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. https://www.markymarkscott.com/product-category/curl-activator/