## 3 LPBF Sensing Techniques
### Ultrasonic Techniques
The B-scan images allowed an accurate evaluation of the flaw's location, size, and depth. [PERSON] et al. [187] utilized a laser ultrasonic technique to detect defects in LPBF-printed NiTi-based alloy samples. Results and validation of the ultrasonic data using XCT indicated that the pulsed laser-generated ultrasonic waves were sensitive to internal defects, and thus, LU testing can identify certain defects at the sub-millimeter level (Figure 33). [PERSON] et al. [188] examined an LU method for detecting subsurface defects in an LPBF aluminum alloy flat plate with artificial defects. Subsurface circular defects could be detected for diameters below 1 mm that were undetectably small from previous studies. [PERSON] et al. [189] proposed an ultra-fast LU imaging method to monitor the LPBF process efficiently. A multi-circle combined scanning strategy and defect location algorithm were constructed to improve detection efficiency. A surface wave focusing algorithm (SWFA) was established to solve the low signal-to-noise ratio (SNR) problem induced by rough surface signals. The scanning efficiency of single-layer inspection was reportedly improved by more than 300% compared with the C-scan imaging method. **Pulsing ultrasonic (PU)** systems collect ultrasonic waves through pulser-receivers, which generate electrical excitations in a transducer to create an ultrasound pulse-echo. Pulsing ultrasonic systems are non-destructive and highly sensitive, allowing for the detection of minimal flaws and high accuracy in determining the depths of internal flaws. However, pulsing ultrasonic systems cannot accurately estimate defects' orientation, size, nature, and shape. [PERSON] et al. [190] utilized a pulser-receiver to cause a shock-impulse excitation in CoCr LPBF-printed samples. The ultrasonic contact pulse-echo velocity measurement method detected minor changes in the porosity concentration, hinting at the ability to detect process deviations such as pore defects in situ. [PERSON] et al. [191] used ultrasonic testing to identify the lack of fusion defects of LPBF-printed parts, as shown in Figure 34. Though the ultrasonic method used to generate the scan images was not mentioned, the data was assumed to be generated using a pulse receiver. [PERSON] et al. [192] presented an ultrasound measurement of the segmental temperature distribution (US-MSTD) method for evaluating the microstructural heterogeneities and spatial variabilities of mechanical properties in LPBF-printed cylindrical 31 AL aluminum alloy samples. The segmental speed of sound in the sample was collected using a square wave pulse/receiver. The excitation pulse was sent to an ultrasonic transducer coupled to the flat base of the sample. The results obtained from the ultrasound were validated using XCT. [PERSON] et al. [193] studied the dependence of ultrasonic phase velocity on the defect formation of LPBF SSS16L components by varying the hatch spacing. A pulse receiver was attached to the sample surface to generate the ultrasonic phase velocity. [PERSON] et al. [92] measured the ultrasonic sound waves of SS316L LPBF-printed components using a pulser/receiver to recognize the spalling response loaded in different scanning orientations. [PERSON] et al. [194] utilized a pulse echo configuration and 10 MHz ultrasonic wave to determine the part quality of a 100 Cr6 LPBF-printed part, determining the location of intentionally-created defects. Positives: non-destructive. Negatives: high sensitivity allows for the detection of minor flaws and high accuracy in determining the depth of internal flaws; negatives: cannot accurately estimate the size, orientation, and shape of the defect; cleaning and removing beforehand prep. Figure 34: Pulsing ultrasonic results of an LPBF-printed specimen showing a lack of fusion locations for (**a**) 65 m\({}^{3}\)/h shielding gas flow rate and (**b**) 32.5 m\({}^{3}\)/h shielding gas flow rate [191].