Microstructure and mechanical optimization of probeless friction stir spot welded joint of an Al-Li alloy

doi:10.1016/j.jmst.2018.03.009

Abstract

Abstract:

In this work, a third generation Al-Li alloy has been successfully spot welded with probeless friction stir spot welding (P-FSSW), which is a variant of conventional friction stir welding. The Box-Behnken experimental design in response surface methodology (RSM) was applied to optimize the P-FSSW parameters to attain maximum tensile/shear strength of the spot joints. Results show that an optimal failure load of 7.83 kN was obtained under a dwell time of 7.2 s, rotation speed of 950 rpm and plunge rate of 30 mm/min. Sufficient dwell time is essential for heat conduction, material flow and expansion of the stir zone to form a sound joint. Two fracture modes were observed, which were significantly affected by hook defect. In addition to mechanical testing, electron backscattering diffraction (EBSD) and differential scanning calorimetry (DSC) were used for microstructure evolution and property analysis. The precipitation of GP zone and Al₃Li as well as the ultrafine grains were responsible for the high microhardness in the stir zone.

Key words: Probeless friction stir spot welding, Precipitation, Mechanical property, Fracture mechanism

Q. Chu, W.Y. Li, X.W. Yang, J.J. Shen, A. Vairis, W.Y. Feng, W.B. Wang. Microstructure and mechanical optimization of probeless friction stir spot welded joint of an Al-Li alloy[J]. J. Mater. Sci. Technol., 2018, 34(10): 1739-1746.

Figures/Tables 12

Fig 1. Schematic illustration of the P-FSSW process: (a) plunge stage, (b) dwell stage and (c) retract stage.

Fig. 2. Schematic illustration of rotation tool.

Table 1 Welding parameters and their levels.

Parameter	Notation	Level
Parameter	Notation	Low (-1)	Middle (0)	High (1)
Dwell time (s)	DT	3	6	9
Plunge rate (mm/min)	PR	10	30	50
Rotation speed (rpm)	RS	750	965 (950)	1180

Table 2 Box-Behnken design matrix.

No.	RS (rpm)	DT (s)	PR (mm/min)
1	1180	9	30
2	1180	3	30
3	750	9	30
4	750	3	30
5	1180	6	50
6	1180	6	10
7	750	6	50
8	750	6	10
9	965	9	50
10	965	9	10
11	965	3	50
12	965	3	10
13	965	6	30
14	965	6	30
15	965	6	30
16	965	6	30
17	965	6	30

Fig. 3. (a) OM macrograph of cross section of P-FSSWed joint (950 rpm/6 s), the microstructure of (b) HAZ and (c) SZ and (d) TMAZ.

Fig. 4. Dependency of hook defect morphology on welding parameters: (a) 950 rpm/3 s, (b) 950 rpm/6 s, (c) 950 rpm/12 s and (d) 1180 rpm/12 s.

Fig. 5. Changes of stir zone depth, effective sheet thickness and tensile/shear strength with welding parameters.

Fig. 6. DSC results for samples from (a) SZ and (b) HAZ for the as-welded condition.

Fig. 7. (a) Cross section of the P-FSSWed joint, and (b) hardness distribution corresponding to the area outlined in red in Fig. 7(a).

Table 3 ANOVA test results.

Source	Sum of squares	df	Mean square	F value	p-value Prob > F
Model	100.37	9	11.15	11.85	0.0018
RS	1.42	1	1.42	1.51	0.2594
DT	40.79	1	40.79	43.35	0.0003
PR	4.805E-004	1	4.805E-004	5.106E-004	0.9826
RS-DT	0.27	1	0.27	0.28	0.6106
RS-PR	2.64	1	2.64	2.80	0.1381
DT-PR	0.88	1	0.88	0.93	0.3660
RS²	8.73	1	8.73	9.27	0.0187
DT²	36.04	1	36.04	38.30	0.0005
PR²	5.08	1	5.08	5.40	0.0531
Residual	6.59	7	0.94
Lack of Fit	6.50	3	2.17	96.46	0.0003
Pure Error	0.090	4	0.022
Cor Total	106.96	16

Fig. 8. Effect of welding parameters on tensile/shear strength for (a) plunge rate of 30 mm/min, (b) rotation speed of 950 rpm and (c) dwell time of 6 s.

Fig. 9. Fractographs of the spot weld with different facture modes in bottom sheets: (a) shear fracture (mode I), (b) plug fracture (mode II), (c) and (d) magnification of regions C and D in Fig. 8(a), and (e) and (f) magnification of regions E and F in Fig. 8(b).

