Analysis of anisotropy mechanism in the mechanical property of titanium alloy tube formed through hot flow forming

doi:10.1016/j.jmst.2021.01.038

Abstract

Abstract:

Anisotropy of mechanical property is an important feature influencing the service performance of titanium (Ti) alloy tube component. In this work, it is found that the hot flow formed Ti alloy tube exhibits higher yield strength along circumferential direction (CD), and larger elongation along rolling direction (RD), presenting significant anisotropy. Subsequently, the quantitative characteristics and underlying mechanism of the property anisotropy were revealed by analyzing the slip, damage and fracture behavior under the combined effects of the spun {0002} basal texture and fibrous microstructure for different loading directions. The results showed that the prismatic slip in primary α grain is the dominant deformation mechanism for both loading directions at the yielding stage. The prismatic slip is harder under CD loading, which makes CD loading present higher yield strength than RD loading. Additionally, the yield anisotropy can be quantified through the inverse ratio of the averaged Schmid Factor of the activated prismatic slip under different loading directions. As for the plasticity anisotropy, the harder and slower slip development under CD loading causes that the CD loading presents larger external force and normal stress on slip plane, thus leading to more significant cleavage fracture than RD loading. Moreover, the micro-crack path under RD loading is more tortuous than CD loading because the fibrous microstructure is elongated along RD, which may suppress the macro fracture under RD loading. These results suggest that weakening the texture and fibrous morphology of microstructure is critical to reduce the differences in slip, damage and fracture behavior along different directions, alleviate the property anisotropy and optimize the service performance of Ti alloy tube formed by hot flow forming.

Key words: Titanium alloy tube, Hot flow forming, Mechanical property anisotropy, Slip behavior, Damage evolution

Zhenni Lei, Pengfei Gao, Xianxian Wang, Mei Zhan, Hongwei Li. Analysis of anisotropy mechanism in the mechanical property of titanium alloy tube formed through hot flow forming[J]. J. Mater. Sci. Technol., 2021, 86: 77-90.

Figures/Tables 23

Fig. 1. Flow forming of Ti alloy tube: (a) schematic of multi-pass backward flow forming; (b) formed tube component.

Table 1 Main forming parameters in flow forming process.

Forming parameters	Values
Roller feed rate (mm/r)	1
Mandrel rotation speed (r/min)	89
Attacking angle of the roller (°)	22.5
Exit angle of the roller (°)	25
Diameter of the roller (mm)	260
Nose radius of the roller (mm)	8
Forming pass	4
Reduced thickness of each forming pass (mm)	3

Fig. 2. The morphology and EBSD results of the microstructures of initial billet (a) and the flow formed component (b): (a1, b1) microstructure morphologies; (a2, b2) inverse pole figures (left) and pole figures (right) of α phase.

Fig. 3. Schematic for extracting and mechanically machining tensile specimen from the formed tube: (a) relative locations between the extracted tensile specimens and the formed tube component; (b) the dimensions of tensile specimen (unit: mm).

Fig. 4. Initial microstructure morphology and the corresponding EBSD results of the observation regions in the quasi-in-situ tensile specimens under RD (a) and CD (b) loading: (a1, b1) microstructure morphologies; (a2, b2) inverse pole figures (left) and pole figures (right) of α phase.

Fig. 5. Schematic of slip system identification.

Fig. 6. Tensile stress vs. strain curves under RD and CD loading.

Table 2 Paused strains during the quasi-in-situ tensile test.

Paused point	Paused engineering strain under RD loading (%)	Paused engineering strain under CD loading (%)
Yielding	1.2	1.2
Hardening	3.0	3.0
Necking	11.0	6.5
Fracture	13.5	8.5

Fig. 7. Slip morphologies at different strains under RD loading: (a) 1.2 %; (b) 3.0 %; (c) 11.0 %; (d) 13.5 %.

Table 3 The identified slip systems and the corresponding Schmid Factors under RD loading.

