## 3 LPBF Sensing Techniques
### Ultrasonic Techniques  
Like acoustic techniques, ultrasonic techniques utilize generated mechanical waves to analyze the quality of LPBF parts. One of the main differences between ultrasonic and acoustic emissions is that for ultrasonic techniques, the ultrasonic waves in the sample are artificially created by forced interaction with an external source. Typically, ultrasonic techniques are performed ex situ. **Immersion ultrasonic (IU)** systems are conducted in a water immersion tank with a pulser/receiver and focused transducer to collect routine incidence pulse/echo measurements. A study used IU-measured ex situ ultrasonic backscattering data induced by the microstructure in LPBF-printed samples using a conventional immersion ultrasonic C-scan system and expected frequencies (see Figure 32) [182]. The ultrasonic backscattering data collected from the immersion ultrasonic system effectively detected the flaws of SS316L samples fabricated using LPBF. Figure 31: Example of FBG acoustic signals confirming pores in LPBF stainless-steel print (modified from [179]). Figure 32: Example provided by [182] showing the immersion ultrasonic C-scan images of an LPBF SS316L printed part segmented by three independent methods: (**a**) time-dependent thresholds, (**b**) 0.5 V fixed threshold, and (**c**) 1.5 V fixed threshold. Lack of fusion and irregular pore defects were observed. [PERSON] et al. [183] performed IU testing using Parametric(r) NDT immersion focus probes with the sample mounted inside a plexiglass tank filled with water. Porosities were observed from the IU data, and the average porosity size was about 0.3 mm. [PERSON] et al. [184] used linear phased array probes to perform IU experiments to detect artificially embedded defects with various shapes and sizes in AlSi10 Mg cubic samples. The IU system could detect defects as small as 0.75 mm, and the IU defect detection capabilities were validated with XCT data. **Laser ultrasonic (LU)** systems typically comprise two pulsed lasers and a recording device such as an interferometer or laser Doppler vibrometer (LDV) recording device. The two pulsed lasers trigger the absorption of the incident laser light pulse, which can cause plasma to form directly above the impact point. Once the energy of the plasma reaches a certain level, the expanding plasma is ejected from the surface of the sample and generates ultrasonic waves from the impulsive recoil force that propagates into and along the sample [185]. The interferometer records two pieces of information: (1) a DC monitor signal, which is proportional to the power of the detector beam signal and is a measure of the light reflected from the sample surface, and (2) an AC voltage corresponding to the instantaneous out-of-plane surface displacement. The returning reflected and diffracted ultrasonic wave signals can distinguish the different propagation modes, which can be used to predict the presence of subsurface or surface defects in the area between or adjacent to the two lasers [186]. LU has high reproducibility, fast scanning, and is usable in harsh environments, though it is less sensitive than piezoelectric-based ultrasonic inspection systems. Another challenge the LU system has is that ultrasonic waves' generation efficiency depends on the material's absorption properties. Additionally, laser safety precautions are necessary. Using a laser ultrasonic system, manually introduced subsurface and surface-breaking micro-defects of LPBF metallic samples were successfully detected [150]. [PERSON] et al. [185; 186] investigated the capabilities of LU techniques to detect manufactured subsurface defects in Ti6 Al4V LPBF samples. At various depths, subsurface porosities and spherical voids below the surface were successfully detected using LU. [PERSON] et al. [153] compared the inspection capabilities of LU and laser thermography on IN 600 LPBF samples. Flaws were purposefully created to establish the sensitivity to detect defects. LU effectively detected the typical micrometric defects of AM products.