Analysis of Shoulder Driven Zone Formation in Friction Stir Welding Using Advanced Scroll Tool

The main objective of this work is to reveal the forming mechanism of the shoulder driven zone during scroll tool friction stir welding (FSW). Scroll tools are widely used in FSW as they offer extra advantages of eliminating the tilted tool axis that is usually employed during the process. Therefore, it is vital to understand the forming mechanism of the shoulder zone when using an advanced scroll tool, which is important for optimizing welding parameters, analyzing welding defects, and controlling microstructure and properties of welds. In this present study, a series of FSW of thick section 6061 Al-T6 and Cu-AstmB370 (Cu foil as tracer) trials was conducted. A novel marker-insert technique by placing the Cu foil outside the welding path was employed, and the zonal flow patterns were characterized via OM, SEM and EDS. For the first time, the material flow behavior and actual flow patterns in the shoulder zone were detected by observing the distribution of Cu foil fragments in the weld zone. It was found that the thickness of shoulder zone varies along the weld transverse cross section with the thicker portion on the advancing side than the retreating side. The flow pattern of the shoulder driven zone reveals that a simple layer-to-layer banded structure appeared in the bottom portion of shoulder driven zone, whereas a featureless flow pattern showed in the top portion of the shoulder zone. Accordingly, the forming mechanism of shoulder driven zone was revealed based on the identified flow patterns.