PSI - Issue 79

Saveria Spiller et al. / Procedia Structural Integrity 79 (2026) 176–181

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3.2. Investigation on the fusion zone of the BoP The observation of the cross-section of BoP shows that the fusion zone has the typical hourglass shape, with minimum width in the middle, and the weld is fully penetrated. The micrograph reported in Fig. 3a depicts the polished and etched cross-section to highlight the minimal presence of porosities inside the fusion zone. These defects are typically connected with the presence of trapped gas. However, as confirmed by the picture and from the general observation of the BoP, no welding cracks were observed, neither in the FZ nor in the HAZ. It must be noted, though, that the extension of the HAZ is very limited, difficult to identify due to the minimal microstructure change. The IPF map obtained with the EBSD technique reported in Fig. 3b shows indeed columnar grains in the FZ growing from the borders of the FZ towards the center. The microstructure in the HAZ and PM is, in comparison, significantly finer. These aspects are reflected in the microhardness profiles that were taken near the top and in the middle of the hourglass and reported in the plot in Fig. 3c: the average PM microhardness is 320 HV0.5, which is in line with other values obtained from the literature about the microhardness of L-PBF IN625 (Li et al., 2015). The value is indeed significantly higher in comparison to what is recorded in the FZ. The plots show a significant drop in the microhardness values with a minimum at 236 HV0.5. Interestingly, there is no significant difference between the two profiles.

Fig. 3: investigation on the fusion zone. a) polished and etched cross-section; b) EBSD map of the fusion zone; c) microhardness profiles taken near the top and in the middle of the cross-section.

To complete the investigation of the material, the microstructure of the fusion zone is observed at higher magnifications to evaluate the peculiar dendritic structure induced by the welding process. Fig. 4a is a high magnification SEM picture representing the center of the FZ: the picture reveals the presence of abundant secondary phases in the interdendritic regions, depicted as shiny particles. The EDS analysis was performed to evaluate the chemical composition of them. The results, reported in Fig. 4b-e, show a significant abundance of Nb and C, a less significant abundance of Mo, and a depletion of Ni. Similar secondary phases were also observed in (Jelokhani Niaraki et al., 2016; Marchese et al., 2016; Qin et al., 2017). The authors claimed the phases were Laves phases and Nb carbides. The abundance of C indeed suggests the identification of the phases as carbides. However, it must be noted that elements such as C, with low atomic number and present in limited quantities, are difficult to detect with precision using systems like EDS (Newbury et al., 2013). The importance of the characterization of the above mentioned phases is related to their impact on the occurrence of welding cracks and the general resistance of the joint (Han et al., 2018; Jelokhani-Niaraki et al., 2016). Indeed, the phases are brittle, and depending on their morphology, they can cause dangerous stress intensification. For this reason, a proper identification of these phases using diffraction methods is of high relevance for further studies.