PSI - Issue 43

Tibor Varmus et al. / Procedia Structural Integrity 43 (2023) 184–189 Tibor Varmus / Structural Integrity Procedia 00 (2022) 000 – 000

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Fig. 3. (a) three types of build orientations of miniature specimens, (b) Type A- surface profile, (c) Type B surface profile, (d) Type C surface profile.

3.3. Fatigue behavior

The results of an experimental determination of the fatigue life of miniature specimens produced by L-PBF with three different orientations and heat treated to T6, and with surfaces in the as-built condition are presented in Fig. 4. The data are presented as S-N fatigue curves in a semi-log plot. As apparent from the figure, the different orientations do not introduce large differences in fatigue strength. The A- specimen (horizontal) shows a higher fatigue strength than the C specimen (vertical), namely 120 MPa vs 100 MPa at 2 x 10 6 cycles which may be explained in terms of surface roughness (Rz = 5.7  m for A- and Rz = 34  m for C orientation). In general, the largely isotropic fatigue behavior is possibly due to the T6 heat treatment that influences the structure making it homogeneous, eliminates residual stresses that typically are generated by L-PBF processing and improved hardness because of strengthening precipitation. The significant impact of the as-built surface roughness on fatigue behavior is readily recognizable when the fatigue data on the hand polished miniature specimens are inserted into Fig. 4. The average surface roughness after hand polishing was decreased to Rz = 1.6 ± 0.3 µm. The corresponding fatigue strength is estimated to be around 200 MPa, which is twice the estimate for the worst as-built specimen direction (i.e., vertical).