3.5.3. Proposed Principles of Formation of Surface Features

This next explanation addresses the correspondence between internal and surface features. Figure 14 presents a flow model to explain this transport deposition. By revolution of the tool the transportation and deposition of the mass layers happens by the speed ratio (ω/*V*). The final flow patterns are observable as the flow-arms at the cross-section of the weld, and the related weld pitch patterns at the surface of the weld-line.

**Figure 14.** The proposed flow patterns for the flow-arms and the weld pitch patterns at the cross-section and surface of the weld region, respectively. The model aims to reveal the flow lines during the revolution of the tool and deposition of the layered mass at the trailing edge of the tool.

In general, the weld pitch patterns at the surface of the weld are directly dependent on the speed ratio between the rotation and advancing actions of the tool, simply as the number of revolution per forward movement, in which can directly influence the formation of the deposited layers at the stirring zone. It should be noted that the pressure imposed underneath the shoulder can affect the shape and size of these bonded layers and compact them together. Also, adding of the new deposited layers into the previous layers leads to increase in compaction condition at the back of the tool.
