °C × 12 h aging) are found to increase the joint strength. Furthermore, low temperature quenching (480 °C) is more beneficial to the joint strength. The joint strength can reach 85% of the base metal. Whereas both low temperature aging (140 °C × 56 h) and stepped aging (100 °C × 12 h + 180 °C × 3 h) processes decrease the joint strength. After heat treatment all the joints show decreased ductility due to the obvious grain coarsening in the nugget zone (NZ) and thermo-mechanically affected zone (TMAZ).

Key words:  2195-T8 Al-Li alloy      Friction stir welding      Welding parameters      Post-heat treatment     
Received:  09 April 2017     
Corresponding Authors:  Zhang J.,Feng X.S.     E-mail:  desternymuyu@163.com;fxsupc@163.com

Cite this article: 

J. Zhang, X.S. Feng, J.S. Gao, H. Huang, Z.Q. Ma, L.J. Guo. Effects of welding parameters and post-heat treatment on mechanical properties of friction stir welded AA2195-T8 Al-Li alloy. J. Mater. Sci. Technol., 2018, 34(1): 219-227.

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https://www.jmst.org/EN/10.1016/j.jmst.2017.11.033     OR     https://www.jmst.org/EN/Y2018/V34/I1/219

Cu Li Mg Zr Fe Si Al
3.97 1.05 0.40 0.11 0.15 0.025 Bal.
Table 1  Chemical compositions of 2195-T8 Al-Li alloy (wt%).
Rolling direction UTS (MPa) YTS(MPa) ε (%)
2195-T8 577 ± 0.3 552 ± 0.3 6.4 ± 0.5
565 ± 0.3 538 ± 0.3 6.3 ± 0.5
Table 2  Measured mechanical properties of 2195-T8 Al-Li alloy.
No. Rotation speed, ω (rpm) Welding speed, v (mm/min) ω/v
1# 1500 500 3.00
2# 1800 500 3.60
3# 2100 500 4.20
4# 2300 500 4.60
5# 2600 500 5.20
6# 2100 700 3.00
7# 2100 600 3.50
8# 2100 400 5.25
9# 2300 548 4.20
10# 1800 429 4.20
11# 1500 357 4.20
Table 3  Welding parameter combinations of FSW.
No. Heat treatment scheme
HT1 520 °C quenching for 50 min + 180 °C aging for 12 h
HT2 480 °C quenching for 25 min + 180 °C aging for 12 h
HT3 500 °C quenching for 25 min + 140 °C artificial aging for 56 h
HT4 500 °C quenching for 25 min + 100 °C aging for 12 h + 180 °C aging for 3 h
Table 4  Schemes of different heat treatment processes.
Fig. 1.  Dimension of the tensile test specimens.
Fig. 2.  (a) Macroscopic view, (b) base metal, (c) NZ, (d) TMAZ(AS), (e) TMAZ(RS), (f) HAZ of metallographic cross-section (2100 rpm, 500 mm/min).
Fig. 3.  Micro-hardness profiles on cross-sections of joints under (a) different rotation speeds at 500 mm/min, (b) different welding speeds at 2100 rpm, (c) different combination of rotation speed and welding speed (ω/v = 4.2).
Fig. 4.  Typical Stress-strain curves of the FSW joint, weld and the base metal (2100 rpm,500 mm/min).
Fig. 5.  Effects of rotation speed (a) and welding speed (b) on tensile strength and engineering elongation of the joints.
Fig. 6.  Effects of welding parameters on tensile strength and engineering elongation of the joints.
Fig. 7.  Image of a fractured FSW joint (2100 rpm,500 mm/min) and a diagram showing the joint fracture locations (between the two dash lines).
Fig. 8.  Microstructures of NZ and TMAZ under different welding parameters (a) 1500 rpm, 500 mm/min (b) 2600 rpm, 500 mm/min.
Fig. 9.  Macroscopic metallographic cross-section of 2195 friction stir welded joint (2100 rpm, 700 mm/min).
Fig. 10.  Tensile strength and engineering elongation of joints after different heat treatments (2100 rpm, 500 mm/min).
Fig. 11.  Micro-hardness profiles on cross-sections of joints before and after HT1 heat treatment (2100 rpm, 500 mm/min).
Fig. 12.  Macroscopic metallographic cross-section of 2195 friction stir welded joints (a) before and (b) after post-heat treatment (2100 rpm, 500 mm/min).
Fig. 13.  Joint fracture location after HT3 post-heat treatment (2100 rpm, 500 mm/min).
Fig. 14.  Microstructures of TAZ/TMAZ on AS under (a) HT1, (b) HT2, (c) HT3, (d) HT4 post-heat treatment (2100 rpm, 500 mm/min).
Fig. 15.  Microstructures of NZ under (a) HT1, (b) HT2, (c) HT3, (d) HT4 post-heat treatment (2100 rpm, 500 mm/min).
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