Joining dissimilar materials often needs adjusting the thicknesses to enable their effective use in transportation industry. Friction stir welding (FSW) is known to be a useful process for joining of dissimilar materials such as aluminum (Al) to steel (St). In FSW in butt configuration, in addition to the dissimilarities in materials, different thicknesses also present further challenges in achieving a perfect joint. Although this challenge does not exist in the lap configuration, the benefits of dissimilar joining are lost due to the increase in weight. In this study, a simple design is presented to join a thin steel sheet with a thickness of 2 mm to an aluminum sheet with a thickness of 5 mm in butt configuration. A S-shape interface of Al/St was obtained due to the offset of the FSW tool into St. Scanning electron microscopy (SEM) observations from the interface revealed that the thickness of Al-Fe intermetallic compounds (IMCs) is highest near the shoulder influenced region with a declining trend towards the bottom of the pin. The mechanism of the formation of this interface during FSW is explored. The contribution of this interface to the mechanical behavior of the joint during tensile testing is analyzed by examining the fracture surface and hardness distribution. The results of the present study provide practical use of FSW for the design and application of dissimilar materials.
