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

21

3

or upper bound of fatigue strength which we can expect for the case without influence of defects. It is well known that there is a very good correlation between fatigue limit and Vickers hardness HV up to HV =~400. For HV >400, fatigue limit drops drastically due to presence of small defects (Murakami, Y. 2002). There is the robust empirical formula (Eq. (1)) between fatigue limit σ w and HV for HV <400 (Fig. 2, Nishijima, S. 1980, and Murakami, Y., 2002). σ w,ideal = 1.6 HV ± 0.1 HV (1) Where, the units are σ w,ideal in MPa and HV in kgf/mm 2 . Since the Vickers hardness of the material investigated in this study ranges from HV =340 to 378, the ideal upper bound of fatigue strength can be estimated to be around σ w,ideal = 544-605 MPa.

N: normalized, Q.T: quenched and tempered. R = -1 zero mean stress specimens. Fig. 2 Relationship between hardness and the fatigue limit (Nishijima, S. 1980. and Murakami, Y. 2002). Figure 3 summarizes S - N data for EBM specimens. Figure 4 summarizes S - N data for DMLS specimens. From Fig. 3 and Fig. 4, the common trend of fatigue strength for all series of specimens is expressed as follows: As-built without HIP < As-built with HIP < Surface polish without HIP < Surface polish with HIP . The surface polished specimens with HIP have the fatigue limit close to the ideal upper bound. On the other hand, the fatigue limits of as-build specimens are only about 27% of the ideal fatigue limit expected from HV . Thus, the surface polish and HIP are definitely effective to improve fatigue properties of AM materials.

800

800

Surface polish without HIP ( =369) Surface polish with HIP ( =345) As built without HIP ( =369) As built with HIP ( =345) HV HV HV HV

Surface polish without HIP ( =378) Surface polish with HIP ( =340) As built without HIP ( =378) As built with HIP ( =340) HV HV HV HV

700

700

600

600

500

500

σ a （ MPa ）

σ a （ MPa ）

400

400

300

300

200

200

100

100

0

0

1.E+04

1.E+05

1.E+06

1.E+07

1.E+08

1.E+04

1.E+05

1.E+06

1.E+07

1.E+08

N f （ cycles ）

N f （ cycles ）

Fig. 3 S - N data for EBM specimens

Fig. 4 S - N data for DMLS specimens