## 3 Results and Discussion  
### Temperature Distribution  
The temperature field distributions during the friction stage of the FRW process of THA to AA are presented in Figure 4. As can be seen in the graphs, the friction welding process begins in the places most distant from the axis of the cylindrical samples. This is where the highest peripheral speed occurs, where the material becomes plasticized as a result of increasing temperature. Figure 4a shows a relatively rapid increase in temperatures during the friction period of the samples. After 3 s, the temperature on the surface reached 278 \({}^{\circ}\)C. After another 3 s, the temperature almost doubled (Figure 4b). It is likely that the high-temperature zone widens from the weld line due to heat conduction within the specimen. As can be seen in Figure 4c, no significant plastic deformation or flash formation occurred during the friction phase.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Material** & **AA** & **THA** \\ \hline Density (kg/m\({}^{3}\)) & 2690 & 18,000 \\ Young modulus (GPa) & 69 & 385 \\ Poisson ratio & 0.33 & 0.28 \\ Thermal expansion (1/C) & 2.2 \(\times\) 10\({}^{-5}\) & 5.2 \(\times\) 10\({}^{-6}\) \\ Specific heat (J/kg K) & 528 & 134 \\ Thermal conductivity (W/m K) & 135 & 90 \\ Melting point (\({}^{\circ}\)C) & 640 & 2173 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Thermophysical properties of the materials.  
Figure 4: Distributions of temperature fields in aluminum alloy during the friction period: (**a**) 3 s (**b**) 6 s (**c**) 9.5 s. (FF = 12.5 kN, n = 1450 rpm, and UF = 50 kN).  
Figure 3: The flow stress of AA (**a**) and THA (**b**) at strain \(\varepsilon=0.1\) during deformation.  
The maximum interface temperature of 581 \({}^{\circ}\)C for the friction weld was achieved in a time of 9.6 s (Figure 5).  
Figure 6 shows the temperature distribution fields of the weld zone during the upsetting stage of the welding process. As can be seen in Figure 5, after reaching the maximum value, the temperature drops rapidly to 280 \({}^{\circ}\)C within a few seconds during the forging stage (see Figure 6a-c).  
It is found that the weld flash begins from 9.6 s to 15 s and that the largest flash is obtained after the upsetting process (Figure 6c). The plastic deformation that accompanies this welding phase causes the significant shortening of the aluminium alloy (Figure 5). It can be seen that the maximum axial shortening of 16.8 mm is achieved during this stage.  
Figure 7 shows the thermal history of the weld interface for various positions. It can be seen that the temperature on the outer surface is the highest, while that on the axis has the lowest value. The highest temperature values were recorded at the welding time of 9.5 s. For all measurement locations, the nature of the temperature distribution is similar. This is because the temperature increases linearly with the increasing friction time. After obtaining the highest value, the temperature drops rapidly, but on the outer surface, this reduction is the smallest.  
Figure 5: Maximum temperature distribution and axial shortening of welded materials during friction welding.  
Figure 6: Temperature distribution of weld zone during the upsetting stage of the FRW process of THA/AA dissimilar alloys: (**a**) 11 s, (**b**) 13 s and (**c**) 15 s. (FF = 12.5 kN, n = 1450 rpm, and UF = 50 kN).  
The radial distribution of temperature for various durations is obtained via simulation in Figure 8.  
It can be noted that an appropriate welding time is required to reach the friction welding temperature of the welding process for specific parameters. The radial temperature was measured for the following welding parameters: rotational speed= 152 rad/s, friction force = 12.5 kN, upsetting time and force of 5 s and 50 kN, respectively. Similar results were obtained for other welding parameters.