## 5 Summary  
The objective of this article is to characterize the ultrasonic properties of the as-fabricated state of additively manufactured IN718 structures. Samples were fabricated using DED technique, and three parameters; ultrasonic velocity, and attenuation and backscatter coefficients; were measured in the as-fabricated state, i.e., no post-processing or heat treatments. This forms the baseline ultrasonic response of DED structures, which were compared with the conventional IN718 reported in the literature. The purpose of the ex situ measurements reported here is to highlight the importance of first understanding the baseline ultrasonic response before we develop in situ characterization, i.e, inspection while the structures are being fabricated. The results and observations from this study are summarized below:  
* The ultrasonic velocity shows that the DED sample exhibits a noticeable anisotropy compared to the conventional sample. Specifically, the build direction has a large influence on the ultrasonic velocities. The DED sample has a lower longitudinal velocity compared conventional samples, which could possibly be attributed to the lack of \(\gamma^{\prime\prime}\) phase. However, the transverse shear velocity along the D1-2 plane was higher than conventional sample, which is conflicting with the hypothesis. * The DED sample exhibits significantly higher attenuation (\(-22\) dB) compared to conventional sample, and the presence of humps were observed. The origin of these humps in the attenuation curve are debatable and presently there is no explanation. * Ultrasonic backscatter results also suggest that the DED sample exhibits higher ultrasonic backscatter compared to conventional sample. Due to the periodic noise, strong peaks could be observed at certain frequencies. However, outside that frequency range, the FOM values are similar to conventional samples. The backscatter along the build direction was noticeably higher than the transverse directions. * Heterogeneity: The DED sample shows a \(-10\) dB spatial variability in amplitude, which is considerable compared to conventional samples. It was also observed that this heterogeneity might be dependent on direction. The high attenuation and strong backscatter suggest that the as-fabricated sample could easily be mistaken for a defective sample. This poses a strong challenge when we use ultrasonic NDE to study the influence of processing parameters or other factors. If the as-fabricated state cannot be understood, any changes in the ultrasonic response based on the changes in processing parameters cannot be understood either. This becomes a major roadblock in moving towards a NDE-based qualification of these structures. The heterogeneity observed in the DED sample can be a major hurdle in developing qualification standards for DED samples. However, the attenuation and backscatter measurements are rich with information that we currently do not understand. Extracting more features from these measurements could possibly allow us to develop more advanced qualifications compared to the existing qualification tools, which use velocity, attenuation and backscatter coefficients. Such an exercise will be explored elsewhere. Finally, we do not believe these observations can be generalized for all metallic structures that are fabricated using DED. These observations maybe specific to IN718 and other structures which have grain boundary precipitation and meta-stable martensitic phases in their microstructure. The results also suggest that more research is required to fully understand the origins of the baseline ultrasonic response, especially wave propagation in complex microstructures. **Supplementary Materials:** The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app13148137/s1](https://www.mdpi.com/article/10.3390/app13148137/s1)]([https://www.mdpi.com/article/10.3390/app13148137/s1](https://www.mdpi.com/article/10.3390/app13148137/s1)), Reference [68] are cited in the supplementary materials. Conceptualization, S.K.C.