Influence of welding parameters on interface evolution and mechanical properties of FSW Al/Ti lap joints

doi:10.1016/j.jmst.2019.04.002

Abstract

Abstract:

Friction stir welding (FSW) was performed to produce Al/Ti lap joints under various welding conditions. More heat was generated when rotational rate increased or traversing rate decreased. Two types of Al/Ti interfaces - mixed interface and diffusive interface - were formed under different welding conditions. The diffusive interface was formed with low heat input, and the mixed interface was formed more heat. The grains at the mixed interface were larger than those at the diffusive interface because of the higher heat input. Moreover, the microstructure of the mixed interface had a lower texture intensity compared with that of the diffusive interface, which was attributed to the enhanced continuous dynamic recrystallization (CDRX). TiAl₃ was formed at the diffusive interface. When the interface varied to the mixed interface as heat input increased, TiAl was fomed within the Al/Ti mixture following the formation of TiAl₃. In addition, TiAl₃ precipitates were observed in the diffusion layer. The hardness value of the mixed interface was higher than 350 HV, due to the larger amount of intermetallic compounds (IMCs). The lap shear strength reached a maximum value of 147 MPa with medium heat input and an interface that exits in a critical state between diffusive and mixed interfaces. All the specimens fractured at the interface, which was attributed to the presence of IMCs.

Key words: FSW, Al/Ti lap joint, Interfacial characteristics, Mechanical properties

Mingrun Yu, Hongyun Zhao, Zhihua Jiang, Zili Zhang, Fei Xu, Li Zhou, Xiaoguo Song. Influence of welding parameters on interface evolution and mechanical properties of FSW Al/Ti lap joints[J]. J. Mater. Sci. Technol., 2019, 35(8): 1543-1554.

Figures/Tables 19

Fig. 1. Microstructure of the BMs: (a) AA6061, (b) Ti6Al4V.

Table 1 Chemical compositions of the base materials (wt%).

	Elements
	Al	Mg	Si	Mn	Ti	V	Fe.
AA6061	Bal.	1.61	0.50	0.15	-	-	-
Ti6Al4V	6.33	-	-	-	Bal.	4.27	0.20

Fig. 2. Schematic illustration of FSW Al/Ti lap joints.

Table 2 Detailed welding parameters for FSW Al/Ti lap joints.

Rotational rate (rpm)	Traversing rate (mm/min)	Tilting angle (deg.)
600	100	3
800
1000
1200
1400
1000	60
	80
	120
	140

Fig. 3. Spindle torques with various welding parameters: (a) v = 100 mm/min, (b) ω = 1000 rpm.

Fig. 4. Measured temperature with various welding parameters: (a) v = 100 mm/min, (b) ω = 1000 rpm, (c) variation of peak temperatures with ω2/v.

Fig. 5. Surface appearance and cross sections of the FSW Al/Ti lap joints at 100 mm/min.

Fig. 6. Surface appearance and cross sections of the FSW Al/Ti lap joints at 1000 rpm.

Fig. 7. Interface evolution with different welding parameters.

Fig. 8. EBSD results of diffusive interface.

Fig. 9. EBSD results of mixed interface.

Fig. 10. SEM micrographs and EDS maps of different interfaces.

Table 3 EDS results of the points shown in Fig.9 (at.%).

Points	Al	Ti	V	Mg	Possible phase
A	96.15	2.02	-	1.83	Al + TiAl₃
B	11.23	85.70	3.07	-	Ti BM
C	94.99	3.10	0.89	1.02	Al + TiAl₃
D	70.26	28.88	0.86	0.70	TiAl₃
E	51.81	46.57	1.35	0.27	TiAl

Fig. 11. TEM results of Al/Ti interfaces: (a) TEM image of the diffusion layer, (b) TEM image of the mixed interface, (c) SAED pattern of the TiAl3 in (a), (d) SAED pattern of the TiAl in (b).

Fig. 12. Hardness distribution of the FSW Al/Ti lap joints with various welding parameters: (a) v = 100 mm/min, (b) ω = 1000 rpm.

Fig. 13. Tensile test results of the Al/Ti lap joints obtained under different welding conditions: (a) variation of lap shear strength with rotational rate, (b) variation of lap shear strength with rotational rate, (c) variation of lap shear strength with ω2/v, (d) comparison with former studies.

Fig. 14. Fracture profiles of the Al/Ti joints: (a) diffusive interface, (b) mixed interface.

Fig. 15. SEM image and elements distributions of different interfaces: (a) diffusive interface, (b) mixed interface.

Fig. 16. XRD results of the fracture surfaces of different interfaces: (a) diffusive interface, (b) mixed interface.

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