PSI - Issue 53

Daniele Cortis et al. / Procedia Structural Integrity 53 (2024) 136–143 Cortis et al ./ Structural Integrity Procedia 00 (2023) 000 – 000

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Fig. 3. SEM micrograph showing the joint between AISI 316L (left) and 16MnCr5 (right).

The 16MnCr5 alloy shows some microcavities and the melting pool boundaries are barely visible (Fig. 4a). The steel appears to have a very fine bainitic microstructure: in fact, as discussed in many papers available in the literature, the microstructure of low alloyed steels produced by additive manufacturing is generally martensitic/bainitic or bainitic [12-14]. Since SLM is a process characterised by high cooling rates reaching 10 6 – 10 8 K/s, the solidification process does not follow the equilibrium diagram. Some studies have shown that while the very rapid solidification of the melt pool promote the formation of martensite, the thermal accumulation promote bainite formation. A recent study [14] demonstrated that during melt pool solidification it is possible to obtain a bainite-like microstructure despite the very high cooling rates. This confirms our finding. The other steel produced in this study is the AISI 316L stainless steel. Fig. 4b shows its microstructure after electrochemical etching. This figure clearly shows the boundaries of the melting pool and the sub grain structure where columnar grains pass through multiple laser tracks, as highlighted by other papers [15,16]. After double etching, it is also possible to analyse the joint area. Fig. 5a shows that the joint area is about 200 µm thick and it appears as an area that separates the two different steels. By observing that area we can notice that into the melt pool there are intermixed phases. This can be explained considering that inside the melt pool there is a rapid variation of the temperature that determines a variation of the surface tension. This promotes Marangoni convection as shown in Fig. 5b. This circular flow favours the bonding at the interface between dissimilar alloys by enhancing element diffusion.

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Fig.4. Optical micrographs showing the microstructure of 16MnCr5 (a) and AISI 316L (b) after etching.