<p>Friction Stir Welding (FSW) has expanded to many metallic materials with higher melting points or much higher strength than the aluminum alloys. If the tool travels too quickly along the welding seam during the welding process or if the melting point of the workpiece is high, the frictional heat generated between the tool and the workpiece may not be sufficient to cause material flow. Insufficient heat input results in the formation of groove or tunnel-shaped defects in the stir zone and also severe wear or breaking of the FSW tool. To solve these problems, a higher heat input is required to soften the materials. Therefore, several preheating methods have been adopted to increase the heat input. In this study, a fiber laser was used as the preheating source during the FSW. In this experiment, the effect of the laser-preheating on the defect formation and tool rotational torque during the FSW was investigated. Additionally, a difference in the material flow during the conventional FSW and laser-preheating FSW was observed by two pairs of x-ray transmission real-time imaging systems. As a result, it was found that the laser preheating reduced the defect formation and the tool rotational torque during the FSW. Furthermore, laser beam irradiation on the retreating side (RS) was the most effective in reducing the defect formation. On the other hand, the irradiation on the advancing side (AS) was the most effective in reducing the tool rotational torque.</p>