PSI - Issue 18

Matilde Scurria et al. / Procedia Structural Integrity 18 (2019) 586–593 Matilde Scurria, Benjamin Möller, Rainer Wagener, Thilo Bein/ Structural Integrity Procedia 00 (2019) 000–000

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smaller compared to the increase of the strain amplitude. Furthermore, the heat treatment ‘C’ results in a positive effect on the material in terms of cycles to crack initiation, even if this is particularly visible as the strain amplitude decreases. This result is aligned with the ones achieved by Scurria et al. (2019) on polished specimens and therefore not directly correlated to the higher surface roughness presented by the XZ specimens after removal of the support structures. Another effect of the heat treatment ‘C’ on the material, for high levels of the strain amplitude (  a,t = 0.8%), is the reduction of the anisotropy of the material, which will be explained in the following paragraph through metallographic investigations. 3. Metallographic investigation The microstructure of the material after the different heat treatments has been evaluated by means of optical micrographs. Since for heat treatments ‘A’ and ‘B’ the microstructure is similar, a direct comparison is here limited to configurations ‘B’ and ‘C’. In Figures 5a and 5b, the material after the heat treatment ‘B’ (720°C) is shown. In Figure 5a, the laser path can clearly be distinguished, with a very fine microstructure and areas of equiaxed dendrites inside the grains. In Figure 5b, as expected, the microstructure is columnar and elongated along the build direction. The melt-pool boundaries and the grain boundaries can be clearly identified, contrary to what is shown in Figure 5c and 5d, representing the material after application of the heat treatment ‘C’. Here, the laser paths and the melt pools are barely visible, the dendritic structure has almost disappeared and segregations (probably metal carbides MC) can be found at the grain boundaries. A more homogeneous microstructure is found for heat treatment ‘C’, which explains the anisotropy reduction of the cyclic stress-strain behavior for different build orientations.

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Fig. 5. Optical micrographs of additively manufactured Inconel ® 718; after heat treatment B (a) orthogonal and (b) parallel to the build direction; after heat treatment C (c) orthogonal and (d) parallel to the build direction.