Microstructure and mechanical properties of double-side friction stir welded 6082Al ultra-thick plates

doi:10.1016/j.jmst.2019.10.005

Abstract

Abstract:

In the present work, 80 mm thick 6082Al alloy plates were successfully double-side welded by friction stir welding (FSW). The relationship between the microstructures and mechanical properties was built for the double-side FSW butt joint with more attention paid to the local characteristic zones. It was shown that a phenomenon of microstructural inhomogeneity existed in the nugget zone (NZ) through the thickness direction. The grain size presented an obvious gradient distribution from the top to the bottom for each single-pass weld, and the microhardness values decreased from both surfaces to the middle of the NZ. The lowest hardness zone (LHZ) exhibited a “hyperbolical”-shaped distribution extending to the middle of the NZ. Similar tensile properties were obtained in the three sliced specimens of the FSW joint, and the joint coefficient reached about 70% which achieved the same level as the conventional FSW Al alloy joints. Finite element modeling proved that the “hyperbolical”-shaped heat affected zone (HAZ) was beneficial to resisting the strain concentration in the middle layer specimen which helped to increase the tensile strength. Based on the analysis of the hardness contour map, tensile property and microstructural evolution of the joints, an Isothermal Softening Layer (ISL) model was proposed and established, which may have a helpful guidance for the optimization on the FSW of ultra-thick Al alloy plates.

Key words: Ultra-thick aluminum alloy plates, Double-side friction stir welding, Finite element model, Microstructural inhomogeneity, Mechanical properties

C. Yang, J.F. Zhang, G.N. Ma, L.H. Wu, X.M. Zhang, G.Z. He, P. Xue, D.R. Ni, B.L. Xiao, K.S. Wang, Z.Y. Ma. Microstructure and mechanical properties of double-side friction stir welded 6082Al ultra-thick plates[J]. J. Mater. Sci. Technol., 2020, 41: 105-116.

Figures/Tables 15

Table 1 Chemical composition of the 6082Al BM (wt%).

Mg	Si	Zn	Cu	Fe	Mn	Cr	Ti	Al
0.90	0.89	0.017	0.045	0.19	0.60	0.042	0.021	Bal.

Fig. 1. (a) Image of the welding tool, schematics of (b) sampling position and (c) sliced tensile specimen.

Fig. 2. Cross-sectional macrostructure of the double-side FSW joint (AS: advancing side; RS: retreating side).

Fig. 3. EBSD maps of different regions in NZ: (a) top region and (b) middle region in the second-pass NZ, (c) overlapped region, (d) middle region and (e) top region in the first-pass NZ.

Fig. 4. Misorientation angle distribution in different regions in NZ corresponding to Fig. 3: (a) top region and (b) middle region in the second-pass NZ, (c) overlapped region, (d) middle region and (e) top region in the first-pass NZ.

Fig. 5. Bright-field TEM micrographs of (a) BM, (b) top region and (c) middle region of the second-pass NZ, (d) overlapped regions. Fig. 5(b-d) is corresponding to rectangles I, II and III in Fig. 3.

Fig. 6. Microhardness profiles of the joint: (a) testing position sketch; (b) hardness profile along centre line through thickness direction; (c) hardness profile on the cross-section in the second-pass weld; (d) hardness profile on the cross-section in the first-pass weld.

Fig. 7. Hardness contour map of the cross-sectional double-side FSW joint.

Fig. 8. Diagrammatic feature of the proposed Isothermal Softening Layer model.

Fig. 9. (a) Tensile result histogram of the double-side FSW joint and BM; (b) optical micrographs showing the fracture position of the sliced tensile specimens.

Fig. 10. FEM model of (a) the second-pass weld and (c) middle layer specimens. The maximum principal strain field after 1.25% strain tensile loading was performed on (b) the second-pass weld and (d) the middle layer specimens. (e) The modeled stress-strain curves of the second-pass weld and middle layer specimens.

Fig. 11. (a) Metallographic image of the weld; (b) magnified metallographic image of the overlapped region; EBSD maps of (c) zone A, (d) zone B and (e) zone C in (b).

Fig. 12. Bright-field TEM micrographs of (a) zone A, (b) zone B and (c) zone C in Fig. 11(b).

Fig. 13. EDS spectrum under STEM mode of the precipitates in zone B.

Fig. 14. (a) Sampling position of the small size tensile specimens; (b) engineering stress?strain curves of different zones.

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