# Solid-State Rotary Friction-Welded Tungsten and Mild Steel Joints
## 3 Results and Discussion  
### Observations of the Joint's Microstructure  
Figure 2a shows the welded tungsten-steel S355 joint. A clearly shaped asymmetrical flash is visible on the side of the steel rod. Figure 2a shows the microstructure of the joints in two characteristic areas near the surface of the welded bars (radius \(\sim\)2 mm), where the linear velocity is the highest and where the most heat is released, and in the second area close to the sample rotation axis, where heating is limited to conduction from neighboring areas, because the linear velocity is close to 0. Figure 3 shows an image of the microstructure obtained without etching for two characteristic areas near the external surface of the bars \begin{table}
\begin{tabular}{c c} \hline \hline
**Process Parameter** & **Value** \\ \hline Spindle rotational speed (in the friction phase) [RPM] & 10,000 \\ Friction phase duration [ms] & 900 \\ Forge phase duration [ms] & 2000 \\ Pressure on the front of the specimens in the friction phase [MPa] & 110 \\ Pressure on joint surface of the samples in the forge (upset) phase [MPa] & 137.5 \\ Shielding gas flow angonium [l/min] & 18 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Process parameters of high-speed friction welding of tungsten + S355 steel. Figure 1: High-speed welding machine. (**a**) General view. (**b**) Assembly of samples to be welded in special argonium atmosphere chamber: (1) rotating sample, (2) stationary sample, (3) atmosphere chamber, (4) shielding gas supply nozzle. and near the axis of the joint. A continuous interface is visible between the pair of welded materials. No incompatibilities in the form of voids, cracks or lack of connection were observed at this interface. On the other hand, the transition zone between the tungsten and the steel varies in width along the entire joint. In the axis of the welded bars, this zone is relatively narrow (Figure 3b), while wider in the radius about 2 mm from the axis (Figure 3). These characteristic areas were subjected to detailed microscopic analysis. Microscopic examinations carried out at a higher magnification revealed the specific microstructure of the tungsten-steel transition zone (Figure 4). It is particularly visible in the area beyond the weld axis, i.e., at the radius where this zone is clearly wider. The microstructure of the tungsten-steel transition zone is two-phase and clearly banded (Figure 4a). The grains of first phase are visible as dark, uniformly colored and irregular, and between them is a second phase with a color and morphology similar to the welded steel. Figure 3: OM microstructure of tungsten-S355 steel friction-welded joint in longitudinal section: (**a**) in the axis of the welded bars, (**b**) at -2 mm radius of the welded bars sample (not etched). Figure 2: Microstructure of a friction-welded joint (tungsten + S355) (**a**); in two characteristic areas, (**b**) near the surface of the welded bars (–2 mm radius), (**c**) close to the sample rotation axis. The surface distributions of W and Fe in these two areas (radius close to the external surface and in the axis of the bars) of the joint (Figures 5 and 6) confirm the dominant share of iron in the transition zone, but with a significant presence of tungsten. Figures 7 and 8 show the linear distribution of elements along a perpendicular line to the W-steel interface. These studies document the participation of tungsten in dark, uniformly colored, irregular grains in the transition zone (Figure 7). The second phase is rich in Fe. The morphology state may be the result of mechanical (plastic) mixing of defragmented grains (on friction surface) of both welded materials and probably combined with the simultaneous occurrence of the phenomenon of diffusion through the interphase boundary in the solid state. Iron dominates in the transition zone. In the weld axis in the transition zone (Figure 8) and in the side part of the joint from the tungsten side (Figure 7), a gentle slope of the concentration profile of both W and Fe was observed.