Observations on metallurgical phenomena and formation of onion ring nugget during friction stir lap welding of dissimilar aluminum by a new 3D technique

The formation of onion ring and the material flow around the tool during welding were visualized by a new technique. The material flow exhibited different characteristics from the retreating side to the advancing side and from the front to the back of the tool. The movement of the horizontal flow from the advancing side to the retreating side was caused by the shoulder and the movement of the convectional flow in front of the tool resulted from the threaded. The formation of the onion ring was attributed to the loads from both the external and internal screws at each rotation. [Display omitted] •The formation of onion ring and the material flow around the tool during welding were visualized by a new technique.•The movement of the horizontal flow from the advancing side to the retreating side was caused by the shoulder.•The movement of the convectional flow in front of the tool resulted from the threaded.•The formation of the onion ring was attributed to the loads from both the external and internal screws at each rotation.