## 3 Results and Discussion
### Surface Roughness  
Table 7 presents the results of surface roughness for the specimen after FSW (base variant) and after SDB according to PS/DC 3\({}^{2}\) plan. Selected roughness parameters such as Height parameters, Spatial parameters, Hybrid parameters, Feature parameters, and Functional parameters were analyzed. The base surface was characterized by a relatively high value of the height parameters of the surface. The average arithmetical mean height of the surface Sa was 6.46 \(\upmu\)m (Figure 6a). The largest maximum height of the surface Sz was 42.5 \(\upmu\)m, which consisted of the maximum height of peaks Sp = 18.8 \(\upmu\)m and the maximum height of valleys Sv = 23.7 \(\upmu\)m. The surface was also characterized by a slight skewness (Ssk = \(-\)0.592), and a negative value of this coefficient indicates a surface with plateau hills. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multicolumn{2}{c}{**Analyzed**} & \multicolumn{8}{c}{**Experimental Run No. **} \\ \multicolumn{2}{c}{**Parameters**} & \multicolumn{8}{c}{**Experimental Run No. **} \\ \hline  & **1** & **2** & **3** & **4** & **5** & **6** & **7** & **8** & **9** & **Base** \\ \hline Height & & & & & & & & & & \\ Parameters & & & & & & & & & & \\ \  
The kurtosis of height distribution Sku, defined as a measure of the probability density of the height of the surface roughness, was 2.32. This parameter is most often a measure of the number of sharp and high peaks or sharp and deep valleys (e.g., scratches or surface scratches). For a perfectly random surface, the Sku parameter takes a value of 3. Values lower than 3 indicate less steep areas of the surface. As a result of the SDB, a clear reduction in the height parameters of the surface was obtained. The parameter Sa was in the range of 0.333-1.7 \(\upmu\)m, and the parameter Szwas in the range of 5.86-36.1 \(\upmu\)m. A similar reduction of parameters was also obtained in the case of Sp, Sv, and the feature parameters such as S10z, S5p, and S5v. It is worth noting that particularly high values of Sa and Sz parameters were expressed in specimens 2 and 3, burnished with the lowest value of the burnishing force \(P=40\) N. The analysis of the geometric surface structure showed that burnished surfaces with a force of \(P=40\) N contained numerous surface defects as a result of the burnishing process. The increase in the value of the burnishing force to \(P=60\) N resulted in the elimination of this unfavorable phenomenon. The scatter of Ssk and Sku parameters of the burnished surfaces was quite large and ranged from \(-0.813\) (experimental run no. 1) to 1.07 (experimental run no. 4) for Ssk. For Sku, it ranged from 2.59 (experimental run no. 7) to 11.7 (experimental run no. 8). The unfavorable impact of the lowest burnishing force value is also evidenced by the fact that individual values of the spatial parameter Str (texture aspect ratio of the surface) were obtained. This parameter expresses the relationship between the shortest and longest decay of the correlation function and takes values in the range of 0-1. Values close to 1 indicate that the GSS has a high level of isotropy, and values close to 0 characterize anisotropic surfaces (Figure 7). The sliding burnishing process is usually characterized by anisotropicity of the obtained surfaces, which was confirmed in the case of the \(P=40\) N. The results of the simulations were obtained in the same way as for the \(P=40\) N.  
Figure 6: Isometric views (**a**) of the base surface and the SDB surfaces: (**b**) \(P=40\) N, \(f=1000\) mm/min (experimental run no. 2), (**c**) \(P=60\) N, \(f=1000\) mm/min (experimental run no. 8). of specimens 6-9. On the other hand, the Str value for specimens 1-5 places them in the range of mixed textures or, as in the case of specimen 3, close to random surfaces. The Sal auto-correlation length is the shortest segment over which the normalized auto-correlation function decreases to a value \(\tau\) which is greater than or equal to zero and less than 1.