## 3 LPBF Sensing Techniques
### Miscellaneous Techniques  
Many other techniques have been employed to determine the part quality and quantify defect characteristics in LPBF-printed parts. These include using electromagnetic induction, cantilevered probes, and gas pycrometers  
**Eddy current testing (ECT)** detects and characterizes subsurface and surface defects of metallic components using electromagnetic induction. A magnetic field at a specified excitation frequency excites eddy currents in the metallic object. The EC flow pattern is perturbed in the presence of a defect, appearing as localized magnetic anomalies [203]. EC testing is ideal for testing surface flaws, has very high accuracy in conductivity measurements, and requires very little sample preparation. However, EC testing cannot detect defects parallel to the surface and is considered unsuitable for complex geometries. [PERSON] et al. [204] performed a feasibility test on an EC testing system's subsurface defect detection capability. The EC method successfully detected subsurface defects in Ti6 Al4V AM parts. The direct laser deposition (DLD) method was used to prepare the specimens, though EC in defect detection applies to LPBF. [PERSON] et al. [205] minimized EC losses in a high-silicon electrical steel (Fe-6.5%wt. Si) fabricated using LPBF to reduce defect population. Various cross-section designs were produced and analyzed regarding the EC losses. Various defects, such as lack of fusion, cracking, excessive surface roughness, electrical shorting between areas that should be isolated, and gas/keyhole porosities, were highlighted regarding unintended EC pathway circulation. **Atomic force microscopy (AFM)** probes operate using a supported, tiny spring-like cantilever with a tip on one end and a piezoelectric element to oscillate the cantilever at its eigenfrequency on the other end. A detector located very close above the cantilever detects any deflection or motion of the cantilever. A 3D image of the topographical shape of the sample surface can be detected at a high resolution from the reaction of the probe to the forces the sample imposes on it. The AFM does not include lenses or beam irradiation, eliminating  
Figure 35: Example of PAUT probe data provided by [202] showing experimental results for (**a**) 0.25 mm and 0.5 mm defects in LPBF sample 1, (**b**) 0.5 mm, 1 mm, and 2 mm defects in LPBF sample 1, (**c**) 2 mm defect in LPBF sample 2, and (**d**) the PAUT experimental setup. spatial resolution limitations caused by diffraction and aberration. The resolution has been noted to be higher than SEM and comparable with TEM, though AFM can only perform a single scan, which can be time-consuming and is affected by thermal drift and the hysteresis of the piezoelectric material. The forces applied to the probe over the sample are a function of their mutual separation and are used to measure its mechanical properties. [PERSON] et al. [146] utilized AFM to characterize the surface roughness of an SS316L LPBF-printed surface, as shown in Figure 36. [PERSON] et al. [95] determined the surface topology of various hatching methods for SS316L LPBF-printed parts using AFM. **Gas pycnometers (GP)** are devices used to measure the density of solid samples. A pathway connects two gas pycnometer chambers with a valve. The first chamber is a gas-tight chamber with a pressure transducer to house the solid sample, and the second chamber is a reference chamber with a fixed internal volume. A valve pushes pressurized gas into the first chamber and passes into the second chamber via the connection with a valve. The second chamber contains a vent valve. The sample volume can be directly estimated from the chamber volume, reference volume, and pressure ratios between the two chambers. The bulk relative density of LPBF-printed parts can be calculated using gas pycnometers. A gas pycnometer can help indicate whether the sample density is within the expected density range, which can help justify whether the part is fully dense or has significant porosities.