Issue 53

P. Ferro et alii, Frattura ed Integrità Strutturale, 53 (2020) 252-284; DOI: 10.3221/IGF-ESIS.53.21 260 although the energy density is linearly increased, the contribution from recoil pressure and Marangoni force are different because of the different temperature-dependence. Figure 7: Illustration of (a) the Marangoni effect and (b) the recoil pressure [34]. Figure 8: Melt pool depth with scan speed of 0.12, 0.20, 0.28 m/s and comparison with the experiment (laser power = 45 W, material = 316L stainless steel) [34] By using fluid dynamic numerical simulation, Willy et al. [46], optimized the overlapping of consecutive molten pool tracks for different laser powers (Fig. 9). It is found that with powers lower than 175 W, a gap is seen between two consecutive molten pool tracks. Zheng et al. [41] used the lattice Boltzmann method (LBM) to solve the Navier-Stock (NS) equations. Compared to finite element method (FEM), finite difference method (FDM) or finite volume method (FVM), LBM describes the fluid dynamics by the collision and streaming of the fluid particles with the great advantage to be much higher efficient than continuum-based approaches (FEM, FVM, FDM). It is found that the surface tension is six orders of magnitude larger than gravity so that fused particles are driven by surface tension rather than gravity to coalesce together (Fig. 10). By using numerical simulation Zheng et al. [41] highlighted the different contribute of interfacial forces on the melted pool morphology. Surface tension, Marangoni convection and recoil pressure effects are shown in Fig. 11. The mere presence of surface tension overestimates the dimension of fusion zone (FZ). When the Marangoni effect is also taken into account, both the FZ size and back material elevation reduce due to the increased convection driven by the tangential Marangoni forces (Fig. 7a). If now the recoil pressure is added to the model, the FZ size decreases further because of the heat loss induced by evaporation while the depression just behind the laser beam and the back-material elevation increase. As depicted in Fig. 12, the laser spot and the depression zone are not found coaxial and this numerical finding was also confirmed by experiments (Fig. 13). For a given laser power, porosities may derive from too high (lack of fusion) or too low (trapped gas) laser speed. Zheng et al. [41] described the lack of fusion porosities formation by numerical simulation (Fig. 14). From 600 mm/s to 1200 mm/s the length of the molten pool increased but the resulting surface profile resulted