Flat friction spot joining of aluminum alloy to carbon fiber reinforced polymer sheets: Experiment and simulation

doi:10.1016/j.jmst.2021.08.043

Abstract

Abstract:

The Al alloy and carbon fiber reinforced polymer (CFRP) hybrid structures, incorporating the performance advantages of the two materials, have been attracting more attention in high-end manufacturing fields. In the current investigation, the flat friction spot joining (FSJ) was employed in joining the AA6061-T6 alloy and CFRP sheets. The significance of temperature distribution in influencing joint quality was highlighted through analyzing interface microstructural features, weld defect formation as well as fractography. To understand the role of thermal energy generation and conduction in the process comprehensively, a 3D thermal-mechanical coupling finite element model was established. The interfacial temperature was characterized by an uneven distribution behavior due to the inhomogeneous heat distribution. The peak temperatures on the top surface and Al alloy to CFRP interface at 1500 rpm rotational speed with 0.1 mm/s plunging speed were 498 °C and 489 °C, respectively. The peak interface temperature was reduced to 286 °C at 250 rpm, which produced an extremely small melted area. Compared with the plunging speed, rotational speed was found to be the predominant parameter for determining the joint property, which could be optimized to simultaneously realize the avoidance of thermal decomposition of CFRP, the sufficient melting duration time, and the wide enough melted area. Simulated thermal histories and melted area profiles were in agreement with experimental ones. The findings could be utilized to provide some feasible guidance for process optimization of dissimilar FSJ of metals and composites.

Key words: Friction spot joining, Numerical simulation, CFRP/Al alloy, Interface microstructure, Thermal conduction

Peihao Geng, Ninshu Ma, Hong Ma, Yunwu Ma, Kazuki Murakami, Huihong Liu, Yasuhiro Aoki, Hidetoshi Fujii. Flat friction spot joining of aluminum alloy to carbon fiber reinforced polymer sheets: Experiment and simulation[J]. J. Mater. Sci. Technol., 2022, 107: 266-289.

Figures/Tables 34

Fig. 1. Schematic diagram of the friction-based filling stacking joining [12].

Fig. 2. Friction-based mechanical riveting process. (a) Friction stir blind riveting [15] and (b) Friction self-piercing riveting [17].

Fig. 3. Schematic diagram of the traditional friction stir spot welding [22].

Fig. 4. Schematic of the refill friction spot joining technique [29].

Fig. 5. Schematic diagram of the flat friction spot joining of Al alloy and CFRP sheets.

Table 1 Chemical composition of AA6061-T6 alloy.

Element (wt%)	Ti	Mg	Mn	Si	Fe	Cu	Zn	Cr	Al
AA6061	0.15	0.8-1.2	0.15	0.4-0.8	0.7	0.15-0.4	0.25	0.04-0.35	Bal.

Table 2 Weld conditions used in the experiment.

Welding condition	Selection
Workpiece material	AA6061-T6, CFRP
Tool material	SKD steel
Solidus temperature of Al workpiece ( °C)	582
Melting and decomposing temperature of resin matrix ( °C)	220 and 340
The tool should diameter (mm)	15
Rotational velocity (rpm)	1000-1500
Plunging depth (mm)	0.3,0.6
Plunge speed (mm/s)	0.1,0.2

Fig. 6. Geometrical dimension of tool and the lapping condition of the two sheets.

Fig. 7. Schematic diagram of lap tensile shear testing specimens.

Fig. 8. Finite element simulation model of the flat FSJ of Al alloy to CFRP.

Table 3 Thermo-physical and mechanical properties of AA6061 and CFRP.

Parameters	Materials	20	100	200	300	400	500	600
Thermal conductivity(W/m•K)	AA6061	195	200	206	208	206	199	195
Thermal conductivity(W/m•K)	CFRP	0.427	0.435	0.512	-	-	-	-
Specific heat(J/kg•K)	AA6061	900	950	1000	1020	1050	1090	1200
Specific heat(J/kg•K)	CFRP	1310	1950	2860	-	-	-	-
Young's modulus(GPa)	AA6061	65	58	51	42.5	30.3	12.6	3
Young's modulus(GPa)	CFRP	11.3	5.35	4.20	-	-	-	-

Table 4 Material constants for the Johnson-Cook constitutive model for AA6061-T6 alloy [46].

A (MPa)	B (MPa)	n	C	m	T_M ( °C)	T_S ( °C)
293.4	121.26	0.23	0.002	1.34	582	22

Fig. 9. Flowchart of the process modeling for the flat FSJ.

Fig. 10. Top surface morphology of the typical flat friction spot joints: (a) w = 1000 rpm, v = 0.1 mm/s, d = 0.3 mm, (b) w = 1250 rpm, v = 0.1 mm/s, d = 0.3 mm, (c) w = 1500 rpm, v = 0.1 mm/s, d = 0.3 mm and (d) w = 1500 rpm, v = 0.1 mm/s, d = 0.6 mm.

