# Imaging Microstructure on Optically Rough Surfaces Using Spatially Resolved Acoustic Spectroscopy
## 3 Results
### Wavy and Rough  
The machined samples presented here are predominantly flat with little _form_, but depending on the machining depth, they can have _waviness_ along with fine-scale _roughness_.  
A commercially pure titanium sample with three surface conditions, polished (top row) and milled to \(R_{\mathrm{a}}\)s of 1.6 and 3.2 \(\upmu\)m, has been measured. The left column of Figure 10 shows the optical maps from each surface, with the right-hand side presenting the corresponding velocity map. The rough surface maps are very similar to that in the polished state. Variations between the pictures are due to several factors; for example, removal of material from the polished state exposes slightly different grains, which is particularly noticeable where the features are small. Additionally, small variations in the sample placement in the instrument lead to small rotational differences that influence the velocity to different degrees depending on the plane of each grain. A small change in the rotation of the sample for the large blue grain, which appears to be aligned with the prism plane, can cause a larger shift in velocity than for other grains.  
A machined titanium (Ti-6 Al-4V) billet slice (\(\mathrm{R_{a}}\approx 0.8\,\upmu\)m) was scanned. A selection of the optical image is overlaid on the velocity map in Figure 11. This sample was scanned by rotating the billet and scanning a ring of data before moving the SRAS head sideways across the centre line of the sample. This was to produce data sets to mimic in silico scanning in a commercial lathe. The wave propagation direction changes for each position on the sample, and in this case, the waves are propagating towards the centre of the sample. Despite the surface roughness, 90% of the surface produces valid SRAS measurements, and the machining lines are not shown in the velocity maps. The structure within the velocity maps shows the fine microstructural texture of the material.  
Figure 9: LPBF: (**a**) Ti-6 Al-4V, acquired using the SKED showing that combining scans with different detector positions can reduce data loss. (**b**) Al sample with designed defects measured on the top and side wall with the Quartet detector.  
The lower half of the billet reports the results of scanning radially. Maintaining a constant angle between scan lines, this means that the pixel size increases from the centre towards the outer edge. In contrast, the upper half of the scan is such that constant pixel size is maintained across the scan area. Therefore, the outer and inner rings are consistent. Careful choice of scan strategy is required to obtain well-sampled data.  
Figure 11: SRAS velocity map of machined Ti-6 Al-4V billet, with surface roughness of \(\rm R_{a}\approx 0.8\,\upmu m\). The lower half of the billet shows a velocity map captured at a fixed angular resolution, leading to an increasing pixel size towards the edge of the sample. In contrast, the velocity maps in the upper half maintain a constant pixel size. A cropped photograph of the sample is inset in the upper half to show the surface finish.  
Figure 10: Commercially pure titanium sample; the sample was polished, then milled twice over, creating a rough surface, with SRAS measurements captured after each step. Optical maps of the surface are shown in the left column with velocity maps to the right.