AMP_04_May_June_2021_Digital_Edition

A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A Y / J U N E 2 0 2 1 2 6 growth oriented along the build direc- tion. Therefore, long epitaxial grains crossing over layers are formed along the V direction. The same grains in an H section appear almost equiaxed. Final- ly, the fine substructure has a cellular morphology in both V and H directions. LOW MAGNIFICATION LM Low magnification light micros- copy (Fig. 2) shows the general view of the LPBF microstructure, revealing the macro-level microstructural features (i.e., solidified melt pools and epitax- ial grains). A sound and regular micro- structure with no major discontinuities (i.e., defects) is evident (Figs. 2a and 2b), proving that processing parame- ters resulted in high-density samples. The anisotropy in the microstructure is revealed as well, which is demonstrated by the different shape of the solidified melt pools along the V and H directions. The layered structure formed by sub- sequent semicircular melt pools is ob- servable in section V (Fig. 2b), while in section H (Fig. 2a), elongatedmelt pools forming the chessboard scanning pat- tern can be seen. Accordingly, polarized light microscopy of V sections (Fig. 2d) highlights large columnar grains cross- ing over layers, due to epitaxy; in H sections (Fig. 2c), this appears as equi- axed grains. HIGH MAGNIFICATION LM High magnification light micros- copy (Fig. 3) facilitates the investigation of micro-level microstructural features (i.e., cellular substructure) and obser- vation of interactions among all mi- crostructural features. Solidified melt pools due to successive laser scans are better highlighted by high magnifi- cation bright field analyses (Figs. 3a and 3b), with shapes defined by the melt pool borders (highlighted by white dashed lines in the figure). The fine cellular substructure can be ob- served by dark filed analyses (Figs. 3c and 3d), which show an extremely fine structure inside the solidified melt pools. To completely resolve the mor- phology of this substructure, scanning electron microscopy is required [8,9] . However, by comparison with high- magnification polarized LM (Figs. 3e and 3f), it can be observed that differ- ently oriented cellular structures be- long to different grains, in the case of both V and H sections. This information is usually acquired by more advanced technologies such as SEM-EBSD. Further, by comparing bright field (Figs. 3a and 3b) and polarized light observations (Figs. 3e and 3f), it is pos- sible to confirm that epitaxial grains in the V section cross over several layers. Additionally, each melt pool is crossed by more than one epitax- ial grain, proving that starting from the border, several grains grew along the single melt pool. The same conclu- sion can be drawn regarding the H sec- tion, where multi- ple grains appear inside a single melt pool and, in many (a) Fig. 1 — Schematic representation of a typical LPBF microstructure shows evidence of the anisotropy of microstructural features in the direction parallel (V) and perpendicular (H) to the build direction. Fig. 2 — Lowmagnification LMmicrographs, bright field and polarized light: (a) and (c), H section; (b) and (d), V section. cases, grains cross over adjacent solid- ified melt pools. CONCLUSION In the present work, convention- al light microscopy (LM) was adopted to characterize the peculiar microstruc- ture of the Co28Cr6Mo alloy processed by the additive laser powder bed fusion (LPBF) process. Results show that the (b) (c) (d)