## 3 Results
### Detected Waveform Evolution  
Rayleigh wave experiments were conducted on a Ti-6 Al-4V specimen in the LU test bed. Process parameter set C (see Table 1) was used for this deposition. During these experiments the generation position remains fixed while the reception point is moved in 4 mm increments. 512 synchronous averages were used to minimize the random noise in the signals received at propagation distances of 4-24 mm. The source for finite amplitude Rayleigh waves was obtained by setting the Nd:YAG Q-switch delay to 185 us, giving a pulse energy of approximately 335 mJ, which will be referred to as the high pulse energy. The high pulse energy ablates the surface, which we deem acceptable for process monitoring since material will be deposited on top of the current surface as the build progresses. Low pulse energy was used to generate predominantly linear Rayleigh wave pulses by increasing the Q-switch delay to 290 us. It is worth noting that filtering can drastically change the waveform shape, therefore in this article filtering has not been used. The received waveforms from three longitudinal scan trials are compiled in Figure 6 for (i) the as-received baseplate using low pulse energy, (ii) the as-received baseplate using high pulse energy, and (iii) the as-built AM deposition using high pulse energy. It is worth reiterating that while the chemical compositions of the baseplate and AM deposition are nominally the same (Ti-6 Al-4V), the microstructures are much different as documented  
Figure 5: Cross-sections of (**a**) Ti-6 Al-4V and (**b**) IN-718 AM depositions on baseplate. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Material** & \(S_{a}\) **(μm)** & \(S_{q}\) **(μm)** & \(S_{z}\) **(μm)** \\ \hline Ti-6 Al-4V baseplate & 1.14 & 1.42 & 10.25 \\ Ti-6 Al-4V Set A & 33.04 & 41.39 & 371.4 \\ Ti-6 Al-4V Set B & 33.09 & 42.26 & 385.7 \\ Ti-6 Al-4V Set C & 30.63 & 38.84 & 397.0 \\ IN-718 Inspalte & 3.72 & 4.64 & 33.94 \\ IN-718 Set D & 19.59 & 25.23 & 264.7 \\ IN-718 Set E & 18.27 & 22.65 & 318.5 \\ IN-718 Set F & 17.52 & 21.76 & 352.1 \\ IN-718 Set G & 13.89 & 18.16 & 237.8 \\ IN-718 Set H & 25.60 & 31.82 & 293.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Profilometer-measured surface roughness. in previous work [60] and the surface roughness is also much different (see Table 3). The waveforms are the particle displacement component normal to the surface, \(u_{3}\), with the laser interferometer detected AC signal normalized with respect to the mean DC level. Note that the variability in the DC level was approximately five times larger for tests on AM depositions than on the baseplate. The line-focused Nd:YAG laser beam generates a low-frequency airborne wave that is detected by the interferometer after the Rayleigh wave arrival. This wave is observed in the A-scan from 4 mm but is not of interest here. The waveforms in Figure 6 were generated by impulse loading rather than a sinusoid and thus do not compare well with the \(u_{3}\) waveforms in Figure 3. The Rayleigh waveform generated by low pulse energy is the most repeatable in Figure 6, both for the three trials and as a function of propagation distance. Thus, the V-shaped waveform represents a linear Rayleigh wave, as the nonlinearity is too small to notice. In contrast, the waveforms generated by high pulse energy, having much higher peak-to-peak amplitudes (note the amplitude scales are different for low and high pulse energy), have a different waveform shape and evolve with propagation distance. These two features could be invaluable for characterizing the material nonlinearity. For low pulse energy, the segments of the V-shaped pulse appear to have nominally the same steepness. However, for high pulse energy, the first segment is significantly steeper (and smoother) than the second segment. Thus, it may be possible to define a steepness ratio to characterize nonlinearity independent from propagation distance. These observations apply to high pulse energy for both the baseplate and the AM deposition.