PSI - Issue 19

Masanori Nakatani et al. / Procedia Structural Integrity 19 (2019) 294–301 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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fatigue limit and Vickers hardness HV up to HV =~400. There is a robust empirical formula (Eq. (1)) for steels between the fatigue limit  w,ideal (unit: MPa) and HV (unit: kgf/mm 2 ) for HV<400. w,ideal = 1.6 ± 0.1 (1) The S-N curves for both AM process showed common trend. The fatigue limit of polished specimens with HIP was close to ideal fatigue limit expected from HV . On the other hand, the fatigue limits of as-built specimens without HIP was only about 30% of ideal fatigue limit. For as-built specimen, HIP treatment increased the fatigue strength slightly. These results indicate that the surface roughness have a significant influence on the fatigue strength even if internal defects are eliminated by HIP treatment. The polished specimens with HIP in both process have almost the same fatigue strength. However, the fatigue strength of as-built EBM specimen with HIP was lower than that of as-built DMLS specimen with HIP. This would be caused by the difference of surface roughness because the rougher surface induces higher stress concentration.

Fig. 4. Bird-eye height images of as-built specimens. The measured area is 4.5 × 6.0 mm.

Fig. 5. S-N curves. The dotted lines indicate the ideal fatigue limits estimated from HV of specimen with HIP.