Issue 62

F. Cantaboni et alii, Frattura ed Integrità Strutturale, 62 (2022) 490-504; DOI: 10.3221/IGF-ESIS.62.33

melt pool, as highlighted from the arrows in Fig. 3d, individuating different sub-grains that grow from the grain boundary following a preferential crystallographic orientation for Co-Cr alloy [39,40]. Since the grain growth is competitive, when they collide, one of them stops [41]. In addition, the micrographs reported in Fig. 3c and Fig. 3d show also examples of sintered particles and spatter, typical defects of components produced by L-PBF, usually characterized by poor surface finish in as-built conditions. In particular, the particle presented in Fig. 3d is characterized by a dendritic structure, indicating a solidification condition very different from the bulk material.

Figure 3: Optical micrographs of longitudinal (a) and transverse (b) cross-sections of AB samples built at 90°. Examples of sintered particles, cellular sub-structure (c), spatter, and lamellar sub-structure (d). Optical micrograph of HT sample (e) and HT sample at higher magnification (f) built at 90° (some examples of equiaxed grains marked by white arrows). Regarding the grain structure, it is also well reported in the literature [32,42] that epitaxial columnar grains grow along the highest temperature gradient [43]. In contrast with AB samples, HT samples show a completely different microstructure, as reported in Fig. 3e. It clearly shows the disappearance of melt pools and the presence of equiaxed grains highlighted by the presence of precipitates along the grain boundaries. These are also dispersed in the matrix, as visible in Fig. 3f at higher magnification. Various phenomena take place during heat treatment. During soaking at 1200 °C, the fine cellular structure is progressively fragmented and dissolved, leading to a more homogeneous distribution of solute atoms and the formation of precipitates [44]. The chemical composition of these particles is still a topic of discussion and further investigations are needed. A strong dependence on the chemical composition of the alloy is evident from the literature [40] and carbide particles [45] or Cr- and Mo-rich particles are identified according to the C level in the considered alloy [40]. In the present study, the extremely low content of C (0.02 wt. %) suggests that precipitates are mainly particles containing Cr and Mo rather than carbides. This is

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