References

[1]	H.Y. Li, D.S. Huang, W. Kang, J.J. Liu, Y.X. Ou, D.W. Li, J. Mater. Sci. Technol. 32(2016) 1049-1053.
[2]	Y. Tao, D.R. Ni, B.L. Xiao, Z.Y. Ma, W. Wu, R.X. Zhang, Y.S. Zeng, Mater. Sci. Eng.A 693 (2017) 1-13.
[3]	Q. Chu, X.W. Yang, W.Y. Li, Y.B. Li, Sci. Technol. Weld. Join. 21(2016)164-170.
[4]	F.F. Wang, W.Y. Li, J. Shen, S.Y. Hu, J. dos Santos, Mater.Des. 86(2015)933-940.
[5]	B. Decreus, A. Deschamps, F. De Geuser, P. Donnadieu, C. Sigli, M. Weyland,Acta Mater. 61(2013) 2207-2218.
[6]	H.J. Liu, Y.Y. Hu, C. Dou, D.P. Sekulic, Mater. Charact. 123(2017) 9-19.
[7]	Y. Yang, L.L. Zhou, J. Mater. Sci. Technol. 30(2014) 1251-1254.
[8]	R.Z. Xu, D.R. Ni, Q. Yang, C.Z. Liu, Z.Y. Ma, J. Mater. Sci. Technol. 32(2016)76-88.
[9]	Q. Chu, W.Y. Li, X.W. Yang, J.J. Shen, Y.B. Li, W.B. Wang, Weld. World 61 (2017)291-298.
[10]	Y.H. Yin, N. Sun, T.H. North, S.S. Hu, J. Mater. Process. Technol. 210(2010)2062-2070.
[11]	T. Rosendo, B. Parra, M.A.D. Tier, A.A.M.D. Silva, J.F.D. Santos, T.R. Strohaecker,Mater. Des. 32(2011) 1094-1100.
[12]	J.Y. Cao, M. Wang, L. Kong, L.J. Guo, J. Mater. Process. Technol. 230(2016)254-262.
[13]	R.Z. Xu, D.R. Ni, Q. Yang, C.Z. Liu, Z.Y. Ma, Sci. Technol. Weld. Join. (2016)1-8.
[14]	W.Y. Li, J.F. Li, Z.H. Zhang, D.L. Gao, W.B. Wang, C.L. Dong, Mater. Des. 62(2014) 247-254.
[15]	D. Bakavos, Y. Chen, L. Babout, P. Prangnell, Metall. Mater. Trans. A 42 (2011)1266-1282.
[16]	S.D. Ji, X.C. Meng, L. Ma, S.S. Gao, Int. J. Adv. Manuf. Techol. 87(2016)1-8.
[17]	Z.L. Liu, H.T. Cui, S.D. Ji, M.Q. Xu, X.C. Meng, J. Mater. Sci. Technol. 32(2016)1372-1377.
[18]	L.C. Campanelli, U.F.H. Suhuddin, A.í.S. Antonialli, J.F.D. Santos, N.G.D. Alcantara, C. Bolfarini, J. Mater. Process. Technol. 213(2013) 515-521.
[19]	H. Chen, L. Fu, P. Liang, F. Liu, Mater. Charact. 125(2017) 160-173.
[20]	B. Cai, Z. Zheng, D. He, S. Li, H. Li, J. Alloys Compd. 649(2015) 19-27.
[21]	H. Sidhar, R.S. Mishra, Mater. Des. 110(2016) 60-71.
[22]	B. Decreus, A. Deschamps, F.D. Geuser, P. Donnadieu, C. Sigli, M. Weyland,Acta Mater. 61(2013) 2207-2218.
[23]	C. Gao, Z.X. Zhu, J. Han, H.J. Li, Mater. Sci. Eng. A 639 (2015)489-499.
[24]	H. Sidhar, N.Y. Martinez, R.S. Mishra, J. Silvanus, Mater. Des. 106(2016)146-152.
[25]	A. Deschamps, M. Garcia, J. Chevy, B. Davo, F. De Geuser, Acta Mater. 122(2017) 32-46.
[26]	Z.X. Zhu, J. Han, C. Gao, M. Liu, J.W. Song, Z.W. Wang, H.J. Li, Mater. Sci. Eng. A681(2017) 65-73.
[27]	Y. Lin, Z. Zheng, Mater. Charact. 123(2017) 307-314.
[28]	I. Nikulin, S. Malopheyev, A. Kipelova, R. Kaibyshev, Mater. Lett. 66(2012)311-313.
[29]	B. Malard, F. De Geuser, A. Deschamps, Acta Mater. 101(2015) 90-100.
[30]	J.H. Cho, H.H. Sang, G.L. Chang, Mater. Lett. 180(2016) 157-161.
[31]	Z.W. Xu, Z.W. Li, S.D. Ji, L.G. Zhang, J. Mater. Sci. Technol. 34(2018)878-885.
[32]	Y. Tozakii, Y. Uematsu, K. Tokaji, J. Mater. Process. Technol. 210(2010)844-851.
[33]	W. Yuan, B. Carlson, R. Verma, R. Szymanski, Sci. Technol. Weld. Join. 17(2012) 375-380.
[34]	C. Gao, R.Q. Gao, Y. Ma, Mater. Des. 83(2015) 719-727.