PSI - Issue 74

Jaromír Brůža et al. / Procedia Structural Integrity 74 (2025) 1–8 Jaromír Brůža / Structural Integrity Procedia 00 (2025 ) 000–000

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3.2. Hardness of L- PBF 316L steels The hardness measurements performed on the sections taken parallel and perpendicular to the building direction showed no substantial differences. Fig. 5 shows the results of statistical treatment of Vickers hardness measureme nts for all three variants of L-PBF 316L steels. The EOS steel with 225 ± 8 HV1 was the hardest, closely followed by SLM steel with 219 ± 6 HV1. The lowest hardness of 200 ± 1 HV1 was measured for Praxair 316L steel. 4. Discussion The EOS steel microstructure exhibited agreement with numerous previous studies, all characteristic repeatedly reported features, namely the complex grain shapes, epitaxial grain growth throughout numerous melt pool boundaries, a high fraction of LAGBs, and a very low or zero fractio n of TBs. This microstructural state is typical for the primary austenitic cellular solidification mode, for which the segregation of Cr and Mo into cell boundaries is characteristic (Fig. 3a) – see Ref. e.g. (Liu et al. 2018), (Godec et al. 2020), (Voisin et al., 2021), (Wang et al., 2025). Since, in this solidification mode, the L-PBF material does not undergo any solid-state transformation during cooling, the entire thermo-mechanical history inherent to the L-PBF process is incorporated into the material. This results in both high dislocation density related to cell structure and a high fraction of LAGBs. As the dislocation strengthening has been repeatedly identified as the most important contribution to the high yield stress (Liu et al. 2018) (Voisin et al., 2021), (Wang et al., 2025), it is not surprising that the EOS steel exhibits the highest hardness of 225 HV1 (see Fig. 5), typical for the adopted printing strategy, contrary to SLM and Praxair steels with a very small fraction of LAGBs (see Fig. 3e and Fig. 3f) and reduced interstitial solid solution strengthening (see Table 1). Although the SLM, Praxair, and EOS steels share a similar melt pool shape (see Fig. 2a, d, g; 2c, f, i) and use the same meander scanning strategy with a 67° rotation (cf. Ma rattukalam et al., 2020; Fonda et al., 2022), the SLM and Praxair steels exhibit a distinctly different, grain-refined microstructure compared to EOS steel. This includes a very low fraction of low-angle grain boundaries (LAGBs) and a high fraction of twin boundaries (TBs), consistent with findings from recent studies (de Sonis et al., 2022; Monier et al., 2023; Fouchereau et al., 2024). The origin of this unique microstructure, due to its incomplete characterization , still represents a challenge. In fact, at present, there are two competing hypotheses trying to explain the above microstructural features. Monier et al. analyzed the microstructure of L-PBF 316L steel manufactured from Praxair powder and proposed that it is a local liquid ordering, known as ic osahedral short-range ordering (ISRO), which is responsible for the observed grain refinement and high fraction of TBs (Monier et al., 2023). In contradiction to this theory, several authors highlighted the importance of chemical composition on the solidification behavior of L-PBF 316L steels. Based purely on the empirical criteria developed for laser welding with no additional supporting experimental evidence, they postulated that a considerable grain refinement is due to the occurrence of ferrite → austenite solidification sequence (Ziri, 2022) (Roirand et al., 2024). As will be discussed below, our experimental results support the latter hypothesis. Our SEM investigations of electrolytically etched specimens manufactured from SLM and Praxair powders showed a particular etching response (Fig. 1b) with a usual cellular substructure occurring only at the melt pool boundaries, while in the remaining part of the melt pools, only a very weak and/or even no cellular substructure was present. Although this specific etching response, sometimes called “fish-scale” structure, has been in the past repeatedly reported for L-PBF Praxair and other 316L L-PBF steels (see e.g., Choo et al., 2019; Dryepondt et al., 2021), a convincing explanation of its origin has been suggested only recently (Godfrey et al., 2022). Godfrey et al. analyzed “ fish-scale” structures in L-PBF 316L steel using various techniques, with the chemical composition very close to our Praxair steel. They concluded that its origin comes from a phase selection phenomenon that occurs during solidification due to spatial and temporal variation of thermal gradients (G) and liquid–solid interface velocity (V) within a melt pool. As the melt pool boundaries have high G and low V, solidification starts in austenitic mode. As the velocity increases, there is a transition to ferritic solidification mode, whereas the subsequent massive solid-state transformation of ferrite during cooling results in a fully austenitic structure (Godfrey et al., 2022). Thus, the present SLM steel started to solidify in austenitic mode, characterized by the segregation of Cr and Mo into cellular walls, while inside the melt pool, no clear segregation was revealed, which corresponds to the massive solid-state ferrite-to-austenite transformation. The periodic repetition of this solidification sequence effectively inhibits an epitaxial grain growth through numerous MPBs, which leads to the considerable grain