Issue 53

P. Ferro et alii, Frattura ed Integrità Strutturale, 53 (2020) 252-284; DOI: 10.3221/IGF-ESIS.53.21 257 Zhang and Zhang [34] found a packing density of 51% against experimental values in between 50% and 61% [39,40] (Fig. 5). Figure 5: (a) DEM simulated powder trajectory during the powder deposition and (b) the powder layer generated by DEM (from [34]). The effects of powder packing densities are studied in Ref. [41]. Authors developed a novel method for the molten pool and porosity modeling in selective laser melting. The general outcome is that, with low packing density values, defects like ‘necking’ areas may arise (Fig. 6a) that are detrimental for the laser to penetrate the subsequent powder layer because of the increased layer local thickness. In this situation, lack of fusion defects are induced as schematized in Fig 6b. Figure 6: (a) melt pool with packing densities of 3.183 (left picture) and 4.366 kg/m3 (right picture) obtained with the same laser power (200 W) and scanning speed (1000 mm/s) (material: Inconel 625). (b) Schematic of lack of fusions defects promoted by breakup of the molten pool (from [41]). It is worth noting that a denudation phenomenon can also worsen the local packing density as described in Ref. [42]. According to process parameters, such phenomenon dramatically reduces the powder packing density next to the melt track during the process. This is due to a competition between outward metal vapor flux directed away from the laser spot and entrainment of powder particles in a shear flow of gas driven by a metal vapor jet at the melt track. Finally, Mindt et al. [35] observed that the power size distribution also is an important parameter to be optimized in order to reduce defects in PBF fabricated components. In general, it is found that the lower the mean particle diameter the lower the porosity since the coating process leads to a more uniform powder layer. Powder-source interaction Irrespective to welding, in PBF processes the heat source travels over a non-flat surface. The source rays are in fact subjected to shadowing, reflection, diffraction and absorption phenomena induced by the powder layer morphology. This