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 5 O ver the past few years, metal addi- tive manufacturing (AM) technol- ogies including laser powder bed fusion (LPBF) have benefited from high interest in both research and industry [1,2] . LPBF relies on localizedmelting of subse- quent layers of finemetallic powder using a laser beam. From a materials perspec- tive, LPBF results in a distinct, extremely fine, and metastable microstructure that guarantees good mechanical properties even in the as-built condition [3] . The ori- gin of this distinctive microstructure lies in the peculiar solidification conditions that the processed alloy undergoes [4] , leading to its characteristicmicrostructur- al features: a layered structure formed by subsequent solidified melt pools, epitaxi- al grains crossing over layers, and a very fine substructure formed within grains. These features, visible from low to high magnification, make the LPBF microstructure hierarchical. Solidified melt pools are usually observed by con- ventional light microscopy, while the other microstructural features often re- quire advanced metallographic tech- niques [5,6] . The present work aims at showing how these major microstruc- tural features can all be observed with conventional light microscopy alone. Further, using a single instrument pres- ents a key advantage—the ability to in- vestigate potential interactions among features. In the research discussed in this article, light microscopy was ap- plied to a Co28Cr6Mo alloy for a bio- medical application processed by LPBF. In addition, this technique has been successfully used with other metal al- loys processed by AM, such as AISI 304L austenitic stainless steel [7] . EXPERIMENTAL SETUP Small blocks were produced start- ing from gas atomized Co28Cr6Mo powder (LPW Technology Ltd., U.K.) us- ing a LPBF machine equipped with a continuous wave laser source (SISMA MYSINT100). Process parameters were chosen in order to obtain high-densi- ty optimized samples, as described in previous works [8,9] . A rotating chess- board scanning pattern was employed and samples were built along the direc- tion perpendicular to the build platform (i.e., vertical direction). Microstructural analyses were car- riedout in the as-built conditiononboth vertical (V, i.e., parallel to the build di- rection), and horizontal (H, i.e., perpen- dicular to the build direction) sections to highlight microstructural anisotropy. Metallographic sections were embed- ded in a phenolic resin and subjected to grinding and polishing to a mirror fin- ish. To reveal the microstructure, elec- trochemical etching was performed at 4 V for 20 s in a 5mL HCl, 10 g FeCl 3 , 100mL H 2 O solution [10] . Conventional light mi- croscopy (LM) was conducted with a Zeiss Imager A1 optical microscope. Po- larized light along with bright and dark fields were used to reveal the micro- structural features unique to LPBF. RESULTS AND DISCUSSION The expected microstructure re- sulting from LPBF is schematically represented in Fig. 1. Three main mi- crostructural features can be identified: borders of the solidified melt pools, epitaxial grains, and a fine cellular substructure formed within grains. As depicted in Fig. 1, these features change their morphology according to the ob- served sections (H or V). The LPBF mi- crostructure strictly depends on the layered manufacturing method and la- ser scanning path. By scanning the layer of powder, the laser beam locally melts metallic powder and forms the melt pool, which rapidly solidifies following the direction of the maximum thermal gradient (i.e., build direction). As a result, the microstructure is also highly oriented [3] . Traces of the laser tracks can be found on both V sections, appearing as semicircular so- lidified melt pools, and H sections, ap- pearing as elongated solidified melt pools that represent the scanning pat- tern followed by the laser beam. As mentioned above, solidification of the molten alloy is driven by epitaxial grain USING CONVENTIONAL LIGHT MICROSCOPY TO REVEAL THE HIERARCHICAL MICROSTRUCTURE OF 3D-PRINTED METAL PARTS This entry won the prestigious Jacquet-Lucas Award for Best in Show in the 2020 International Metallographic Contest. Lavinia Tonelli University of Bologna, Italy JACQUET-LUCAS AWARD