Effect of thermal induced porosity on high-cycle fatigue and very high-cycle fatigue behaviors of hot-isostatic-pressed Ti-6Al-4V powder components

doi:10.1016/j.jmst.2021.04.066

Abstract

Abstract:

The present work reports the effect of thermal induced porosity (TIP) on the high-cycle fatigue (HCF) and very high-cycle fatigue (VHCF) behaviors of hot-isostatic-pressed (HIPed) Ti-6Al-4V alloy from gas-atomized powder. The results show that the residual pores in the as-HIPed powder compacts present no obvious effect on the HCF life. The regrowth of the residual pores can be observed after solution heat treatment. The pore location ranks the most harmful for the fatigue life compared with the other initiating defects. The maximum stress intensity factors were calculated. The plastic zone size of fine granular area (FGA) is much less than the characteristic size of the microstructure, and the crucial size of the internal pores in this study is about 40 μm. The failure types of fatigue specimens in the VHCF regime were classified, and the competition of different failure types was described based on the modified Poisson distribution.

Key words: Titanium alloy, Hot isostatic pressing, Fatigue life, Crack initiation type, Porosity

Min Cheng, Zhengguan Lu, Jie Wu, Ruipeng Guo, Junwei Qiao, Lei Xu, Rui Yang. Effect of thermal induced porosity on high-cycle fatigue and very high-cycle fatigue behaviors of hot-isostatic-pressed Ti-6Al-4V powder components[J]. J. Mater. Sci. Technol., 2022, 98: 177-185.

Figures/Tables 13

Fig. 1. Dimensions of (a) HCF and (b) VHCF fatigue specimens (unit: mm).

Fig. 2. SEM images showing the microstructures of (a) as-HIPed and (b) heat-treated Ti-6Al-4V alloys.

Fig. 3. Micro-CT images of Ti-6Al-4V alloys: (a) 2D slice showing the presence of pores in the as-HIPed state; (b) 2D slice showing the TIP in the heat-treated state.

Fig. 4. Statistics of extremes distribution of pores contained in Ti-6Al-4V powder compacts.

Fig. 5. The stress-strain curves of Ti-6Al-4V powder compacts.

Fig. 6. S-N curves of Ti-6Al-4V powder compacts in the HCF regime.

Fig. 7. S-N curves of heat-treated Ti-6Al-4V powder compact in the VHCF regime.

Fig. 8. SEM images showing the fracture surfaces of as-HIPed Ti-6Al-4V powder compacts: (a) σa=540 MPa, Nf=3.01 × 106; (b) σa=590 MPa, Nf=6.96 × 105.

Fig. 9. SEM images showing the fracture surfaces of heat-treated Ti-6Al-4V powder compacts: (a) crack initiated at surface (σa=575 MPa, Nf=6.87 × 105); (b) crack initiated at surface with facets (σa=550 MPa, Nf=4.46 × 106); (c) crack initiated at subsurface with facets and pores (σa=500 MPa, Nf=2.99 × 106); (e) crack initiated at surface pores (σa=575 MPa, Nf=1.13 × 105); (d, f) high magnification images of (c, e) (symbol with black arrows: facets, symbol with white arrows: pores).

Fig. 10. SEM images showing the VHCF fatigue fracture surfaces of heat-treated Ti-6Al-4V alloys: (a) crack initiated at surface with facets (σa=500 MPa, Nf=3.81 × 105); (b) crack initiated at surface pores (σa=380 MPa, Nf=4.50 × 105); (c) crack initiated at interior with pores and facets: (σa=380 MPa, Nf=4.24 × 108); (d) crack initiated at interior with facets (σa=390 MPa, Nf=1.87 × 108).

Fig. 11. S-N curve showing the effect of pore size and stress concentration (Kt) on the VHCF fatigue life. The color markers denote the pore size measured by SEM observation.

Fig. 12. (a) The defect size versus fatigue life and (b) the ΔKini versus fatigue life of the heat-treated Ti-6Al-4V powder compact in the VHCF regime.

Fig. 13. Schematic diagram of the competition of three failure types and the failure morphology in the VHCF regime.

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