# Abrasive Surface Finishing on SLM 316L Parts Fabricated with Recycled Powder
## 3 Results and Discussion
### Microstructure  
The 3D surface roughness results of the four specimens above can be seen in the macro scale in the below pictures. Take into account that for each measurement, an appropriate height scale was selected to better visualize the maximum height and depth of the surface irregularities.  
From Figures 10 and 11 below, it can be observed that the surfaces of the SLM parts are characterized by droplets of melted metallic powder. It can be noticed that in Figure 11(a1), there is a spherical metallic droplet standing out from the others. The reason for this is that the used recycled powder contains unfiltered longitudinally sintered powder particles, as described in [15]. Those pre-melted pieces are accumulated with droplets in this run, subsequently forming the spherical outlier. Besides, the ones with the worst roughness are the angled sides, Figure 10(a2) and Figure 11(a2), with the presence of several peaks and  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Scanning** & **No.** & **Sa [\(\mu\)m]** & **SaZk** & **SaTop** & **Sz [\(\mu\)m]** & **SzZk** & **SzTop** \\
**Strategy** & **No.** & **Sa [\(\mu\)m]** & **[\(\mu\)m]** & **[\(\mu\)m]** & **Sz [\(\mu\)m]** & **SzTop** [\(\mu\)m]** & **SzTop** [\(\mu\)m]** \\ \hline \multirow{2}{*}{Meander} & 1 & 14.1 & 32.1 & 10.9 & 134.6 & 225.6 & 153.8 \\  & 7 & 0.9 & 6.7 & 1.6 & 68.4 & 98.4 & 62.0 \\ \hline \multirow{2}{*}{Stripes} & 1 & 14.9 & 46.5 & 13.4 & 284.6 & 361.0 & 123.6 \\  & 8 & 0.9 & 3.7 & 1.0 & 44.8 & 87.8 & 67.5 \\ \hline \hline \end{tabular}
\end{table}
Table 5: As-built and best 3D surface roughness results, grouped according to the types of surfaces for the meander and stripes strategy.  
Figure 9: As-built and best 3D surface roughness results grouped according to the types of surfaces for the meander and stripes strategy: (**a**) Sa, (**b**) Sz.  
valleys. As aforementioned, thanks to the Upskin function, the top sides as the as-built condition, Figure 10(a3) and Figure 11(a3), appear to be the smoothest.  
Figure 10: 3D surface roughness with the meander strategy of: (**a**) as-built, (**b**) best post-treated. From top to bottom: **1**. vertical, **2**, angled, and **3**, top faces.  
One of the obvious traits for post-treated surfaces is the disappearance of the peaks, while valleys remain, which is due to the nature of the abrasive process. After the elimination of the peaks, the depth of the valleys of the angled sides is revealed and is the most severe in comparison with the two other types of surfaces, proving the significant impact of the \"staircase\" effect on the SLM 316L parts. It is worth noting that the machine built-in functions, such as Upskin and Downskin, can help to improve significantly the surface roughness of the exposed surfaces, which can be obviously observed on the top sides, Figure 10(a3) and Figure 11(a3). Additionally, post-processing can significantly reduce the  
Figure 11: 3D surface roughness with stripes strategy of: (**a**) as-built, (**b**) best post-treated. From top to bottom: **1.** vertical, **2.** angled, and **3.** top faces.  
waviness of these as-built top sides, from Figure 10(a3-b3) and Figure 11(a3-b3). In general, the peaks and the waviness of the as-built surfaces undergoing the proposed treatment process were notably reduced. Especially for the vertical sides of both scanning strategies, the roughness reduction of Sa down to 0.9 \(\upmu\)m was recorded.