PSI - Issue 38

Hugo Roirand et al. / Procedia Structural Integrity 38 (2022) 149–158 Author name / Structural Integrity Procedia 00 (2021) 000 – 000

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3. Results and discussion 3.1. Microstructure comparison

In order to understand the role of AM parameters and especially laser scan strategy on microstructure generation in LPBF process, specimens from the different platform has been analyzed with OM after etching. These cartography illustrated in Figure 3, show both melt pool and grains morphology. First, the influence of rotations between layers on melt pool positions is obvious by comparing Figure 3 (a) to other ones. This melt pool alignment for strategies without rotation allows the grain growth from the bottom of melt pools across several layers thanks to epitaxial growth. On the other hand, the constant superposition of the meltpools overlaps along the building direction promotes the generation of larger grains following a chevron pattern as it is well explained by (Andreau et al., 2019a). Finally, the periodic alternation of fine and elongated grains with the larger grains creates a crystallographic lamellar microstructure with excellent mechanical and corrosion properties as it has been already mentioned by Sun et al. (Sun et al., 2019, 2018). By comparing Figure 3 (a) and (b), the effect of laser pattern on melt pool morphologies and grain structure can be studied. Melt pools seems to be larger and less deep with the chessboard strategy than with the meander strategy. The large meltpools shown in Figure 3 (b) have already been observed for 316L parts built with a similar chessboard strategy by Liverani et al. (Liverani et al., 2017). The large melt pools of SLM_chess could be the reason why grains are larger and more elongated compared to SLM_Rot. Indeed, large and flat melt pools promote grain growth rather than grain germination (Andreau et al., 2019a). Concerning the comparison with Orlas specimen (Figure 3 (d)), melt pools are thinner and less deep as it was expected because of the low layer thickness and laser spot diameter. In consequence, grains are thinner, but still elongated as epitaxial growth still take place.

(b)

(a)

(c)

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Fig. 3 Optical microscopy images after etching (building direction upwards) (a) SLM_SR ; (b) SLM_Chess; (c) SLM_Rot ; (d) Orlas.

These differences in macro and microstructures will probably influence the mechanical properties as the grain size or grain orientations are key parameters in monotonic or fatigue behavior.