Improving Joint Morphologies and Tensile Strength of Al/Mg Dissimilar Alloys Friction Stir Lap Welding by Changing Zn Interlayer Thickness

doi:10.1007/s40195-019-00937-9

Abstract

Abstract:

The pure Zn foils with different thicknesses (0.02, 0.05, 0.1, 0.2 and 0.3 mm) were selected as interlayers to improve the quality of friction stir lap welding joint of 7075-T6 Al and AZ31B Mg dissimilar alloys. The effects of the interlayer thickness on joint formation, microstructure and tensile strength were analyzed. The results displayed that the maximum length of the boundary between stir zone (SZ) and thermo-mechanically affected zone in lower plate was obtained by the addition of the Zn interlayer with 0.05 mm thickness. The Mg-Zn intermetallic compounds (IMCs) were discontinuously distributed in the SZ, replacing the continuous Al-Mg IMCs. The size of Mg-Zn IMCs increased with the increase in the thickness of the Zn interlayer. The maximum tensile shear strength of 276 N mm^-1 was obtained by the addition of 0.05 mm Zn foil, which increased by 45.6% of that of the joint without the Zn foil addition.

Key words: Al/Mg dissimilar alloys, Friction stir lap welding, Zn foil thickness, Microstructure, Tensile shear strength

Jinglin Liu, Shiyu Niu, Rong Ren, Shude Ji, Lei Wang, Zan Lv. Improving Joint Morphologies and Tensile Strength of Al/Mg Dissimilar Alloys Friction Stir Lap Welding by Changing Zn Interlayer Thickness[J]. Acta Metallurgica Sinica (English Letters), 2019, 32(11): 1385-1395.

Figures/Tables 16

Table 1 Chemical compositions of 7075-T6 Al and AZ31B Mg alloys (mass%)

Material	Cu	Fe	Si	Mg	Mn	Zn	Cr	Ni	Al
7075-T6	1.64	0.24	0.42	2.49	0.3	5.72	0.21	-	Bal.
AZ31B	0.0015	0.0027	0.12	Bal.	0.48	0.88	-	0.0005	3.1

Fig. 1 Schematic of the Zn-added FSLW process

Fig. 2 Cross sections of the joints: a the conventional joint, b 0.02 mm, c 0.05 mm, d 0.1 mm, e 0.2 mm, f 0.3 mm Zn-added joints

Fig. 3 ELWs and the boundary lengths between SZ and TMAZ in lower plate under different processes 3.2 Microstructure The microstructure also significantly affects the joint quality [24]. For further investigate, the effects of Zn foil thickness on the tensile property of the joint, the microstructure is analyzed based on the SEM, EDS and XRD results in Figs. 4, 5, 6, 7, 8 and 9.

Fig. 4 SEM images of the regions at the conventional joint marked in Fig. 2a: a region 1, b region 2, c region 3

Fig. 5 SEM images of the regions at the 0.02 and 0.05 mm Zn-added joints: a, b regions 4 and 5 marked in Fig. 2b, c, d regions 6 and 7 marked in Fig. 2c

Fig. 6 EDS results: a-c regions A-C marked in Fig. 2, d-f points a-c marked in a-c

Fig. 7 Enlarged views of the regions at the 0.1, 0.2 and 0.3 mm Zn-added joints: a, b regions 8 and 9 marked in Fig. 2d, c, d regions 10 and 11 marked in Fig. 2e, e, f regions 12 and 13 marked in Fig. 2f

Fig. 8 EDS results of the regions marked in Fig. 2: a, b regions D and E

Fig. 9 XRD patterns on the cross sections: a the conventional joint, b the 0.05 mm Zn-added joint

Table 2 EDS results of points 1-5 marked in Fig. 2

Point	Composition (at.%)				Possible phase
Point	Mg	Al	Zn	O	Possible phase
1	93.3	3.2	0.9	2.6	Mg substrate
2	55.3	42.1	0.8	1.8	Al₁₂Mg₁₇
3	37.2	60.3	0.8	1.7	Al₃Mg₂
4	50.5	40.1	7.7	1.7	Al-Mg-Zn?+?Al₁₂Mg₁₇
5	47.1	24.9	25.7	2.3	Mg-Zn?+?Al-Mg-Zn

Table 3 EDS results of points 1-3 marked in Fig. 7

Point	Composition (at.%)				Possible phase
Point	Mg	Al	Zn	O	Possible phase
1	47.3	19.6	30.6	2.5	Mg-Zn?+?Al-Mg-Zn
2	60.1	9.7	28.2	2.0	Mg-Zn
3	53.9	6.4	34.4	2.3	Mg-Zn

Fig. 10 Tensile shear strength of joints under different welding processes

Fig. 11 Fracture locations: a the conventional joint, b 0.02 mm, c 0.05 mm, d 0.1 mm, e 0.2 mm, f 0.3 mm Zn-added joints

Fig. 12 Fracture surface morphologies of the regions marked in Fig. 11: a region a, b region b, c region c

Fig. 13 XRD patterns on the fracture surfaces: a the conventional joint, b the 0.05 mm Zn-added joint

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