PSI - Issue 68

Xiangnan Pan et al. / Procedia Structural Integrity 68 (2025) 1038–1044 X. Pan et al. / Structural Integrity Procedia 00 (2025) 000–000

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generally comparable, and AM without HIP being the worst. For EM, BM, AM with HIP and without HIP, their fatigue resistances are as: σ f-8 = 550, 568, 530 and 240 MPa; σ f-9 = 550, 454, 454 and 172 MPa. Surface crack initiation dominates the fatigue failure of EM in both HCF and VHCF regimes, which is consistent with the literature (Pan et al., 2018; Pan and Hong, 2019), but for the BM and the AM with HIP, it dominates only in HCF regime. Internal crack initiation dominates the fatigue failure of AM without HIP in both HCF and VHCF regimes, but for the BM and AM with HIP, it dominates only in VHCF regime. It is obvious that specimen surface roughness will have a significant influence on the surface crack initiation induced fatigue failure. If the roughness is small enough, the fatigue resistance does not vary with the specimen surface and is defined as the intrinsic fatigue resistance. The AM with and without HIP are two LM. For the HIP one, the interior induced VHCF is similar to the literature (Pan et al., 2020a and 2024d), where the material is with coarse LM fibers and fine EM matrix produced by a method of conventional manufacturing. We developed a pre-torsion technique (Pan et al., 2020b) to modulate the elongation of a Ti-6Al-4V with EM while maintaining almost the same tensile strength. The S-N results suggest that the VHCF is closely related to the material plasticity, and the poorer the plasticity, the poorer the performance. This conclusion is also supported by Fig. 2. 4. Viewpoints and perspectives In summary, the intrinsic fatigue resistance depends on the external loads ( σ m and σ a ) and material properties ( σ u and δ f ), regardless of specimen size, geometry and surface roughness. For titanium alloys exemplified by Ti-6Al-4V: the directly printed state of AM (PBF-LB) one is with an LM of short and thin martensitic α’ and/or α’’ laths, which has the highest UTS, lowest elongation and worst fatigue performance; the HIP state is with a coarsened LM with α lamellae, which has the medium UTS, elongation and fatigue performance; the conventionally made EM has the lowest UTS, highest elongation and best VHCF performance. The tensile quantities σ u , δ f and/or their function E 1 can be adopted to indicate HCF and VHCF of titanium alloys with different microstructures. References Abe, T., Furuya, Y., Matsuoka, S., 2004. Gigacycle fatigue properties of 1800 MPa class spring steels. Fatigue Fract. Eng. Mater. Struct. 27(2), 159–167. Ashby, M., 2016. Materials Selection in Mechanical Design. 5th ed. Butterworth-Heinemann, Oxford, UK. Atrens, A., Hoffelner, W., Duerig, T., Allison, J., 1983. 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