## 3 LPBF Sensing Techniques
### Optical Techniques
#### 3.1.2 Techniques Using an X-ray Source  
**X-ray computed tomography (XCT)** devices utilize an X-ray source to produce 2D radiographic slice imagery of the investigated object from many directions. Using the stack of cross-sectional 2D radiograph slices, a reconstruction algorithm is employed to  
\begin{table}
\begin{tabular}{l c c c c c c} \hline \hline
**Material** & **SEM** & **TEM** & **EBSD** & **EDS** & **XPS** & **EPMA** \\ \hline \multicolumn{8}{c}{Stainless-Steel-Based Alloys} \\ \hline SS316L [41; 47] & & & & ✓ & & & \\ \hline GH3536 [46] & ✓ & ✓ & & ✓ & & ✓ \\ \hline Custom austenitic SS. [37] & ✓ & ✓ & & & & ✓ \\ \hline SS316L [48; 49; 50; 51], 1.2767L tool steel & ✓ & & & & & \\ \hline SS316L [53; 54], Maraging steel, 2205 & & & & & & \\ Duplex SS. [55; 56] & ✓ & ✓ & ✓ & ✓ & & \\ \hline
17-4 PH SS [57] & & ✓ & & ✓ & ✓ & \\ \hline Maraging steel [58] & ✓ & ✓ & ✓ & ✓ & & \\ \hline AISI 420 Steel [59] & ✓ & & & ✓ & & \\ \hline \multicolumn{8}{c}{Titanium-Based Alloys} \\ \hline Ti6 Al4V [43], IN718 [60] & ✓ & ✓ & & ✓ & & \\ \hline TiB/Ti6 Al4V [44] & ✓ & ✓ & & ✓ & ✓ & \\ \hline TiNi [61], NiTi [62] & ✓ & ✓ & ✓ & ✓ & ✓ & ✓ \\ \hline Ti6 Al4V [63], Ti and Ti-13 Nb-13 Zr [63], Ti–43 Al–9V-0.5Y [64] & ✓ & ✓ & ✓ & & \\ \hline \multicolumn{8}{c}{Nickel-Based Alloys} \\ \hline IN625 [65] & ✓ & & ✓ & ✓ & ✓ & \\ \hline Hastelloy X [66] & ✓ & & & ✓ & & \\ \hline Ni + SAF2205-22 Cr [67], IN718 & & & & & & \\ \hline
[68; 69] & ✓ & & ✓ & ✓ & & \\ \hline Nickel-based alloy [38], IN718 [70; 71] & ✓ & & ✓ & & \\ \hline IN718 [60], alloy 247 LC [72] & ✓ & ✓ & & ✓ & & \\ \hline \multicolumn{8}{c}{Aluminum-Based Alloys} \\ \hline AlSi7 MgO.3 [73], AlSi10 Mg [72; 73; 74], Al-33 Cu [75] & ✓ & & & & \\ \hline Al2024 [74] & ✓ & & ✓ & ✓ & \\ \hline AlSi10 Mg [76; 77] & ✓ & & & ✓ & \\ \hline AlSi10 Mg [78; 79] & ✓ & & ✓ & & \\ \hline Al-3.6 Zn-0.6 Mg [79] & ✓ & ✓ & ✓ & ✓ & ✓ \\ \hline AddalloyTM [80], AlSi10 Mg [81] & ✓ & ✓ & ✓ & ✓ & \\ \hline \multicolumn{8}{c}{Other alloys/elements} \\ \hline Ta [82] & ✓ & & & ✓ & ✓ & \\ \hline Zn alloy [83] & ✓ & & & ✓ & \\ \hline Nb [84] & ✓ & & ✓ & & \\ \hline Ag [85], W [86] & ✓ & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Combinations of optical techniques using an electron source for various customized or less investigated LPBF-printing powder materials.  
generate a compiled 3D morphology image of the object [87]. The advantages of XCT are that it requires little or no sample preparation and is non-destructive to the sample. The disadvantages of XCT are that it is very time-consuming and expensive, and it can produce blurred material boundary images. An example of XCT analysis of LPBF-printed part porosities is provided in Figure 20 [88].  
**X-ray diffraction (XRD)** machines bombard incident X-ray beams at the investigated crystalline sample to analyze the crystallographic condition. The sample's crystalline structure causes the X-rays to diffract in many specific directions. Information on the angles and intensities can be used to determine the 3D electron density distribution. This distribution can estimate the average position of the atoms in the crystal and provide knowledge regarding the presence of defects within the crystal. The advantages of XRD are that it is a non-destructive technique, requires little to no sample prep time, and does not need vacuum operating conditions. The disadvantage of XRD is that it is considered less accurate for small crystalline structures and can produce peak overlays when performing non-isometric crystalline structure pattern indexing. Figure 21 shows an example of XRD pattern results for LPBF SS316 powder [89].