## 3 Results and Discussion
### Zones, Flash, and Material Stirring  
A common feature observed in all welded joints, except for the non-tilted condition, is the almost complete absence of entry flash (as shown in Figure 7a,b), while exit flash is clearly visible, as illustrated in Figure 7a,c, for a specimen from the P1 group with filler material and a tilted tool. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**ID** & **Rotation Speed** & **Welding Speed** & **Tilt Angle (\({}^{\circ}\))** & **Material Feed** & **SOD** & **Welding** \\  & **(rpm)** & **(mm/min)** & **Tilt Angle (\({}^{\circ}\))** & **(mm/min)** & **(mm)** & **Direction** \\ \hline
**P4. 100\% infill. Cylindrical tool pin. Tool inclination 8\({}^{\circ}\). No material addition** & & & & & \\ P4.0 to P4.7 & 2500 & 20 & 8 & - & 1 & Forward \\
**P5. 100\% infill. Truncated conical pin. Tool inclination 8\({}^{\circ}\). No material addition** & & & & & \\ P5.0 to P5.5 & 2500 & 20 & 8 & - & 1 & Forward \\ P5.6 & 2500 & 20 & 8 & - & 1 & Backward \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._  
Figure 7: Specim P1_2. (**a**) The front and bottom views of the welded specimen. (**b**) The absence of an entry flash on the front side. (**c**) The exit flash observed from the bottom side. (**d**) Homogeneous mixing in the central weld zone. (**e**) The details of the central weld zone. Figure 7c shows an intermediate zone of the weld. Additionally, Figure 7d,e show the detail of the intermediate zone. The material mixing in this zone is noticeable, where the different colors are clearly stirred. Figure 7a shows that, on the side opposite the weld, material built up in internal areas that were not fully filled. Then, the joint is mainly composed of filler material (black), creating a clearly defined and sharp transition between the two parts. This happens because the high porosity in those regions allows the filler material to gather there. As a result, the weld zone likely ends up with less porosity than the surrounding material, which could lead to better mechanical behavior. Two distinct stages were identified during the welding process in all the joints produced:  
* The first stage began at the start of welding and continued until the tool reached a stable temperature. During this stage, material flow was inconsistent and irregular, with no clearly defined pattern. This behavior is highlighted in red in Figure 8a,b. * The second stage corresponded to a steady-state regime (the yellow zone in Figure 8), during which the tool temperature remained nearly constant throughout the process. At this point, the material flow became homogeneous and predictable. Both stages were observed regardless of the specimen's infill percentage. Figure 8a,b show a detailed view of the unstable and stable stages in 100%-filled and 15%-filled specimens, respectively. These stages were also evident in the welding equipment's behavior. During the transient regime, a slight drop in the main motor's rotation speed was observed, caused by the increased energy demand needed to heat the tool. Once the steady-state regime was reached, the tool operated at a slightly higher and more stable speed. Then, the length of the first stage was longer for specimens with a higher infill density, as these required more energy to initiate material flow due to the higher temperatures reached. Regarding the non-tilted welding condition (P0 specimens), very poor material mixing was found, as shown in Figure 8c. Although the filler rod heated and plasticized the weld zone, the plastic material flow was enough to create the joint. These findings were also reported in ref. [42], where low angles were associated with insufficient material flow, and a void was created. It was observed that the specimens that were welded at slower welding speeds showed shorter transient regime zones. Therefore, it was found that the temperature in the tool reached stable values faster at lower welding speeds.