## 3 Experimental Methods and Data Acquisition  
Friction stir welding is essentially a welding method using friction heat as the welding heat source, so it is an effective way to evaluate the quality of joints by using heat input. The heat input of friction stir welding can be expressed as:  
\[q_{k}=k^{\prime}n/v \tag{11}\]  
where \(k^{\prime}\) is a constant, \(n\) is the rotation speed of the tool, and \(v\) is the welding speed. It can be seen that the parameter \(n/v\) directly represents the size of the welding heat input. For a given FSW tool structure and plunge depth, the quality of the friction stir welded joint is primarily dependent on the heat input factor. Therefore, the adjustment of the process parameters can effectively control the weld microstructure of the heat-affected zone and improve the performance of the welded joint. The base metal for this investigation was a 6-mm-thick 2A14-T6 aluminum alloy plate with dimensions of 300 mm long by 150 mm wide. The chemical composition and mechanical properties of the base metal are listed in Tables 2 and 3[36]. The original 2A14-T6 aluminum alloy base material has a coarse strip grain structure, and massive second phases are uniformly distributed in grain boundaries and grains. When the second phases are uniformly distributed in the matrix phase as fine dispersed particles, a significant strengthening effect occurs, which is called second phase strengthening. The atomic percentages of Al, Cu, Mg, and Si, the main elements of 2A14-T6 aluminum alloy base metal, are 66.22%, 32.23%, 1.08%, and 0.47%, respectively [37]. The massive second phase is essentially an aggregate of precipitated phase Al2 Cu, which has a certain strengthening effect. The main reason for the strengthening of the second phase is the interaction between them and dislocation; which hinders dislocation movement and improves the deformation resistance of aluminum alloy. The FSW tool used for welding was a shoulder concave structure; the tool pin was a conical thread structure, the diameter of the shoulder of tool was 16.3 mm, the cone angle of the tool pin was 15\({}^{\circ}\), the length of the tool pin was 5.6 mm, and the overall size was consistent with the parameters of the simulation model. The welding process was completed on a self-developed high-precision friction stir welding robot. During the welding process, the spindle speed was 800 r/min, the welding speed was 80 mm/min, the plunge depth of the tool shoulder was 0.25 mm, and the tool was titled by 2.5\({}^{\circ}\) towards its trailing edge. The overall welding parameters were consistent with the simulation parameters. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Cu** & **Si** & **Mn** & **Mg** & **Fe** & **Zn** & **Ti** & **Ni** & **Al** \\ \hline
3.9–4.8 & 0.6–1.2 & 0.4–1.0 & 0.4–0.8 & \(\leq\)0.7 & \(\leq\)0.3 & \(\leq\)0.15 & \(\leq\)0.1 & marginal \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Chemical composition of 2A14 aluminum alloys (wt %). Figure 3: Simulation results of welding process: (**a**) Plunging phase, 43 s; (**b**) dwelling phase, 64 s; (**c**) welding phase, 79 s; (**d**) welding phase, 98 s.  
The FEM numerically simulates the temperature field on the workpiece, so the temperature measurement on the workpiece can be compared with the finite element simulation data, and the FEM can be modified to ensure the correctness of the FEM simulation. During the welding process, K-type sheathed thermocouples were used to synchronously collect the temperature of the measuring points, and the first set of measuring points was set at 80 mm from the initial welding position, because the temperature near the initial position did not reach a steady state during the welding process, while the temperature also reached a relatively steady state value with the stable welding seam formation.