## 3 Results and Discussion
### Multi-Layer Formation Test  
As shown in Section 3.1, the optimal laser power and scan rate (200 W and 128.6 mm/s, respectively) were determined through the single-line formation test (Figure 5). These values were used during the multi-layer formation test to fabricate 3D-printed Co-Cr alloy products. Figure 6 shows the surfaces of the 3D-printed bodies in accordance with the scan-line spacing used. In general, the overhang areas have a higher maximum temperature, a higher tensile stress, and a larger distortion than the areas above a solid substrate [21]. In addition, a higher energy density input (a lower scan rate or a higher beam current) may cause severe curling at the overhang area [21]. Prominent overhang area formation when the scan-line spacing values were 60 and 20 \(\upmu\)m indicates that a high energy density input was supplied to the bodies because of the high overlap ratios. Figure 5: Various bead shapes produced during the single-line formation test under six different conditions (\(F\) = 1–5, and 8) (see Figure 3). When the laser power and scan rate were 200 W and 128.6 mm/s, respectively (\(F\) = 4, see 2.2 to read the symbols), an optimal bead shape was achieved. As also shown in Figure 6, the overlap ratio tends to decrease as the scan-line spacing increases (Equation (6)) [22]. Considering the various scan-line spacing values used, the surface shape of the melted powder was classified into three different groups according to the overlap ratio, as follows [12]: (1) a clear mesh-structured surface at small overlap ratios (24% and 43%); (2) a flat and smooth surface by optimal overlapping (62%); and (3) excessive overlapping of the lines of the melted material (77% and 92%) with occurrence of swelling [23]. When the overlap ratios were 24% and 43%, the SEM image showed rough surfaces with many small melt balls [23]. In contrast, when the overlap ratios were excessively high (77% and 92%), swelling, which is the rise of solid material above the plane of powder distribution and melting, was prominent on the SEM surface images [23]. When the overlap ratio was 62%, the SEM image revealed uniform-shaped lines. Although a slight melt ball formation was also detected on the surface, this does not seem to have an obvious detriment to SLM quality [24]. In the present multi-layer formation test, therefore, the most stable deposition was achieved when the scan-line spacing was 100 \(\upmu\)m; as a result, the overlap ratio was 62%. Figure 7 shows the \(R_{\mathrm{a}}\) values according to the scan-line spacing values and overlap ratios. When the scan-line spacing was 200 and 150 \(\upmu\)m, the surfaces were uneven and had higher \(R_{\mathrm{a}}\) values (6.2 and 4.1 \(\upmu\)m, respectively); this was probably due to small overlap ratios and the occurrence of the non-connected line formation (Figure 6). When scan-line spacing values of 60 and 20 \(\upmu\)m were used, the 3D-printed bodies had rough surfaces (\(R_{\mathrm{a}}\) = 3.3 and 3.9 \(\upmu\)m, respectively); this was probably due to the excessive melting, swelling, and excessive overlap ratios. The smoothest surface (\(R_{\mathrm{a}}\) = 1.8 \(\upmu\)m) was obtained when the scan-line spacing was 100 \(\upmu\)m (overlap ratio = 62%). These findings suggest that  
Figure 6: Photograph and map showing the multi-layer formation test results. The scanning electron microscopy (SEM) surface images are also shown (100\(\times\), scale bar = 100 \(\upmu\)m). The optical and topography (2D and 3D) images are obtained from the phase-shift interferometry. Optimal melting was achieved when the scan-line spacing was 100 \(\upmu\)m.  
use of the optimal scan-line spacing results in an overlap ratio that is slightly more than 50%, which enhances the surface quality of the 3D-printed Co-Cr alloy products. It should be noted that the results of the multi-layer formation test (Figures 6 and 7) were derived from the simplified experimental design (in particular, the number of deposited layers).