Abstract: Force components serve as the information carriers for defect evolution in aluminum alloy friction stir welding, playing a crucial role in defect prediction and identification. This paper utilizes a force measuring platform to facilitate in-situ measurement of the traverse force, lateral force, and plunge force throughout the welding process. The variations in force signals and defect sizes observed at different welding parameters are analyzed, and the responses of force components to defect evolution are examined. The results indicate that force signals initially increase, stabilize, and then rapidly decrease during the welding process, reflecting the mechanical movement of the stirring head and the changes in the thermoplastic state of the weld materials. When the rotation speed is low or the welding speed is high, tunnel defects emerge within the weld; in these cases, the plunge force is relatively low while the lateral force is comparatively high. Even with constant process parameters, a decrease in axial force and an increase in lateral force can indicate the formation of tunnel defects. Furthermore, the reduction in plunge force is attributed to the diminished extrusion effect of the shoulder on the weld material, making it difficult for the material to be wrapped and migrated to the weld surface, thereby leading to the formation of tunnel and groove defects. The increase in lateral force results from the reduced ability of the stirring pin to drive the material, causing a buildup of material in front of the stirring pin on the retreating side of the weld. This exacerbates the extrusion of materials on the retreating side by the stirring pin, making it challenging for the materials behind the stirring pin to backfill, which ultimately leads to the formation of tunnel defects.