Fig. 11. Cross-sectional view of friction spot joined AA6061-T6 and CFRP at w = 1500 rpm, v = 0.1 mm/s, d = 0.3 mm.

Fig. 12. Cross-sectional morphology and microstructures of friction spot welded AA6061-T6 and CFRP joint at w = 1500 rpm, v = 0.1 mm/s, d = 0.3 mm.

Fig. 13. Lap tensile shear testing results of friction spot welded AA6061-T6 and CFRP joint.

Table 5 The average shear strength of joints at different conditions used in the study.

Welding parameters	w = 1000 rpm, v = 0.1 mm/s, d = 0.3 mm	w = 1250 rpm, v = 0.1 mm/s, d = 0.3 mm	w = 1500 rpm, v = 0.1 mm/s, d = 0.3 mm	w = 1500 rpm, v = 0.1 mm/s, d = 0.6 mm
Shear strength (MPa)	5.96	5.69	4.56	4.35

Fig. 14. Fractured surface morphology of friction spot welded AA6061-T6 and CFRP joint (a).

Fig. 15. Comparison between numerically predicted and experimentally measured thermal data: (a) thermal history at the top surface of AA6061-T6 sheet close to the tool edge, and (b) melted area profile at the surface of CFRP side.

Fig. 16. Comparison between numerical predicted and experimentally analyzed melted and decomposed area profiles at three welding parameters. (a) w = 1000 rpm, v = 0.1 mm/s, d = 0.3 mm, (b) w = 1500 rpm, v = 0.1 mm/s, d = 0.3 mm (c) w = 1500 rpm, v = 0.1 mm/s, d = 0.6 mm.

Table 6 Numerical predicted error of the melted and thermal decomposition areas.

Case	Diameter of melted area (mm)		Error (%)	Diameter of decomposition area (mm)		Error (%)
Case	Predict	Experiment	Error (%)	Predict	Experiment	Error (%)
w = 1000 rpm, v = 0.1 mm/s, d = 0.3 mm	12.8	11.2	14.3	8.6	8.5	1.2
w = 1500 rpm, v = 0.1 mm/s, d = 0.3 mm	13.7	12.7	7.9	8.8	8.9	1.1
w = 1500 rpm, v = 0.1 mm/s, d = 0.6 mm	17.0	15.8	7.6	10.1	10.8	6.9

Fig. 17. Transient heat flux distribution at the Al surface (a) and the weld surface of the CFRP side (b) at different heating times and cooling times. (w = 1500 rpm, v = 0.1 mm/s, d = 0.3 mm).

Fig. 18. (a) Vector field of thermal conduction inside the joint during the process and (b) Distribution of the heat flux on the CFRP surface (w = 1500 rpm, v = 0.1 mm/s, d = 0.3 mm).

Fig. 19. Dissipation of friction work and plastic work in process (a) evolution at w = 1500 rpm, v = 0.1 mm/s, d = 0.3 mm, and (b) effect of welding parameters on friction dissipation energy.

Fig. 20. Temperature field of friction spot joint of 6061 Al alloy and CFRP sheets at w = 1500 rpm, v = 0.1 mm/s, d = 0.3 mm: (a) top surface of Al alloy, (b) weld surface of CFRP, (c) cross-sectional view of joint.

Fig. 21. Evolution of peak temperature (a) and interfacial temperature at on the interface (b) in friction spot joint of 6061 Al alloy and CFRP sheets. (w = 1500 rpm, v = 0.1 mm/s, d = 0.3 mm).

Fig. 22. Effects of welding parameters on (a) the weld surface of CFRP and (b) the duration time of surface temperature higher than the melting or decomposition temperatures.

Fig. 23. Predicted melted area on the CFRP surface by the developed numerical model at different welding parameters: (a, c) varied rotation velocity, (b, d) varied plunging speed.

Fig. 24. Variation of the melted and decomposition area dimensions on the CFRP surface with welding parameters: (a) varied rotation velocity, (b) varied plunging speed.

Fig. 25. Schematic diagram of the optimization solution for the friction spot joining of Al alloy to CFRP.

Fig. 26. Fractured surface of the joints by confirmatory experiments and validated simulation results at (a) w = 500 rpm, v = 0.1 mm/s, d = 0.6 mm and (b) w = 250 rpm, v = 0.05 mm/s, d = 0.3 mm, and (c)-(d) tensile shear testing results.

Fig. 27. Schematic diagram of the optimization solution for the friction spot joining of Al alloy to CFRP

Fig.28. Fractured surface of the joints by confirmatory experiments and validated simulation results at (a) w=500 rpm, v=0.1mm/s, d=0.6 mm and (b) w=250 rpm,v=0.05 mm/s, d=0.3 mm, and (c, d) tensileshear testing results.

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