# Control of Porosity in Parts Produced by a Direct Laser Melting Process
## 3 Results and Discussion
### Porosity in Multi-Track DLM Process
The effect of different amounts of laser power on porosity was investigated using a laser scanning speed of 110 mm/s and line spacing of 0.06 mm. Figure 14 shows cross-sectional views obtained from the multi-track test using different test powders. In the case of powders 3 and 4, 100 W laser power leads to unstable deposition, owing to insufficient heat input. Figure 13: Illustration of RT model: (**a**) powder 1, (**b**) powder 2, (**c**) powder 3, and (**d**) powder 4. A relation between the apparent density of the respective powders and the porosity of the molten layers are shown in Figure 15. As shown in the figure, higher porosity has been obtained with decreasing apparent density. The porosity decreases with increasing laser power, and the porosity of the respective powders differ significantly at the same laser power. As explained previously, the difference in laser absorption and scattering is one of the main causes. Another possible cause of these differences in porosity susceptibility may be the presence of a powder-depleted (PD) zone in the powder bed during the DLM process. [PERSON] et al. (2019, 2020) reported that the laser beam directly interacts with the powder, as demonstrated in Figure 16. The melt pool formed around the laser beam contacts the substrate by its central part only. The lateral parts of the melt pool detach from the neighboring powder to form a round, free surface with reduced surface energy. Thus, a rearrangement densification occurs in the center of a considerably larger powder-consumed band by the action of viscous flow under capillary pressure ([PERSON] et al., 2019). The importance of this consolidation mechanism lies in the presence of a PD zone in the powder bed. The layered powder prior to laser irradiation may be changed significantly during powder consolidation, and the porosity generation may be directly influenced by the PD zone. The differences in motion dynamics between the large non-spherical powder and small spherical powder will change the width and irregularity of the PD zone. The PD zone is expected to increase with an increase in the non-spherical powder size. Therefore, the increased porosity densities in powders 2, 3, and 4 are assumed to be related to the increased PD zone. The effect of defocusing distance on porosity formation was investigated using laser power of 200 W, laser scanning speed of 110 mm/s, and line spacing of 0.06 mm. The cross-sectional views obtained from focal distances of \(-\)3 mm, \(-\)1 mm, 0 mm, +1 mm, and +3 mm are presented in Figure 17. The plane porosity shown in Figure 18 increases as the non-spherical powder size becomes larger, regardless of the power. When the laser beam meets the focus, the plane porosity becomes low (Kal  
Figure 19 illustrates the effects of layering thickness and scan rate on plane porosity. A multi-track melting test was performed with laser power of 200 W and line spacing of 0.06 mm. The porosity increases with the layering thickness and scan rate. The densification of a laser-melted layer is strongly dependent on heat concentration, and increased layering thickness and scan rate reduce the heat concentration and increase the possibility of porosity. In order to obtain a dense layer, the track being scanned must re-melt a portion of the previous layer. The layer creation is partly a result of the deposited powder and partly re-melting of the previously formed layer [28]. In the case of high powder layer thickness, a relatively small overlapped area is obtained, as illustrated in Figure 20a, where the  
Figure 16: Schematic of rearrangement consolidation kinetics. Figure 15: Relationship between the apparent density of the respective powders and the porosity of the molten layers (scan rate: 110 mm/s, line spacing 0.06 mm). porosity creation is mainly a result of insufficient melting of the deposited powder. In Figure 20b, the powder layer is significantly thinner than that in Figure 20a, and a far denser layer is obtained.