3.5.4. Integrated Weld Transport Model

In Figure 15, the simultaneous formation of the surface flow patches (pitch layers) and the internal flow layers has been demonstrated as a discontinuous flow model for the plasticized batches of the mass which are formed by the revolution of the tool. As the shoulder action is directly responsible for the formation of the surface flow patches at the sub-shoulder region, the rotation and the advancement of the pin at the mid-SZ area generates the plasticized flow batches at the proximity of the tool, at the breadth of AS-RS. The deposited flow layers at the trailing edge of the tool form the bended flow layers at the borders of the AS and RS, which are revealed as the hourglass-boundaries within the cross-section of the weld.

**Figure 15.** Schematic of the cross-section of the weld with a proposed model for the formation mechanism of the flow-arms at the breadth of the stirring zone, between the AS and RS.

#### **4. Discussion**

#### *4.1. Originality*

The work makes the following original contributions to the understanding of the welding of AA6082-T6.

## 4.1.1. Characterization of the Flow Layers at the Hourglass-Borders

The combination of the metallographic delineation in optical microscopy and the metallurgical analysis by the SEM and EDS, confirms that the dark curved patterns in the AS and RS hourglass borders of flow-arms, being oxidation layers (AS) and elongated grains with a high-density of accumulation of the sub-grain boundaries as the result of the shearing and DRX (RS).

The original aim of this work was to understand the relationship between flow regimes and physical defects. The results show that the flow layers are important in understanding the internal flow, and the resulting defects. Failed welds show more flow lines, and show in specific locations in the cross section. Also these flow lines are associated with oxidisation. OM methods are relatively quick and simple to apply, but identification of these characteristics of the flow lines required electron microscopy. Now that the metallurgical nature has been elucidated, it is potentially possible that the simpler OM method could be used in future as an industrial quality control tool.
