# Solid-State Rotary Friction-Welded Tungsten and Mild Steel Joints
## 3 Results and Discussion
### Instrumented Indentation Test  
The HV0.1 hardness distribution revealed changes in the properties of the friction-welded joint. However, due to the size of the indentation and the transition zone, its  
Figure 10: The hardness distribution as a function of the distance from the tungsten–S355 steel interface.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Spectrum Label** & **Spectrum 1** & **Spectrum 2** & **Spectrum 3** & **Spectrum 4** \\ \hline Mn & - & 1.10 & 0.88 & 1.09 \\ Fe & 1.02 & 93.70 & 81.71 & 97.91 \\ W & 98.98 & 5.19 & 17.40 & 1 \\ Total & 100 & 100 & 100 & 100 \\ \hline \hline \end{tabular}
\end{table}
Table 3: The quantitative share of elements in the areas of the reaction zone marked in Figure 4a.  
properties have not been determined precisely enough. Therefore, to determine the properties of the transition zone, the IIT method with a lower test force was used. The results for the \"0\" area characterize the properties of the reaction zone. Additionally, this method made it possible to obtain values describing the elastic deformation coefficients: \(E_{r}\) and the elastic part of the indentation \(\eta_{IT}\).  
The measurements were conducted in the area containing the transition zone in the axis of the friction-welded bars (Figure 3a). The distribution of mechanical properties was determined along four measurement lines, from which mean values were calculated. The measurement results as a function of the distance from the W-S355 interface are shown in Figures 11-13. The depths of the indenter imprints were recorded from 162 to 303 nm. The hardness change tendency in the area near the W-S355 interface was confirmed. The average indentation hardness value of the transition zone was at the same level of steel on the section up to 150 \(\upmu\)m from the border with tungsten. This value was ~5.7 GPa. At a distance of 200 \(\upmu\)m from the interface, the indentation hardness value of the steel drops to about 5 GPa. The values for tungsten ranged from 6.5 to 6.8 GPa. The exception was the area 150 \(\upmu\)m away from the interface, for which a value of ~5.9 GPa was recorded.  
Figure 11: The indentation hardness distribution as a function of the distance from the tungsten–S355 steel interface.  
Figure 12: The distribution of reduced modulus of the indentation contacts as a function of the distance from the tungsten–S355 steel interface.  
E\({}_{r}\) values for tungsten were 390 GPa. The area 150 \(\upmu\)m away from the interface is an exception, for which a value of just over 350 GPa was recorded. For S355, the reduced modulus values ranged from 237 for the area 50 \(\upmu\)m away from the interface to 284 GPa for the 800 \(\upmu\)m distance. By contrast, \(E_{r}\) for the reaction zone was 291 GPa.  
While making imprints in the friction-welded sample, the plastic deformation work (\(W_{plastic}\)) was dominant. For tungsten, \(\eta_{IT}\) was about 14-15%, while for steel, it was about 13%. In the reaction zone, the W\({}_{elastic}\) part was 16%. It remained at an elevated level (14-18%) in the friction-welded zone at 400 \(\upmu\)m from the interface on the S355 side.  
The reaction zone was formed from the combination of W-S355 materials and is characterized by intermediate properties between them. Compared to W, lower indentation hardness, reduced modulus and a greater part of plastic deformation work during the indentation process were recorded. However, in relation to steel, higher values of \(H_{IT}\), \(E_{r}\) and \(\eta_{IT}\) were registered. This tendency was also present farther away from the W-S355 interface. The gradual changes in the measured properties (HV0.1, \(H_{IT}\), \(E_{r}\) and \(\eta_{IT}\)) are visible at the following distances from the interface: 550 \(\upmu\)m for steel, and 150 \(\upmu\)m for tungsten. It was also observed that the results for the reaction zone showed the greatest dispersion. This proves that the dynamic joining process caused greater heterogeneity in this area in relation to the base materials.