Correlation between microstructures and mechanical properties of high-speed friction stir welded aluminum hollow extrusions subjected to axial forces

doi:10.1016/j.jmst.2017.11.015

Abstract

Abstract:

The AA6005A-T6 aluminum hollow extrusions were friction stir welded at a high welding speed of 2000 mm/min and various axial forces. The results show that the nugget zone (NZ) is characterized by fine equiaxed grains, in which a low density of equilibrium phase β is observed. The grains in the thermo-mechanically affected zone (TMAZ) are elongated, and the highest density of dislocations and a low density of β' precipitates can be found in grains. The heat affected zone (HAZ) only experiences a low thermal cycle, and a high density of β” precipitates and a low density of β' precipitates remain in the coarsened grains. The microhardness evolutions in the NZ, TMAZ and HAZ are governed by the grain refinement and dislocation strengthening, the dislocation and precipitation strengthening, and the precipitation and solid solution strengthening, respectively. When increasing the axial force, the changing trend of one strengthening mechanism is contrary to the other in each zone, and the microhardness increases in different zones. As a result, the tensile strength roughly increases with raising the axial force, and all joints show good tensile properties as the high welding speed inhibits the coarsening and dissolution of strengthening precipitates significantly.

Key words: Aluminum hollow extrusions, High-speed friction stir welding, Microstructures, Mechanical properties

Xiangqian Liu, Huijie Liu, Tianhao Wang, Xiangguo Wang, Si Yang. Correlation between microstructures and mechanical properties of high-speed friction stir welded aluminum hollow extrusions subjected to axial forces[J]. J. Mater. Sci. Technol., 2018, 34(1): 102-111.

Figures/Tables 19

Table 1 Chemical compositions and mechanical properties of AA6005A-T6 aluminum hollow extrusions.

Chemical composition (wt%)									Mechanical properties
Si	Mg	Mn	Fe	Cu	Cr	Zn	Ti	Al	Tensile strength	Elongation	Vickers hardness
0.7	0.6	0.5	0.35	0.3	0.3	0.2	0.1	Bal	294 MPa	6.2%	90-95 Hv

Fig. 1. High-speed friction stir welding process: (a) schematic view, (b) experimental set up.

Table 2 Dimensions of tool and welding parameters used in the experiments.

Dimensions of tool			Welding parameters
Shoulder diameter	Pin diameter	Pin length	Rotation speed	Welding speed	Axial force	Tilt angle
10 mm	7.4 mm	3.8 mm	2000 r/min	2000 mm/min	15-21 kN	1.5°

Fig. 2. Macrostructures of the HSFSW joints produced using different axial forces: (a) 15 kN, (b) 17 kN, (c) 19 kN, (d) 21 kN.

Fig. 3. Widths of NZ at different depths and depth of NZ of the welds welded using various axial forces.

Fig. 4. Optical pictures showing grain structures in different zones of the HSFSW joints: (a) BM, (b) HAZ, (c) TMAZ on the RS, (d) TMAZ on the AS.

Fig. 5. TEM picture showing fine recrystallized grain in the NZ.

Fig. 6. Welding thermal cycles in different zones of the HSFSW joints.

Fig. 7. Optical pictures showing grain structures in the NZ obtained at different axial forces: (a) 15 kN, (b) 17 kN, (c) 19 kN, (d) 21 kN.

Fig. 8. Evolutions of the average grain size with the axial force in the NZ.

Fig. 9. TEM microstructures in the BM: (a) dislocations, (b) precipitates observed in low magnification, (c) precipitates observed in high magnification, (d) precipitate-free zone (PFZ) along the grain boundary.

Table 3 Peak temperature and dwelling time at high temperature in different zones obtained using various axial forces.

P(kN)	T_P (°C)			t_D (s)
	NZ	TMAZ	HAZ	NZ	TMAZ	HAZ
15	434	370	305	2.9	2.6	2.2
17	473	401	332	3.8	3.5	3.0
19	500	423	351	4.5	4.2	3.6
21	522	441	367	5.2	4.9	4.2

Fig. 10. Precipitates and PFZs observed in the HAZ produced at different axial forces: (a, b, c) 15 kN, (d, e, f) 21 kN.

Fig. 11. Distribution characteristics of dislocations and precipitates in the TMAZ fabricated using different axial forces: (a, b, c) 15 kN, (d, e, f) 21 kN.

Fig. 12. Distribution characteristics of dislocations and precipitates in the NZ obtained at different axial forces: (a, b) 15 kN, (c, d) 21 kN.

Fig. 13. Effect of axial force on the microhardness distributions of the HSFSW joints.

Fig. 14. Effect of axial force on the tensile properties of the HSFSW joints.

Fig. 15. Fracture locations of the HSFSW joints welded at different axial forces: (a) 15 kN, (b) 17 kN, (c) 19 kN, (d) 21 kN.

Fig. 16. Fracture surfaces of the HSFSW joints obtained at different axial forces: (a) 15 kN, (b) 17 kN, (c) 19 kN, (d) 21 kN.

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