PSI - Issue 83

Daniele Cortis et al. / Procedia Structural Integrity 83 (2026) 3–13

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(b) ε ' = 0.00007 s -1

(a) ε ' = 0.00007 s -1

Fig. 6. Poisson ratio (a) and Elastic Modulus (b) at different temperatures: -100 °C; +22 °C; +100 °C; +200 °C.

3.2. Microstructure Microstructure analysis was performed along the build direction of one of the flat cross-section tensile test pieces by means an optical microscope (i.e., KEYENCE VHX 7000). The test piece was mechanically ground with silicon carbide (SiC) abrasive papers and then polished with diamond suspensions of 6 µm, 3 µm and 1 µm. Finally, it was chemically etched according to the ASTM E407 (2023) standard using the solution N. 3 (i.e., 2 ml HF, 3 ml HCl, 5 ml HNO ₃ , 190 ml water).

(a)

(b)

Equiaxed grains

Columnar grains

Melt pools morphology

Al 3 (Sc, Zr) particles

Build direction

Build direction

Fig. 7. Optical micrographs showing the material microstructure along the build direction at different magnifications: (a) 500x; (b) 1500x.

Fig. 7 (a) shows the typical microstructure of a PBF-LB material, with the overlapping of melt pools formed by successive laser scan tracks in each layer. In this case, melt pools have a broad morphology, likely associated to the high thermal conductivity of the α (Al) matrix. Observing the micrograph, it is possible to notice a double grain structure with fine equiaxed grains located at the boundary of molten pools and columnar grains at their center, Fig. 7 (b). The fine equiaxed grain boundaries are enriched with Al 6 Mn and Al ₃ (Sc,Zr) nano precipitates. During the melt pool solidification, Al ₃ (Sc,Zr) nano particles are formed first and when columnar grains are growing along the solidification path, Mn elements are segregated from the grains forming Al 6 Mn nano precipitates at boundaries of both equiaxed and columnar grains [Tang et al. (2024)]. The aging heat treatment (i.e., 4 h at 350 °C) did not alter this grain