## 3 LPBF Sensing Techniques
### Miscellaneous Techniques
Gas pycnometry has been used alongside the Archimedean method to determine the bulk density of LPBF-printed parts [90; 92]. Positives of a gas pycnometer include high volumetric measurement accuracy, high reproducibility, and operation in a closed system, which eliminates many external factors. Negatives: It requires significant sample volumes, is challenging to clean and dry, is potentially time-consuming, and is expensive. **Conversion electron Mossbauer spectroscopy (CEMS)** is a susceptible method for investigating the surface of complex, highly absorbing samples. Mossbauer spectroscopy examines iron's valence state, which cannot be differentiated using EPMA. CEMS is a non-destructive technology that can provide high-resolution data; however, it is known to be vibrationally sensitive and requires cryogenic conditions. [PERSON] et al. [97] investigated corrosion-resistant alloy 800H steel samples fabricated using LPBF, as shown in Figure 37. CEMS proved that phase separation does not occur during the LPBF process and that the oxidation of the steel does not cause hot cracking in the samples. **Neutron diffraction (ND)** uses an accelerator to produce an incident neutron beam that can create a wavelength spectrum. The diffracted neutrons can be measured using detector banks. The through-thickness resolution of the specimen can be specified using a set of radial collimators in front of the detectors. Neutron diffraction is more reliable in determining elemental composition than X-ray or electron beam methods and can detect residual stresses beneath the surface of the investigated sample. However, significant energy is required to produce the large number of neutrons needed. [PERSON] et al. [206] used neutron diffraction to measure the longitudinal and transverse residual stresses in pre  
Figure 36: AFM-generated surface roughness profiles comparing SS316 wrought and LPBF-printed samples [146]. and post-annealed SS316L LPBF parts, as shown in Figure 38. [PERSON] et al. [68] measured bulk residual stresses in IN718 LPBF samples using the neutron diffraction technique. The sensitivity of the measured interplanar distance for stress-free sample strains in the three orthogonal directions could be attributed to the local variation in the microstructure defects. **Electrochemical impedance spectroscopy (EIS)** is used to analyze the effects of corrosion on the microstructure and electrochemical behavior. The electrochemical value measured at the electrode interface reflects the magnitude of the property, and the concentration of chemical species is measured. The steady-state technique utilizes small signal analysis and probe signal relaxation over various applied frequencies. The corrosion rate is highly dependent on the microstructure, the difference between the constituent phases, and the density of the beta-grain boundaries [206]. Due to the layered structure present in AM, mechanical properties and corrosion resistance differ from those of standard manufacturing. The corrosion resistance of LPBF samples is usually higher than that of standard manufactured parts [206]. EIS is a quick and non-destructive method for analyzing corrosion effects on LPBF-printed microstructures. However, accuracy depends on prior measurements and developed models, which can be pretty complex. [PERSON] et al. [207] confirmed using EIS  
Figure 37: Example of Mössbauer spectrum data provided by [97] showing (**a**) comparison of data mixture of austenite non-magnetic steel (dark-yellow singlet), magnetic steel (orange sextet), and Fe–O oxide (red doublet), (**b**) CEMS spectral data of A800H steel industrial plate, and (**c,d**) LPBF-manufactured plates of 1O, and 1M. Figure 38: Longitudinal stress map generated using ND along the central longitudinal plane of SS316L LPBF-printed sample [206]. that LPBF-printed parts have better corrosion performance than EBM due to the higher charge transfer resistance of the LPBF-fabricated Ti6 Al4V alloy at all immersion times. [PERSON] et al.