## 3 Results and Discussion
### Surface Morphology and Roughness Analysis
The results showed that the waviness features were directly due to the presence of unmelted and partially melted Ti64 metal powder particles sintered to the surface. As can be seen, the large-scale variations in the surface's height, such as form and waviness, should be neglected as the length scale of FOV is similar to the minimum cut-off value of 0.1 mm. Therefore, this small evaluation length (146.52 \(\upmu\)m), about five times the mean particle size of the Ti64 powders, allows us to directly evaluate the areal roughness parameters from the SLM printed Ti-6 Al-4V surface's actual contour. Figure 5 shows the 3D surface texture parameters, including S\({}_{\text{a}}\), S\({}_{\text{q}}\), S\({}_{\text{sk}}\), and S\({}_{\text{ku}}\), for untreated and heat-treated specimens, where the corresponding values of p are 0.14, 0.06, 0.02, and 0.04, respectively. As can be seen, values of S\({}_{\text{a}}\) and S\({}_{\text{q}}\) were not sensitive to the surface topography of the untreated or heat-treated Ti64 specimens; however, values of S\({}_{\text{sk}}\) and S\({}_{\text{ku}}\) revealed distinct differences between the two surfaces. The heat-treated sample had an S\({}_{\text{sk}}\) value of 0.3 \(\pm\) 0.2, which is within the range of \(-\frac{1}{2}\) and \(+\frac{1}{2}\); i.e., the height distribution was approximately symmetrical. However, the untreated sample had an S\({}_{\text{sk}}\) value of 0.7 \(\pm\) 0.2, which is within the range of \(+\frac{1}{2}\) and \(+1\); i.e., the height distribution was moderately skewed. On the other hand, the heat-treated sample had an S\({}_{\text{ku}}\) value of 2.6 \(\pm\) 0.3, which is smaller than 3, and so the height distribution curve is said to be platykurtic and have relatively few high peaks or low valleys. However, the untreated sample had an S\({}_{\text{ku}}\) value of 3.3 \(\pm\) 0.6, which is larger than 3, and so the height distribution is spiked, and the surface has relatively many high peaks and low valleys. Accordingly, this finding also depicts the same microscale features as SEM images. Figure 4: A Gaussian high-pass filter with a cut-off value of 0.1 mm was used to suppress the longer wavelength components for determining the surface features. (**a–c**)/(**d–f**) respectively show the real surfaces, the roughness features, and the waviness features of untreated/heat-treated Ti64 specimens. Roughness features obtained by a high-pass filter with a cut-off value of 0.1 mm were further analyzed to compare the areal surface texture parameters via the measured surface in the absence and filtering presence. Table S1 showed that after removing the waviness portion from the measured surface, values of S\({}_{\mathrm{a}}\), S\({}_{\mathrm{q}}\), and S\({}_{\mathrm{ku}}\) were not sensitive to the surface topography of the untreated or heat-treated Ti64 specimens, where the corresponding values of p are 0.94, 0.66, and 0.35, respectively; however, the value of S\({}_{\mathrm{sk}}\) revealed distinct differences between the two surfaces (\(p<0.01\)). After removing the waviness portion from the measured surface, values of S\({}_{\mathrm{sk}}\) for the untreated sample and the heat-treated sample were 0.4 \(\pm\) 0.1 and 0.0 \(\pm\) 0.2, respectively, which was within the range of \(-\frac{1}{2}\) and \(+\frac{1}{2}\); i.e., the height distribution was approximately symmetrical. Interestingly, values of S\({}_{\mathrm{sk}}\) for untreated and heat-treated Ti64 specimens were larger than 3, which implied the surface has relatively high peaks and low valleys. As can be seen in Figure 4, these spikes were localized around the surface of the unmelted and partially melted Ti64 metal powder particles. These spikes' contribution to the S\({}_{\mathrm{sk}}\) became significant after removing the waviness portion from the measured surface.