PSI - Issue 74

Jaromír Brůža et al. / Procedia Structural Integrity 74 (2025) 1–8

4

4

Jaromír Brůža / Structural Integrity Procedia 00 (2025 ) 000–000

3. Results 3.1. Microstructure of L- PBF 316L steels

The EOS microstructure exhibits the usual L-PBF grain structure. Grains with complex torturous boundary geometry are macroscopically elongated along the y-axis, which coincides with the building direction. An epitaxial grain growth is present as grains may easily cross melt pools either directly or via so called side branching, see e.g. (Wang et al., 2025), sometimes even through multiple layers, as seen in Fig. 1a and Fig. 2a,b. The IPF colored map shows a relatively weak <011> fiber texture with a contribution <111> texture component as confirmed by the pole figures showing a texture index of 2.6 (see Fig. 2c). As apparent from the SEM micrograph of etched surface shown in Fig. 1a, all grains consist of cellular substructure which corresponds with the occurrence of austenitic solidification mode in the whole volume of the EOS L-PBF steel. The cell walls in this solidification mode are characterist ically enriched by Cr and Mo (ferrite stabilizing elements) (Wang et al., 2018), (Godec et al., 2020), (Voisin et al., 2021), (Wang et al., 2025). Here, it should be highlighted that solidification cells resemble 3D honeycombs elongated along <001> easy gr owth direction (Voisin et al., 2021; Wang et al., 2025). Depending on the inclination of the section plane, they thus may appear in a 2D representation as quasi-hexagonal cells, elongated cells, or planar features.

Fig. 1 SEM micrographs of the microstructure of L-PBF 316L steels manufactured from (a) EOS, and (b) Praxair powder as revealed using electrolytic etching. A clear change in chemical segregation within individual melt pools in Praxair L-PBF steel is noticeable. The building direction is vertical, upward oriented in both micrographs. EBSD mapping of 316L steels manufactured from SLM and Praxair powders has brought different microstructural pictures. Contrary to EOS 316L steel, SLM and Praxair steels exhibit visibly grain-refined microstructure as apparent from Fig. 2d-e and Fig. 2g-h, respectively. The grains are also elongated, but their length, due to inhibition of the epitaxial growth, only rarely exceeded the depth of a single melt pool (see Figs 2d and 2g). Large area EBSD mapping revealed a fiber <011> texture type in both steel s, see Fig. 2f and Fig. 2i. The texture index of 3.3 evidences a weak -to-moderate texture of SLM 316L steel, while the weakest texture with a texture intensity of 2 has been found for Praxair 316L steel. Finally, a noticeable and significant difference between SLM, Praxair, and EOS 316L steels was revealed during SEM examination of electrolytically etched cross-sections – c.f. Fig. 1a and Fig. 1b. Contrary to EOS 316L steel with characteristic cellular structure within the whole volume of melt pools (see Fig. 1a), the etching of Praxair steel (Fig. 1b) revealed the areas with usual cellular substructure present only at the melt pool boundaries while in the remaining part of melt pools only a very weak and/or even no cellular substructure in noticeable. A very similar picture, not shown here, was also found for SLM 316L steel. This finding has indicated a more complex solidification behavior of SLM and Praxair 316L steels with a possible occurrence of other solidification modes in addition to the usually detected austenitic solidification mode. Results of grain boundary (GB) mapping (see Fig. 3) revealed other significant differences among the three variants of L-PBF 316L steels. While the EOS microstructure (Fig. 3a,d) exhibits a typical character of GB misorientation distribution with a high fraction of low-angle grain boundaries (LAGBs) around 40% and a very low fraction of 60° Ʃ3 twin boundaries (TBs) around 2.5%, nearly the opposite trend was found for the two remaining L PBF steels. Meanwhile, only ~5% of the grain boundaries are LAGBs, in the case of the SLM (Fig. 3b,e) and Praxair (Fig. 3c,f). Moreover, a significant number of high- angle boundaries are Ʃ3 and Ʃ9 twins. LAGBs tracing the cell boundaries are present predominantly in areas where clear cellular substructure is apparent.