PSI - Issue 65

Boris Voloskov et al. / Procedia Structural Integrity 65 (2024) 302–309 Voloskov et al./ Structural Integrity Procedia 00 (2024) 000–000

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with significantly higher porosity NP110 showed the minimum elongation at break. In general, the relative elongation at break was higher for the specimens with smaller values of hatch distance, which is 50  m.

Fig. 4. (a) tensile diagrams; (b) mechanical characteristics (c) photo of tested specimens.

4. Discussion

A common observation for both NP and chessboard strategies for the cases of hatch distances of 50 µm and 110 µm is that the number of pores on the left side is significantly larger than on the right side. It is assumed that this is influenced by the specimen’s position relative to the rake on the baseplate (Fig. 1a). The distribution of pores from the surface to the center for specimens with a diameter of 4 mm is shown in Fig. 5a and Fig. 5b for a hatch distance of 80 µm, comparing the NP and chessboard strategies. It is well seen that the pores are mostly located in the contour region (Fig. 5a). For chessboard strategy, 72% of porosity is concentrated in the region of 135  m. For NP strategy, 95% of porosity is concentrated in the region of the same size. Indeed, the surface region is constructed using different parameters, known as contour parameters. This approach is recommended by the manufacturer. Additionally, all internal geometries and contours are printed with a counter set of parameters. Most probably, this causes such porosity distribution. The contour regions often show accumulation of voids due to insufficient overlap between contour and bulk scans or/and high amount of turning points of laser path (Martin et al. 2019). The contour parameters used during printing are shown in Table 1. Formula 1 allows one to quantify how much energy is being delivered to the powder bed during the PBF-LB process (Y. Kuzminova et al. 2020).

P

E



V

v d t  

(1)

where E V is volumetric energy density, P is output laser power, v is scan speed, d is hatch distance and t is layer thickness. The values of volumetric energy density were calculated for the contour and main (Table 1, hatch distance 80  m) printing parameters. The results obtained were 140 J/mm 3 and 101 J/mm 3 for the contour and main parameters, respectively. As energy density increases, the melt pool becomes deeper and wider, which can result in a phenomenon known as keyholing. This occurs when vaporization of the material happens at the bottom of the melt