**3. Results and Discuss**

## *3.1. Verification of Finite Element Model*

The axial force was tested throughout the FSW process. Figure 7 shows the comparison of the test results with the simulation results. It can be seen from the curve that the temperature gradually rises and reaches a peak in the first phase of the curve. In other words, the tool pin is just in contact with the workpiece, and some of the material is extruded due to plastic shear. The temperature rise softens the base material around the tool pin and promotes the pressing down of the tool. The material around the second stage tool gradually softens, and the external force of the tool plunged into the material increases slowly. In the third stage, as the tool shoulder comes into contact with the extruded material, the axial force begins to gradually increase, mainly due to the need for a greater axial force due to the tool shoulder squeezing the spilled material. At the peak of the axial force, the tool is prone to wear. Fracture can easily occur under extreme conditions. However, the axial force of the simulation output is slightly lower than the actual test results, but the overall trend of each step is in good agreement.

**Figure 7.** Axis force curve in the *Z* direction.

The spindle torque gives important information about the power consumption of the welding process, so torque is a decisive factor in the power requirements of the friction stir welding robot spindle. The change in the torque curve is gradual compared to the axial force, and the torque curve and the axial force curve have similar trends. As shown in Figure 8, the torque of the tool is relatively small at the beginning of the plunging. As the contact area between the tool pin and the material

increases, the spindle torque also increases. During the dwelling phase of the tool, the torque is gradually reduced, and the torque variation during the welding phase is relatively flat. The simulated data and the experimental data have certain errors, and the simulated curve changes relatively gently during the plunging phase, but the overall values and trends are close to the experimental data, thus verifying that the finite element model is relatively accurate.

**Figure 8.** Spindle torque variation curve.
