# Imaging Microstructure on Optically Rough Surfaces Using Spatially Resolved Acoustic Spectroscopy
## 2 Materials and Methods
### Materials and Samples
#### 2.3.4 Additive Manufacturing Specimens  
This article includes results from three specimens produced using the WAAM process with Ti-6 Al-4V welding wire feedstock onto a titanium substrate. A pulsed gas tungsten arc welding torch, with argon shielding, was used to deposit structures made of 20 single-bead layers, giving an approximate final width of \(\sim\)6 mm and height of \(\sim\)24 mm. Following the deposition and cooling of each layer, a 100 mm diameter roller was passed over the top surface to create the deformed specimens. The rolling applies vertical forces of 50 kN or 75 kN, which is monitored by a load-cell. More information on the build process used to produce these specimens can be found in [40], whilst further detailed information on the WAAM technique can be found in [PERSON] et al. [41]. The Quartet detector was used to scan a 8 \(\times\) 12 mm area using 2000 averages per point with a step size of 100 \(\times\) 100 \(\upmu\)m. A final WAAM specimen was made from a single deposition bead of Ti-6 Al-4V. An area of 36 \(\times\) 16 mm was scanned using 500 averages and a step size of 100 \(\times\) 100 \(\upmu\)m.  
A further three specimens manufactured by L-PBF are also presented in this work. Two (Al-block and Ti-cube) were manufactured using a Realizer SLM50 (ReaLizer GmbH, Borchen, Germany) equipped with a 100 W continuous wave laser at 1064 nm wavelength. The laser was used to melt and fuse powder feedstock layer-by-layer to build the sample.  
In the case of the Al-block, AlSi10 Mg powder was used, and Ti-6 Al-4V powder was used for the Ti-cube.  
The titanium sample was scanned with the SKED detector over an area of \(10\times 10\,\mathrm{mm}\), using 128 averages per point and a step size of \(50\times 50\,\mathrm{\upmu m}\).  
The Al-block was designed with six side holes, with diameters ranging from \(0.5\,\mathrm{mm}\) and \(1\,\mathrm{mm}\). The sample was scanned with the Quartet detector, and the top face used 200 averages and a \(17\times 9\,\mathrm{mm}\) scan area with a step size of \(100\times 200\,\mathrm{\upmu m}\). A \(7\times 5\,\mathrm{mm}\) area on the side wall was also measured, using 8000 averages and a step size of \(100\times 67\,\mathrm{\upmu m}\). The side wall was particularly challenging to work on due to the extreme level of roughness caused by unmelted particles sticking to this surface and so required more averages to obtain a good SNR than the top face. Further detail on the Al-block and Ti-cube can be found in [PERSON] et al. [42] and [PERSON] et al. [43], respectively.  
The third L-PBF sample, an SRAS-sample, was manufactured in Ti-6 Al-4V using a Renishaw AM250 (Renishaw plc., Wotton-under-Edge, UK). This was measured with the Quartet detector over an area of X by Y mm, using 4000 averages per point and a step size of \(100\times 100\,\mathrm{\upmu m}\).  
A final WLAN sample was fabricated from Ti-6 Al-4V, with the addition of boron towards the build centre. The deposition was built in a circular pattern on a titanium substrate. This was scanned with the Quartet detector over an area of X by Y mm using 2000 averages per point and a step size of \(100\times 100\,\mathrm{\upmu m}\).