## 3 Results  
### SAW Velocity Images on As-Deposited SLM Samples  
In order to realise an in-situ additive manufacturing inspection system, the technique must be able to operate on the rough surfaces produced by the stochastic manufacturing process. The additive manufacturing process considered for this study was selective laser melting (SLM)--using commercially available SLM machines, the flatness of parts in the macro zone can be well controlled  
Figure 6: A large grain T16246 block (**a**) was scanned using both the smooth surface SRAS setup and rough surface SRAS setup. The values shown in (**a**) indicates the roughness (in \(R_{a}\)) in that area of the sample. The measured SAW velocity (m/s) of the sample is shown in (**b**) using the smooth surface setup (**c**) using the rough surface setup. A box in (**b**) indicates the area of the sample that were scanned using both systems. Images (**d**,**e**) show the maximum signal amplitude (arb. units) detected using the smooth and rough surface setups respectively—both images show a decrease in response in the rougher region (which supports the findings from the previous experiment), however, a measurable signal is still observable when using the rough surface setup in the rough region.  
(i.e., topology), with the micro level roughness of the part defined by various built parameters such as powder size, material, build pattern regime, and applied energy [6].  
A \(10\times 10\times 10\) mm\({}^{3}\) cube was built using Ti64 powder feedstock (powder size: \(\sim\)\(\diameter\)30 \(\upmu\)m) on a Renishaw AM250 machine. The sample was made using a meander-90 \({}^{\circ}\) hatching pattern, with the laser build power set to 110 W.  
As stated, the finished-part surface roughness is dependent on a number of machine and environmental factors--the same is also true for part-built components, a consideration for inspecting in-situ. A reasonable assumption would be that the finished-part surface roughness would be comparable with the inter-layer profile, given unchanging and controlled build parameters.  
The surface profile of the built part was measured using a focus variation microscope (Alicona InfiniteFocus)--a reconstruction of the sample surface profile is shown in Figure 7b, with the surface profile measurements made by this instrument shown in Figure 7c. The \(S_{a}\) parameter, an areal roughness measurement (mean height deviation of a surface area), of the centre area is highlighted in the figure--this is roughly \(S_{a}\approx 6\)\(\upmu\)m.  
Before performing a SRAS scan on the sample, a line was scored into the surface to provide an identifying mark and was then optically imaged using a microscope, shown in Figure 8a. A small area SRAS scan (3 mm \(\times\) 5 mm) was conducted with 25 \(\upmu\)m steps along both the x and y axes using a \(\lambda_{g}=250\)\(\upmu\)m pattern, producing a frequency \(f_{s}\approx 12\) MHz. The produced SRAS velocity image is shown in Figure 8b. The image shows the variation in the texture across the sample, with the velocity plotted within the expected range for Ti64. This velocity map is consistent with other Ti64 samples produced in a previous publication [10], as well as SLM Ti64 texture images seen in literature [33; 34]. The images also show areas of low signal amplitude as a grey colour. This could be due to a low light return due to a large surface height variation--these features are seen in the optical micrograph. Additionally, this could indicate the presence of subsurface pores or cracks which do not allow SAW propagation [9].  
Figure 7: **(a)** Photograph of the Ti64 selective laser melting (SLM) sample under inspection, **(b)** a surface profile map of the SLM sample measured using the focus variation microscope, **(c)** height parameters measured using the microscope, which includes the highlighted \(S_{a}\) value (\(\sim\)6 \(\upmu\)m).