Combined strengthening mechanism of solid-state bonding and mechanical interlocking in friction self-piercing riveted AA7075-T6 aluminum alloy joints

doi:10.1016/j.jmst.2021.07.026

Abstract

Abstract:

A recently developed friction self-piercing riveting (F-SPR) technique based on the combination of friction stir processing and riveting has been reported to possess both solid-state bonding and mechanical fastening characteristics. However, there is still a lack of quantitative understanding of the hybrid enhancement mechanism, hindering its engineering application. To fill in this gap, the current research investigated the microstructure evolution, microhardness distribution, and miniature-tensile performance of the aluminum alloy AA7075-T6 F-SPR joints by experiments. An accurate numerical simulation model was established to quantitatively evaluate the individual contributions of microstructure, local bonding strength, and macro interlocking to the performance of the joint, which could well explain the experimental results. It was found that due to the friction stirring of the rivet, solid-state bonding driven by dynamic recrystallization is realized between the trapped aluminum in the rivet cavity and the bottom aluminum sheet. The solid-state bonding zone has 75% yield strength, 81% ultimate tensile strength, and 106% elongation compared to the base material. This solid-state bonding enables the internal interlocking between the trapped aluminum and the rivet to withstand the additional load, which forms a novel dual-interlock fastening mechanism and increases the peak cross-tension force by 14.3% compared to the single-interlock joint.

Key words: Combined strengthening, Friction self-piercing riveting, Miniature tensile test, Solid-state joining, Dual-interlocking, Numerical simulation

Yunwu Ma, Bingxin Yang, Shanqing Hu, He Shan, Peihao Geng, Yongbing Li, Ninshu Ma. Combined strengthening mechanism of solid-state bonding and mechanical interlocking in friction self-piercing riveted AA7075-T6 aluminum alloy joints[J]. J. Mater. Sci. Technol., 2022, 105: 109-121.

Figures/Tables 21

Fig. 1. Friction self-piercing riveting (F-SPR) process

Fig. 2. Rivet and die. (a,?b) Physical images; (c) rivet dimensions; and (d) die dimensions.

Table 1. F-SPR process parameter combinations.

Process number	Stage-I		Stage-II		D_switch (mm)	D_plunge (mm)	Δt (s)
	f₁ (mm/s)	ω₁ (rpm)	f₂ (mm/s)	ω₂ (rpm)
#1	2.0	3600	11	0	2.5	5.3	1.50
#2	2.0	3600	11	0	3.0	5.3	1.71
#3	2.0	3600	11	0	4.0	5.3	2.12
#4	2.0	3600	11	0	5.3	5.3	2.65

Fig. 3. Miniature tensile test. (a) Extraction location of the miniature tensile bar; (b) dimensions of the miniature tensile bar; and (c) testing fixture with a miniature sample.

Fig. 4. Dimensions of the cross-tension sample (unit: mm).

Fig. 5. Macro cross-section profiles of the F-SPR joints under different switch depths. (a) 2.5 mm; (b) 3.0 mm; (c) 4.0 mm; and (d) 5.3 mm.

Fig. 6. F-SPR joint indexes under different switch depths. (a) The calculated heat input; (b) the measured peak riveting force; and (c) the measured interlock amounts.

Fig. 7. Microstructures of the F-SPR joint with 3.0 mm switch depth. (a) Micro profile of the etched joint; (b)-(d) correspond to locations b-d in Fig. 7(a). The FGZ refers to “fine grain zone”, and the CGZ refers to “coarse grain zone”.

Fig. 8. EBSD images of (a) base material, (b) the CGZ in Fig. 7(b), (c) the FGZ; (d) zoom-in region of the black box in (c); (e)-(g) fractions of misorientation angles in (a)-(c), respectively. RD, TD, and ND represent the rolling, transverse, and normal directions, respectively.

Fig. 9. Vickers microhardness distribution of the F-SPR joints under different switch depths. (a) 2.5 mm; (b) 3.0 mm; (c) 4.0 mm; and (d) 5.3 mm. The dimensions denote the width of the softening zones.

Fig. 10. True stress-true stain curves of solid-state bonding zone and base materials.

Fig. 11. Strength and elongation of solid-state bonding zone and base material.

Fig. 12. Microhardness distribution on the miniature tensile test samples of different F-SPR switch depths. (a) 2.5 mm; (b) 3.0 mm; (c) 4.0 mm; and (d) 5.3 mm.

Fig. 13. Comparison of peak cross-tension load and energy absorption of F-SPR joints with different switch depths.

Fig. 14. (a) Typical cross-tension load-displacement curves; and (b) macro profiles of the joint with 3.0 mm switch depth at the displacements marked with i-iv in (a).

Fig. 15. Finite element model for the cross-tension test of an F-SPR joint

Table 2. Average hardness values and hardening parameters of different zones.

Region	Average hardness	Hardness ratio	Swift law constants
			K (MPa)	ε₀	n
AA7075-T6 BM(Zones A & B)	169	1.0	835.7	0.015	0.12
Zone C	154	0.91	760.5
Zone D	161	0.95	793.9
Zone E	151	0.89	743.8
Zone F	159	0.94	785.6
35CrMo (Rivet)	470	-	3432.5	0.0026	0.14

Fig. 16. Case study for the cross-tension modeling of F-SPR joints.

Fig. 17. The simulated cross-tension load-displacement curves of Cases1-3 in comparison with the experimental curve.

Fig. 18. Joint geometry comparison and the simulated Von-Mises stress distribution. (a) Experimental cross-section profiles of the joint obtained by interrupting the cross-tension tests; (b) Case-1; (c) Case-2; and (d) Case-3. Subfigures ii-iv correspond to the loading stages marked with ii-iv in Fig. 17.

Fig. 19. Comparison of the contact forces in cross-tension tests. (a) Location of the three interfaces; (b) Case-1; (c) Case-2; and (d) Case-3.

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