• Monitoring Signal Definition

F and M represent the axial thrust exerted on the tool in the Z-axis direction and the rotational torque measured along the X-Y plan, respectively. B_2 corresponds to the force acting along the welding feed direction as the FSW tool traverses along the weld seam, this moment drives the tool through the workpiece material. B_1 is generated by the rotational motion of the tool during the FSW process, which characterizes the reaction forces exerted on the tool by the surrounding material.

• Basic Trend

F increases sharply as the stirring pin penetrates the 6061. This rapid increase during penetration is due to the stirring pin's need to overcome the material's resistance to deformation. As the penetration depth increases, the contact area between the stirring pin and the material expands, leading to a continuous increase in F until it reaches its peak. The maximum force is generated during penetration, making the pin prone to buckling and failure. Therefore, monitoring this peak value helps prevent tool failure. Users can set a value in the software. When the actual value approaches this value, the software will issue an alarm, allowing the user to stop testing and avoid material and tool losses, thus improving production efficiency. The gradual decrease in F is due to the tool remaining in a certain position (2 s), at which point the material reaches a sufficiently high temperature, and F decreases as the local softening around the stirring head improves. During welding, if the welding is uniform, F will gradually stabilize.

The M variation follows a similar pattern to the F, increasing continuously during penetration and gradually reaching its maximum value, then decreasing during the dwell phase and stabilizing during welding. The absolute values of the B1 and B2 also gradually increase during the penetration process, reaching the first peak value.