PSI - Issue 7

Hiroshige Masuo et al. / Procedia Structural Integrity 7 (2017) 19–26 Hiroshige Masuo et Al./ Structural Integrity Procedia 00 (2017) 000–000

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fatigue test. Even if a specimen ran out at a stress for N =10 7 cycles, we cannot define the stress as the fatigue limit of the specimen. The fatigue limit of a specimen varies depending on the most detrimental defect in the specimen which is difficult to identify prior to fatigue failure test. Figures 5 and 6 show the surface morphologies of as-build specimens for EBM and DMLS, respectively. The surface roughness is different between EBM and DMLS due to the different particle size. Figure 7 shows the microstructure and defects of a transverse cross section for an as-built specimen of EBMwithout HIP. Many defects are visible in Fig. 7. Figure 8 shows the microstructure and defects of a transverse section for an as-built specimen of EBM with HIP. Most of defects disappeared by the effect of HIP.

(a) EBM as-built

(b) DMLS as-built

Fig. 11 Surface morphology of as-built specimens after HIP.

HIP does not improve the surface condition of as-built specimens Figures 9 and 10 show defects which were observed at fracture origin in contact with specimen surface. Compared to rolled steels, the direction of defects in AM materials is random and not aligned in the identical direction. The varieties of defect shape are mostly originated from lack of fusion or various sizes of particle. Figure 11 shows the fatigue fracture surface near specimen surface of as-built specimens. The surface roughness in both EBM and DMLS is much larger in size than the size of defects shown in Figs. 9, 10, 14 and 15. It must be noted that defects opened to specimen surface cannot be eliminated by HIP.

(a)

(b)

(d) e < d 2 → √ area eff

√ area eff

√ area eff

d

e

(c) e < d → √ area eff

(e) √ area eff = Dotted area

Fig. 12 Estimation method for the effective size (dotted line) of irregularly shaped defects and defects near surface. (Murakami, Y. 2002, and Murakami, Y., Nemat-Nasser, S., 1983) (a) Irregularly shaped internal defect. (b) Irregularly shaped surface defect. (c) Irregularly shaped internal defect in interaction with surface. (d) Interacting adjacent two defects. (e) Inclined defect in contact with surface.