## 3 Results and Discussion
### Scanning Electron Microscope (SEM) Analysis  
The surface morphology of as-built AM components from four different AM techniques is shown in Figure 3. Irregularities such as loosely attached powders, partially melted powders, balling melts, semi-welded structures, staircase effect, and melt zones due to heat treatment are identified based on the size difference, nature of the irregularity (loose powder or partial melts) [8; 18]. Currently, these irregularities are of high interest to researchers working in AM domain. Mostly the data available in SEM analysis are under-used or not given much attention. Figure 3 shows the asperities, such as loosely attached particles (SLM, DMLS, LENS), partially melted powders (SLM, DMLS, EBM, LENS), balling melts/semi-welded structures (SLM, DMLS, EBM), staircase effect (SLM, DMLS, LENS) and melt zones due to heat treatment (DMLS, EBM) identified in the surfaces under investigation. (Note that categorizing each type of irregularity may vary from user to user, some asperities in AM components overlap on each other and are difficult to separate. This is considered a challenge in this technique.)  
Figure 3: Surface morphology of as-built AM components.  
Size Distribution of Surface Irregularities  
SEM images provide clear visual evidence of the asperities present on any surface. SEM images of AM components can be used to understand the shape and size of each type of asperity present on the surface. The \(\varphi_{\mathrm{m}}\) of the asperities are identified through image processing using ImageJ (open-source image processing software). Five SEM images are analyzed. Figure 4 shows the steps followed to determine \(\varphi_{\mathrm{m}}\). Raw SEM images were converted into an 8-bit image, and the outlines were enhanced via a built-in function in the software ImageJ. Figure 4b shows the resulting image after enhancing the outline of the asperities present. Figure 4c shows the method of \(\varphi_{\mathrm{m}}\) measurement of a single structure. Since most of the asperities present are irregular in shape, \(\varphi_{\mathrm{m}}\) is calculated by drawing a circle with a diameter as shown in Figure 4d equal to the largest length available in a single structure. Asperities present in five different SEM images are measured using this technique, and the frequency of occurrence was obtained for each type of asperity.  
From the image processing analysis, the various types of asperities identified in each AM process and their frequency of occurrence are shown in Figure 5. The frequency of smaller-sized asperities in all the AM techniques is high (five times more) than larger-sized asperities.  
The calculated maximum diameter of each type of asperity present in an AM surface is given in Table 2. It can be noticed that the standard deviations of the results in Table 2 are high. This is because the size of asperities present in the component depends mainly on the AM process conditions. The measurement values (\(\varphi_{\mathrm{m}}\)) can be used to calculate filtering values in profile and areal surface texture measurements to filter out each type of asperity from the AM surface. The following section will explain the significance of measuring \(\varphi_{\mathrm{m}}\) of asperity present on the surface.  
Figure 4: Image processing steps to determine the maximum diameter of asperities. (**a**) raw SEM image; (**b**) post-processing to determine the edges; (**c**) definition of maximum diameter; and (**d**) measuring the maximum diameter of asperities.