PSI - Issue 68

Matias Jaskari et al. / Procedia Structural Integrity 68 (2025) 480–485 M. Jaskari et al. / Structural Integrity Procedia 00 (2025) 000–000

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3.3. Fracture initiation Fracture surfaces, examined using FESEM, consistently revealed that fracture initiation occurred at the sample surface despite electropolishing, which enhanced the surface quality equal of or even below 0.3 µm. As shown in Fig. 5, the electropolished sample fatigued at a stress amplitude of 550 MPa exhibited surface-initiated fracture, as is indicated by a red arrow. After initiation, the crack initially propagated by cleavage, then shifted to trans-granular propagation, continuing through the material’s cross-section.

Fig. 5. Fracture surface of electropolished sample, fatigued at 550 MPa. Fracture initiation location marked by red arrow.

These results confirm that Inconel 718 exhibits a small critical defect size, making it highly vulnerable to defect driven fatigue failure. Although electropolishing enhanced the fatigue limit by 90%, it still falls significantly short of the theoretical ideal, achieving only 0.58 times the ideal theoretical value of 1.6*HV. With all fractures initiating at the surface, the material appears to be nearly defect-free internally. However, the improvement in surface quality alone was insufficient to fully mitigate fatigue failure under these conditions. To further optimize fatigue performance, additional surface treatment techniques, such as severe shot peening or methods aimed at controlling residual stresses, should be explored. 4. Summary This study demonstrates the significant role of surface quality and microstructural characteristics in the fatigue behavior of PBF-LB Inconel 718. The key findings can be summarized as follows: • Microstructural analysis revealed epitaxial grain growth in the build direction and a homogenous face-centered cubic (FCC) phase, with a possible presence of brittle Laves phase after heat treatment. • Electropolishing improved surface roughness from 5.2 µm (Ra) to 0.3 µm, leading to a more than 90% increase in the fatigue limit, with a notable improvement in high cycle fatigue (HCF) behavior, though the fatigue limit still only reached 0.58 times the theoretical ideal value of 1.6*HV. • Hardness testing showed slight cyclic softening after fatigue at 800 MPa, with hardness values decreasing from 450 HV (pre-fatigue) to 436 HV (post-fatigue). • Fracture consistently initiated at the surface, even after electropolishing, with cracks propagating through cleavage before transitioning to trans-granular propagation, suggesting that while surface treatments significantly enhance fatigue resistance, further optimization is necessary to address defect-driven fatigue failure in PBF-LB Inconel 718 components.