Joint formation mechanism of high depth-to-width ratio friction stir welding

doi:10.1016/j.jmst.2019.01.016

Abstract

Abstract:

High depth-to-width ratio friction stir welding is an attractive method for the joining demands of aluminum profiles, which is sparked with its extremely low heat input and high mechanical performance. In this study, the joint formation mechanism was studied by a numerical model of plastic flow combined with experimental approaches. A fluid-solid-interaction algorithm was proposed to establish the coupling model, and the material to be welded was treated as non-Newtonian fluid. The thread structure and the milling facets on tool pin promoted drastic turbulence of material. The thread structure converged the plasticized material by its inclined plane, and then drove the attached material to refill the welds. The milling facets brought about the periodic dynamic material flow. The thread structure and the milling facets increased the strain rate greatly under the extremely low heat input, which avoided the welding defects. The condition of the peak temperature of 648 K and the strain rate of 151 s^-1 attributed to the lowest coarsening degree of precipitate. The tensile strength of the joint reached 265 MPa, equivalent to 86% of base material. The amelioration via the material flow model inhibits the welding defects and optimizes the parameter intervals, providing references to extracting process-structure-property linkages for friction stir welding.

Key words: Friction stir welding, Joint formation, Material flow, Al-Mg-Si alloy, Microstructure, Mechanical properties

Yongxian Huang, Yuming Xie, Xiangchen Meng, Junchen Li, Li Zhou. Joint formation mechanism of high depth-to-width ratio friction stir welding[J]. J. Mater. Sci. Technol., 2019, 35(7): 1261-1269.

Figures/Tables 15

Fig. 1. (a) Depth-to-width ratio data of welding tools (The green points and the green blocks were the depth-to-width ratio data in the published open literature) and (b) illustration of hollow aluminum profiles with a large thickness-to-width ratio.

Table 1 Chemical composition and mechanical properties of Al-Mg-Si alloy sheets.

Chemical composition (wt%)									Mechanical properties
Mg	Si	Cu	Fe	Mn	Zn	Cr	Ti	Al	Tensile strength (MPa)	Elongation (%)
1.10	0.81	0.23	0.75	0.15	0.31	0.07	0.19	Bal.	310	15.6

Fig. 2. Schematic diagram of high depth-to-width ratio FSW.

Fig. 3. Schematic view of high depth-to-width ratio FSW numerical model including solid structure and fluid domain.

Table 2 Material properties of H13 high speed steel.

Density (kg/m³)	Young’s modulus (GPa)	Poisson’s ratio	Specific heat [J/(kg K)]	Thermal expansion coefficient (10^-6 K^-1)	Thermal conductivity [W/(m K)]
7800	211	0.28	560	37	9.1
A (MPa)	B (MPa)	C	m	n	T0 (K)
908.5	321.4	0.28	1.18	0.278	300

Fig. 4. Characteristics of weld surface appearances by high depth-to-width ratio FSW (The pentagons were the trough thickness reduction points at different welding speeds).

Fig. 5. Macrostructure and microstructures of the typical joint: (a) joint macrostructure; microstructures of (b) “B”, (c) “C”, (d) “D”, (e) “E” and (f) “F” in Fig. 5(a); (g) EBSD pattern at advancing side.

Fig. 6. Distributions of equivalent strain rate, velocity and their comparison: (a) equivalent strain rate contour, (b) equivalent velocity contour and (c) comparison between simulation and experiment at advancing side.

Fig. 7. (a) Distributions of the 200 sampling points at the red line, (b) curves of equivalent strain rate, (c) curves of equivalent velocity and (d) curves of peak temperature at different welding speeds.

Fig. 8. (a) Illustration of plasticized material flow via high depth-to-width ratio FSW and (b) velocity distributions on three Cartesian axes.

Fig. 9. Comparisons of defect distributions between experiments and simulations: (a) and (b) 30 mm/min; (c) and (d) 100 mm/min; (e) and (f) 300 mm/min; (g) and (h) 400 mm/min; (i) and (j) 600 mm/min.

Fig. 10. Partly fracture surface morphologies of high depth-to-width ratio FSW: (a) 30 mm/min, (b) 300 mm/min, (c) 400 mm/min and (d) 600 mm/min.

Fig. 11. XRD pattern showing precipitations on fracture surfaces.

Fig. 12. Response surface of the average void diameter based on the temperature and the equivalent strain rate.

Fig. 13. Tensile properties of the high depth-to-width ratio FSW joints at different welding speeds.

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