# Control of Porosity in Parts Produced by a Direct Laser Melting Process
## 3 Results and Discussion
### Porosity in Multi-Track DLM Process  
In order to investigate the porosity generated during the DLM process, a quadrangle structure was fabricated by means of a multi-track melting test. The multi-track melting test was performed at a laser power of 200 W, scan rate of 110 mm/s, and line spacing of 0.06 mm. As illustrated in Figure 11, the cross-hatching technique was applied in the laser scanning pattern in order to fabricate the quadrangle structure. The fill pattern was rotated by 90\({}^{\circ}\) when scanning the subsequent layer to reduce the impedamental effect of a larger uneven layer produced during the previous scanning process. Figure 10: H/W ratio of single-track bead: (**a**) powder 1 and (**b**) powder 2. Figure 9: Cross-sectional views of single-track beads processed at different scan rates: (**a**) powder 1 and (**b**) powder 2. The effects of processing parameters on porosity generation were investigated, including scan rate, layering thickness, line spacing, and powder size. Figure 12 shows cross-sectional views obtained from the multi-track test. The powder layering thickness was adjusted based on the DLM operational stability. The layering thicknesses of powders 1, 2, 3, and 4 were 0.03 mm, 0.03 mm, 0.06 mm, and 0.09 mm, respectively. As illustrated in the figure, the porosity varies significantly according to the powder size and morphology. In the case of powder 1, a fully dense layer without porosity was obtained. In the case of deposited layers using powders 2, 3, and 4, the porosity increased with the size of the non-spherical powder mixed with the spherical powder. The variation in porosity densities is mainly caused by the powder packing density [25]. When larger powders with non-spherical morphologies are mixed with spherical powder, numerous pores are generated as a result of the reduced powder packing density. Figure 11: Fabrication of quadrangle structure by cross-hatching technique. Figure 12: Porosities in deposited layers obtained using: (**a**) powder 1, (**b**) powder 2, (**c**) powder 3, and (**d**) powder 4. Laser radiation penetrates into the powder through the pores, to a depth of several particle diameters, because of multiple reflections. If sufficient laser radiation is applied, the powder particles begin to coalesce and the powder layer structure changes, which has a considerable impact on the effective radiative and thermal properties and thereby influences the radiation, heat transfer, and consolidation kinetics [26]. When the laser beam radiates towards the powder layer, laser beam energy is dissipated by means of absorption and scattering. Part of the laser energy is absorbed by the powder particles and substrate, while the remaining energy is scattered back from both. Therefore, understanding the interaction between the powder and laser beam is key to laser penetration and powder bed absorption. The RT simulation indicated that metal powder layers with higher packing density exhibit increased absorptivity, owing to multiple reflections of laser radiation between powder particles. As illustrated in Figure 13, the laser beam is taken as the ray penetrating into the powder bed, which is reflected and absorbed by the powders. This illustration was based on a simulation of three rays. The metal layer was divided into six layers and the total absorption points were added. For example, referring to Figure 13a, in the first sub-layer, there are six absorption points. Having calculated the absorption points in each sub-layer, the total absorption points of each layer can be obtained [16]. Assuming that the absorbed energy at each absorption point is the same for the sake of simplicity, the number of absorption points can provide an index of energy absorption at each layer. As the number of total absorption points for powders 1, 2, 3, and 4 are 53, 42, 34, and 28, respectively, less energy is absorbed with increasing powder numbers. Therefore, powder 1 exhibits a dense layer, while powder 4 exhibits the most porous layer.