This study presents a simple and innovative design to join a 2 mm thick steel sheet to a 5 mm thick aluminium sheet in a butt configuration. Thickness differences were addressed using support plates, while an aluminium run-on plate was employed to prevent the FSW tool from plunging into the steel. The process produced a unique S-shaped Al/St interface, the formation mechanism of which is analysed in this study. Scanning electron microscopy (SEM) observations revealed a gradient in the thickness of intermetallic compounds (IMCs) along the joint interface, decreasing from the top to the bottom. This S-shaped interface led to a 150% increase in the ultimate tensile strength (UTS) of the joint. The mechanism underlying this enhancement, attributed to the curved geometry of the interface and its alignment with the loading direction, is discussed in detail. These findings highlight the potential of this approach for improving the performance of dissimilar material joints in lightweight structural applications.