Strain (%)	No.	Activated slip system	Slip plane and direction	Schmid Factor
1.2	1	prism<a>	(10-10)[-12-10]	0.45
	2	prism<a>	(01-10)[2-1-10]	0.47
	3	prism<a>	(1-100)[-11-20]	0.42
	4	prism<a>	(10-10)[-12-10]	0.45
	5	prism<a>	(10-10)[-12-10]	0.49
	6	prism<a>	(10-10)[-12-10]	0.31
	7	prism<a>	(01-10)[2-1-10]	0.47
	8	prism<a>	(10-10)[-12-10]	0.49
	9	basal<a>	(0002)[-12-10]	0.42
	10	prism<a>	(10-10)[-12-10]	0.49
	11	prism<a>	(01-10)[2-1-10]	0.48
	12	prism<a>	(01-10)[2-1-10]	0.42
3.0	1	prism<a>	(10-10)[-12-10]	0.50
	2	prism<a>	(10-10)[-12-10]	0.44
	3	prism<a>	(01-10)[2-1-10]	0.40
	4	prism<a>	(01-10)[2-1-10]	0.38
	5	basal <a>	(0002)[-12-10]	0.01
	6	prism<a>	(10-10)[-12-10]	0.43
	7	prism<a>	(1-100)[-11-20]	0.46
	8	prism<a>	(01-10)[2-1-10]	0.44
	9	prism<a>	(10-10)[-12-10]	0.41
	10	pyram<a>	(10-11)[-12-10]	0.32
	11	pyram<a>	(01-11)[2-1-10]	0.37
	12	basal <a>	(0002)[-12-10]	0.22
	13	prism<a>	(01-10)[2-1-10]	0.44
	14	pyram<a>	(-1011)[-12-10]	0.46
	15	pyram<a+c>	(-1101)[-1213]	0.50
	16	prism<a>	(01-10)[2-1-10]	0.39

Fig. 8. Slip morphology at different strains under CD loading: (a) 1.2 %; (b) 3.0 %; (c) 6.5 %; (d) 8.5 %.

Table 4 The identified slip systems and the corresponding Schmid Factors under CD loading.

Strain (%)	No.	Activated slip system	Slip plane and direction	Schmid Factor
1.2	1	prism<a>	(10-10)[-12-10]	0.44
	2	prism<a>	(1-100)[11-20]	0.45
	3	basal <a>	(0002)[-12-10]	0.38
	4	prism<a>	(01-10)[2-1-10]	0.39
	5	prism<a>	(1-100)[11-20]	0.40
	6	prism<a>	(01-10)[2-1-10]	0.33
	7	basal <a>	(0002)[-12-10]	0.38
	8	prism<a>	(10-10)[-12-10]	0.47
	9	prism<a>	(1-100)[11-20]	0.34
	10	prism<a>	(1-100)[11-20]	0.35
3.0	1	prism<a>	(01-10)[2-1-10]	0.47
	2	prism<a>	(10-10)[-12-10]	0.44
	3	pyram<a>	(1-101)[11-20]	0.49
	4	basal <a>	(0002)[-12-10]	0.27
	5	prism<a>	(01-10)[2-1-10]	0.47
	6	prism<a>	(01-10)[2-1-10]	0.45
	7	prism<a>	(01-10)[2-1-10]	0.48
	8	pyram<a>	(-1011)[-12-10]	0.36
	9	basal <a>	(0002)[11-20]	0.39
	10	prism<a>	(10-10)[-12-10]	0.44
	11	prism<a>	(1-100)[11-20]	0.34
	12	prism<a>	(10-10)[-12-10]	0.46
	13	basal <a>	(0002)[2-1-10]	0.47
	14	prism<a>	(01-10)[2-1-10]	0.38
	15	prism<a>	(1-100)[11-20]	0.37
	16	prism<a>	(1-100)[11-20]	0.42

Fig. 9. An example of cross-slip occurring in αp under CD loading: (a) morphology of cross-slip at strain of 3.0 % (hardening); (b) EBSD result of (a); (c) morphology of cross-slip at strain of 6.5 % (necking).

Fig. 10. Fraction of αp grains where slip is activated at different deformation stages under different loading directions.

Fig. 11. Damage distribution under RD loading: (a) void distribution in the interior of specimen; (b) microcrack distribution on the surface of specimen.

Fig. 12. Damage distribution under CD loading: (a) void distribution in the interior of specimen; (b) microcrack distribution on the surface of specimen.

Fig. 13. Fractography under RD loading: (a) overall morphology of fracture surface; (b) magnified morphology of center region; (c) detailed morphology of fracture surface; (d1, d2) snake sliding patterns on fracture surface.

Fig. 14. Fractography under CD loading: (a) overall morphology of fracture surface; (b) magnified morphology of center region; (c) detailed morphology of fracture surface.

Fig. 15. Fraction distribution of SF values of prismatic slip in αp grains under different loading directions.

Fig. 16. Schematic of the dominant effects of the developing trend of resultant stress on slip plane for the slip separation and cleavage separation.

Fig. 17. Micro-crack path under different loading directions: (a) RD; (b) CD.

Fig. 18. Schematic of the mesoscopic deformation and damage evolution under different loading directions: (a) RD; (b) CD.

Fig. 19. Analysis results of the deformation texture and recrystallization texture of the spun microstructure shown in Fig. 2(b2): (a) distribution of the recrystallized α grains (green grains); (b) pole figures of αp grains; (c) pole figures of the recrystallized α grains.

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