PSI - Issue 33

Maria Beatrice Abrami et al. / Procedia Structural Integrity 33 (2021) 878–886 / Structural Integrity Procedia 00 (2019) 000–000

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Fig. 3. Microhardness of Scalmalloy after soaking at test temperature for 7 h.

To verify this assumption, microstructural analyses were performed by optical microscope for the sample tested at room temperature and for the samples tested at the highest temperature (i.e. 200 °C). The comparison of the material microstructure of these two different conditions are reported in Fig. 4. The peculiar microstructural features of L-PBF process can be identified for AlMgScZr alloy. In particular, for both the samples, the scan tracks following the laser path can be observed in the cross section along the x-y plane (Fig.4 a,c), while the melt pools can be detected along the building direction z, together with the multiple overlaps of successive depositions (Fig.4 b,d). Melt pools have a semi-circular shape, due to the progressive melting of small volumes of powder. The boundaries of melt pools and scan tracks can be easily observed as darker areas, where the Al 3 (Sc,Zr) particles are concentrated, identifiable as dark points (Tocci et al. 2019, Zhang et al. 2018, Spierings et al. 2017). Porosities appear as almost spherical voids with extremely fine sizes, which form as result of the inert gas entrapment during the building process (Milewski 2017). By comparing different testing temperatures, it emerges that there are no microstructural variations after the soaking at high temperature. In fact, the melt pool/scan track boundaries can still be recognized, meaning that Al 3 (Sc,Zr) particles does not solubilize, contributing to the strengthening of the alloy also at high temperatures. Moreover, their presence at grain boundaries hinder grain growth in temperature (Spierings et al. 2017). These microstructural features allows the microhardness stability detected in Fig. 3. Lastly, the size of the porosities is found to be constant, as it does not increase after test in temperature. Therefore, defects are stable and do not worsen with temperature exposure. These observations agree with what previously detected in microhardness analyses, confirming therefore the very high microstructural stability of the alloy with temperature.

Fig. 4. Micrographs of the studied alloy after pin-on-disk test performed at room temperature (a-b) and at 200 °C (c-d).