## 3 LPBF Sensing Techniques
### Optical Techniques
#### 3.1.1 Techniques with Electron Source
In EBSD, the diffracted electrons at each point form a pattern that can be detected using a backscatter detector (BSD) that produces kernel average misorientation (KAM), pole figures (PF), and inverse pole figure (IPF) maps [40]. KAM maps display the average misorientation between each pixel concerning its surrounding pixels [41]. It enables semi-quantitative analysis of strain distributions at near-surface depths in deformed microstructures. PFs and IPFs are stereographic projections where the orientations are plotted relative to the sample and crystal axes [42]. Often, PFs provide texture, and IPFs are used as a color key for crystal orientation maps (COMs). The advantages of EBSD include its relatively fast detection and analysis rate, its ability to scan and map large sample areas (mm) of interest, and its non-destructive nature. Additionally, it determines grain boundaries without etching and evaluates crystallographic textures. The disadvantages of EBSD include its need for vacuum compatibility and potential extensiveness. It requires precise surface preparation, including polishing, and multiple attempts using the EBSD to achieve high-quality results. Figure 16 displays an example of EBSD IPF imagery, highlighting changes in crystallographic texturing indicating grain boundaries and potential defects [43]. **Energy-dispersive X-ray spectroscopy** (EDS, EDX) is a surface analytical technique that measures the electronic structure behavior of the material when exposed to an electron beam. The exposure of an electron beam causes an electron in the inner shell of the material to be ejected, forming a vacant inner shell electron position. The produced X-rays from the ejection can be accumulated at all produced wavelengths from the sample using a solid-state semiconductor detector. Figure 15: Example of TEM imagery provided by [37], showing locations of stacking faults and interstitial atomic ordered structures of an LPBF supersaturated austenitic stainless-steel-printed part. EDS is used to determine and quantify the material's elemental composition. The advantages of EDS include its short measurement time, no need for pre-treatment processes such as surface polishing, and compact size. The disadvantages of EDS are that it could potentially provide poor energy peak resolution due to bremsstrahlung background interference and is somewhat inaccurate for heterogenous materials. An example of how EDX is utilized in defect detection is provided in Figure 17, showing how EDX can be used in tandem with SEM to analyze defects [44]. **X-ray photoelectron spectroscopy (XPS)** devices use the photoelectric effect like EDS to generate signature signals produced by the photoelectrons akin to a specific element. Figure 16: Examples of EBSD IPF imagery provided by [43] showing the change in crystallographic texturing along the build direction for various LPBF power (P) scan speeds (V). Figure 17: An example provided by [44] of EDX spectral data cross-analyzed with SEM micrographs, showing the worn surfaces after friction testing of an LPBF-printed part (large arrows indicate scan direction). (**a,b**) 0 wt% TiB2 sample showing parallel furrows and wear debris; (**c,d**) 3 wt% TiB2 sample showing a delaminated region defect; (**e,f**) corresponding EDX analysis results for point 4 (**b**) and point 5 (**d**). Unlike EDS, which is typically coupled with SEMs or TEMs, the XPS analyzer is a separate device that explicitly uses the detection data of the ejected electrons to determine the near-surface regional chemical composition. The advantages of XPS are that it collects data quickly, is efficient in detecting surface contaminants, and is non-destructive to the sample. The disadvantages of XPS are that significant relative errors are common and that the reproducibility of the same data at different facilities is unsteady. Figure 18 shows an example of XPS data for an LPBF-printed part [45].