PSI - Issue 66

Grzegorz Glodek et al. / Procedia Structural Integrity 66 (2024) 331–336 Author name / Structural Integrity Procedia 00 (2025) 000 – 000

335

5

17.5 kN. Material failure was also observed due to plain fatigue, which occurred outside the contact zones. For the AM material, these failures were attributed to internal defects and predominantly occurred either at the curved edge of the dovetail section or in the transition zone between the curved and flat edges.

Force [kN]

20.0

Failure Outside of Contact

Wrought

19.5

AM

19.0

Wrought, FOC

Fretting Fatigue Failure

AM, FOC

18.5

18.0

17.5

17.0

10 6

10 4

10 5

N [cycles]

Figure 4 Force-life data for wrought and AM Ti-64 samples (FOC refers to failure outside of contact).

Different factors contribute to the high fretting fatigue load sensitivity observed in experimental results. The highly localized nature of fretting fatigue stresses means that small variations in applied load can drastically alter the magnitude of these stresses. This can generate zones of localized plastic deformation of varying sizes, affecting the crack initiation rate and overall fatigue performance. The microstructure of Ti64 is predominantly composed of the alpha phase with an HCP crystal structure, which has fewer slip systems available for deformation. This limited deformation mechanism can result in more uniform cyclic loading behavior and a smoother transition between LCF and HCF regimes. However, this also makes the material more sensitive to applied stress, as plastic deformation is less easily accommodated. Additionally, early crack propagation in fretting fatigue occurs in mixed-mode conditions (Mode I and Mode II). Depending on the specific stress state at the contact interface, either mode can dominate, leading to variations in crack propagation rates and contributing to the material’s sensitivity to load changes. 3.3. Internal defects Porosities and internal defects are a common issue encountered in components manufactured using LPBF [12]. Two of the most common types of internal voids are small, spherical porosities caused by gas entrapment and larger defects caused by the incomplete of fusion between successive layers. In the case of the AM-Ti64 components tested in this study, the lack of fusion defects was responsible for the early fatigue failure outside the contact regions. Figure 5 shows an example of a fracture surface with visible lack of fusion defects. The critical defect present at the edge of the sample was a subsurface defect before the EDM process and was brought to the surface afterwards. This defect has an approximate area of 0.115 mm 2 , and is irregular in shape with a circularity of 0.61, which further contributes to increasing the stress concentration in its vicinity. Fatigue crack initiation occurred around this defect, rapidly propagating as it combined with other internal voids, as seen in Figure 5, ultimately leading to failure in the transition zone between the neck and curved edge of the dovetail.