Effects of welding parameters and post-heat treatment on mechanical properties of friction stir welded AA2195-T8 Al-Li alloy

doi:10.1016/j.jmst.2017.11.033

Abstract

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

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]. J. Mater. Sci. Technol., 2018, 34(1): 219-227.

Figures/Tables 19

Table 1 Chemical compositions of 2195-T8 Al-Li alloy (wt%).

Cu	Li	Mg	Zr	Fe	Si	Al
3.97	1.05	0.40	0.11	0.15	0.025	Bal.

Table 2 Measured mechanical properties of 2195-T8 Al-Li alloy.

	Rolling direction	UTS (MPa)	YTS(MPa)	ε (%)
2195-T8	∥	577 ± 0.3	552 ± 0.3	6.4 ± 0.5
2195-T8	⊥	565 ± 0.3	538 ± 0.3	6.3 ± 0.5

Table 3 Welding parameter combinations of FSW.

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 4 Schemes of different heat treatment processes.

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

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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