## 4 Recommended Sensing Technologies for Defect Types
### Recommended Acoustic and Ultrasonic Sensing Techniques for Defect Types  
Acoustic and ultrasonic techniques have been reported to capture the majority of reported defects. Acoustic techniques can detect surface quality, microstructure, and mechanical defects, while ultrasonic defects can only detect microstructure and mechanical defects. Balling surface-quality defects can be detected using AES and NRS. LOF porosity microstructure defects can be detected using acoustic techniques such as FBG and NRS and all ultrasonic techniques, including IU, LU, PU, RUS, and PAUT. Gas porosities can be detected using IU and LU. Keyhole porosities can be detected using IU. Liquidation and solidification cracking can be detected using AES. IU and LU can detect solidification cracking and delamination defects. Table 9 summarizes the possible sensing technologies for each defect using acoustic and ultrasonic techniques.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline
**Defect Type** & **Defect** & **SEM** & **TEM \({}^{*}\)** & **EBSD** & **EDS** & **XPS** & **EPMA** \\ \hline \multirow{4}{*}{Mech. defects} & Liq. cracking [29; 64; 232] & ✓ & & ✓ & ✓ & & \\ \cline{2-8}  & Sol. cracking [29; 65] & ✓ & & ✓ & & & \\ \cline{2-8}  & Delamination [44] & ✓ & & & ✓ & & \\ \hline \end{tabular}
\end{table}
Table 6: \(Cont.\)  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Defect Type** & **Defect** & **XCT** & **XRD** & **OM** & **CLSM** & **DSLR** & **DIC** & **OES \({}^{*}\)** & **APT \({}^{*}\)** \\ \hline \multirow{4}{*}{Surface-quality defects} & Balling [12; 191] & & & ✓ & ✓ & & & \\ \cline{2-8}  & Surface oxidation [11; 82; 108; 128] & ✓ & ✓ & & & & ✓ & ✓ \\ \cline{2-8}  & Surface roughness & & & ✓ & ✓ & ✓ & & & ✓ \\ \cline{2-8}  & [13; 49; 91; 125; 146] & & ✓ & ✓ & ✓ & ✓ & & ✓ \\ \cline{2-8}  & Denudation [15; 16] & & & ✓ & ✓ & ✓ & ✓ & & \\ \cline{2-8}  & Vaporization [15; 17] & ✓ & & ✓ & & & & \\ \hline \multirow{4}{*}{Microstructure defects} & LOF porosities [59; 66; 191] & ✓ & & ✓ & ✓ & & & \\ \cline{2-8}  & Gas porosities [20; 190; 197] & ✓ & ✓ & ✓ & & & & \\ \cline{2-8}  & Keyhole porosities [98; 156] & ✓ & & ✓ & & & & \\ \hline \multirow{4}{*}{Mech. defects} & Liq. cracking [29; 32; 115] & ✓ & & ✓ & & & ✓ & \\ \cline{2-8}  & Sol. cracking [32; 118; 190] & ✓ & & ✓ & & & ✓ & \\ \cline{2-8}  & Delamination [17; 33; 191] & & ✓ & & ✓ & ✓ & & \\ \hline \end{tabular}
\end{table}
Table 7: Applicability of optical sensing technologies (X-ray, light, and other sources) to detect LPBF defects.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Defect Type** & **Defect** & **TIC** & **IRTC** & **SWIR** & **STWIP** & **IPS** & **DSC \({}^{*}\)** \\ \hline \multirow{4}{*}{Surface-quality defects} & Balling [25] & & ✓ & & & & \\ \cline{2-8}  & Surface oxidation [82; 128] & & ✓ & & & & \\ \cline{2-8}  & Surf. roughness [158] & & & & ✓ & & \\ \cline{2-8}  & Denudation [150] & ✓ & & & & & \\ \cline{2-8}  & Vaporization [150] & ✓ & & & & & \\ \hline \multirow{4}{*}{Microstructure defects} & LOF porosities [155; 159] & & & ✓ & ✓ & & \\ \cline{2-8}  & Keyhole porosities [155; 154; 155] & ✓ & ✓ & & ✓ & \\ \hline \multirow{2}{*}{Mech. defects} & Sol. cracking [165] & & & & & & \\ \hline \end{tabular}
\end{table}
Table 8: Applicability of other optical and thermal sensing technologies to detect LPBF defects.