## 3 Results and Discussion  
### Single-Line Formation Test  
To fabricate defect-free SLM products, it is important to initially optimize the laser power and scan rate [16]. Figure 3 shows the results of the single-line formation test, which was performed to determine the two process parameters. Under all the applied laser powers and scan rates, three different line patterns could be observed, along with no melting [17]. The three different patterns can be described as follows: Single fused lines with significantly coarsened balling (type 1), single fused lines that are smooth and continuous (type 2), and single lines with a small amount of balling and/or discontinuous partial melting (type 3). These three different line patterns can be explained based on the energy density of the laser beam supplied to the metal powders [16]. The energy density describes the laser energy applied to the powders (Equation (1)). The range of energy densities is shown in Figure 4. When the energy density of the applied laser is sufficient to melt the powder, the melting zone enlarges, and the temperature in the vicinity of the melted zone remains high [17]. However, when the energy density of the applied laser is insufficient to fully melt the metal powder, unmelted powder remains on the layer, forming defects, because the temperature of the melting zone is insufficient. In this study, perfect bonding between the powders and the substrate was achieved when the energy density was 1.4-5.1 J/mm (type 2, Figure 3). Under this condition, it seems that a sufficient amount of liquid, with a reasonable stability, was formed; after solidification, this yielded continuous and smooth melted lines without the occurrence of balling. In contrast, when the energy density was lower than 1.3 J/mm, there was insufficient energy supplied to the powders, which caused poor adhesion between the powders and substrate (type 3, Figure 3). When the surface energy was reduced, the melt became highly instable; this resulted in the production of rough lines consisting of a number of small balls and/or partial melting on their surfaces. However, the line formation was also compromised when the energy density was excessive (type 1, Figure 3) [13; 17]. When the energy density was greater than 6.6 J/mm, a single fused line with significantly coarsened balling was observed (Figure 3). This balling phenomenon indicates that the melted powder grains stick together due to the surface tension force, forming a series of spheres, when the energy density is high [16; 17]. Such high energy densities can be generally considered to be due to the high laser power and slow scan rate. Figure 2: Schematic illustration of the multi-layer formation test. In addition, as shown in Figure 5, the optimal laser power and scan rate were determined using the single-line formation test, based on the objective function, \(F\)[12]. The beads produced on the substrate should form a stable line, where the melted zone and substrate are highly stable. Depending on the laser power and scan rate applied, \(F\) values between 1 and 8 could be determined (Equations (1)-(5)). When the \(F\) values increase, the average angle increases. The two angles (\(\alpha_{1}\) and \(\alpha_{2}\), Figure 5) indicate the line symmetry, which is a favorable characteristic for the overlapping of the lines. When \(F\) = 1-3, the melting zone was unstable because of the shallow penetration depth (low \(H_{2}\) value), which was  
Figure 4: Energy density ranges under various laser powers and scan rates (see Figure 3). Figure 3: Photograph (**a**) and map (**b**) showing the single-line formation test results. The beads obtained under six different conditions (\(F\) (objective function) = 1–5, and 8), which resulted in the production of continuous and smooth lines (type 2), were further examined to determine the optimal process conditions (laser power and scan rate). probably due to the overly high scanning rate. When the \(F\) value increases, the \(H_{1}\) value increases but the \(H_{2}\) value decreases.