Improvement of Formation Quality for Friction Stir Welded Joints

The weld, ambient temperature, and direction of a friction stir welding (FSW) tool play roles as important as that of the welding parameters in the FSW process. On the assumption that the weld parameters are the same under different directions, the joint welded using a stir tool rotated in a counterclockwise direction exhibits better formation quality than does the joint welded in a clockwise direction. The formation quality of the joints welded underwater clearly improves compared with that of the joints welded in air. Excellent weld joints free from defects are obtained in the present range of the FSW parameters for joints welded underwater and the stir tool rotated in the counterclockwise direction. The investigation of the flow path of the softened metal around the FSW tool reveals that the flow pattern of the softened metal driven by the shoulder and the pin varies with the rotational direction of the FSW tool, weld ambient temperature, and weld parameters. An excessively high weld input is detrimental to sound flow and avoidance of weld defects. By contrast, moderately decreasing the ambient temperature around the weld zone can improve the formation quality of the weld joints, regardless of the other weld conditions.