# Imaging Microstructure on Optically Rough Surfaces Using Spatially Resolved Acoustic Spectroscopy
## 3 Results
### Wavy Samples  
WAAM samples have varying degrees of _form_, with high levels of _waviness_ but less overall fine-scale _roughness_. Figure 7 shows the optical micrographs from three WAAM samples with different processing parameters. The top sample is built layer-by-layer without a rolling pass. The middle and bottom samples have an interpass rolling stage with 50 kN and 75 kN force applied, respectively. The optical images clearly show the layer-by-layer nature of the build and for the undeformed sample, additional significant \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Finish Type** & & & & & \multicolumn{3}{c}{**\% Drop-Out**} & & & \\ \hline \(R_{a}\) / **um** & **0.05** & **0.1** & **0.2** & **0.4** & **0.8** & **1.6** & **3.2** & **6.4** & **12.5** & **25** \\ Honing & 35 & 41 & 38 & 35 & 41 & 45 & \(\circ\) & \(\circ\) & \(\circ\) & \(\circ\) \\ Grinding & 0 & 1 & 1 & 0 & 1 & 2 & \(\circ\) & \(\circ\) & \(\circ\) & \(\circ\) \\ Lapping & 0 & 1 & 1 & \(\circ\) & \(\circ\) & \(\circ\) & \(\circ\) & \(\circ\) & \(\circ\) \\ Shot blasting & \(\circ\) & \(\circ\) & \(\circ\) & 0 & 5 & 5 & 17 & 13 & 23 & \(\circ\) \\ Grit blasting & \(\circ\) & \(\circ\) & \(\circ\) & 0 & 5 & 10 & 16 & 26 & 41 & \(\circ\) \\ Turning & \(\circ\) & \(\circ\) & \(\circ\) & 57 & 21 & 21 & 27 & 45 & 59 & \(\circ\) \\ Horizontal milling & \(\circ\) & \(\circ\) & \(\circ\) & 1 & 2 & 20 & 6 & 3 & 3 & \(\circ\) \\ Vertical milling & \(\circ\) & \(\circ\) & \(\circ\) & 1 & 1 & 0 & 2 & 13 & 4 & \(\circ\) \\ Spark erosion & \(\circ\) & \(\circ\) & \(\circ\) & 0 & 1 & 2 & 2 & 13 & 30 & 38 \\ \hline \(R_{a}\) / **um** & **0.5** & **1.4** & **2.8** & **4.6** & **6.8** & **9.6** & **12.8** & **16.4** & **20.5** & \\ Cast & 10 & 20 & 44 & 55 & 42 & 41 & 36 & 51 & 75 & \\ \hline \hline \end{tabular} \(\circ\)—surface of this roughness and finish type not available for testing.  
\end{table} 
Table 4: Drop out for each roughness comparator, i.e., ratio of unmeasurable positions to measurable positions on the sample surface. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Finish Type** & & & & & & **Single Shot SNR (dB)** & & & \\ \hline \(R_{a}\) / **um** & **0.05** & **0.1** & **0.2** & **0.4** & **0.8** & **1.6** & **3.2** & **6.4** & **12.5** & **25** \\ Honing & 23.7 & 23.2 & 24.9 & 23.7 & 21.9 & 21.7 & \(\circ\) & \(\circ\) & \(\circ\) & \(\circ\) \\ Grinding & 21.7 & 19.5 & 21.4 & 23.5 & 23.6 & 27.3 & \(\circ\) & \(\circ\) & \(\circ\) & \(\circ\) \\ Lapping & 25.7 & 24.8 & 24.5 & \(\circ\) & \(\circ\) & \(\circ\) & \(\circ\) & \(\circ\) & \(\circ\) & \(\circ\) \\ Shot blasting & \(\circ\) & \(\circ\) & \(\circ\) & 21.9 & 17.5 & 18.7 & 12.5 & 13.7 & 16.4 & \(\circ\) \\ Grit blasting & \(\circ\) & \(\circ\) & \(\circ\) & 22.6 & 19.5 & 13.9 & 10.2 & 8.8 & 10.8 & \(\circ\) \\ Turning & \(\circ\) & \(\circ\) & \(\circ\) & 21.0 & 24.9 & 23.1 & 22.5 & 19.0 & 14.0 & \(\circ\) \\ Horizontal milling & \(\circ\) & \(\circ\) & \(\circ\) & 24.4 & 23.2 & 17.4 & 20.6 & 24.7 & 25.9 & \(\circ\) \\ Vertical milling & \(\circ\) & \(\circ\) & \(\circ\) & 24.2 & 25.8 & 21.9 & 24.9 & 21.1 & 23.7 & \(\circ\) \\ Spark erosion & \(\circ\) & \(\circ\) & \(\circ\) & 21.9 & 18.5 & 13.9 & 13.2 & 14.7 & 14.0 & 16.1 \\ \hline \(R_{a}\) / **um** & **0.5** & **1.4** & **2.8** & **4.6** & **6.8** & **9.6** & **12.8** & **16.4** & **20.5** & \\ Cast & 17.0 & 15.8 & 13.9 & 11.3 & 11.6 & 13.2 & 14.3 & 14.2 & 13.2 & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Single-shot average SNR (dB) for all valid data points taken on each roughness comparator using the Quartet detector. colouration of the surface due to surface oxide/nitride layers forming during the time spent at elevated temperatures. The velocity maps show the underlying texture of the samples; for the undeformed case, there is the presence of large prior-\(\beta\) grains, i.e., the long columnar growth that continues up through the layers as the sample is built.