The aim of this study is to investigate friction stir welding (FSW) to join A304 austenitic stainless steel and low carbon steel A283 Gr. C in-lap configuration to clad the carbon steel with highly corrosion-resistant stainless steel. Thus, a wide range of FSW parameters were investigated such as FSW tool rotation rate from 200 to 400 rpm, tool traverse speed from 25 to 75 mm/min, and vertical forces of 20 to 32 KN. The FSW parameters combination of high welding rotation rate (400 rpm) and high vertical forces (32 KN) results in rejected joints in terms of surface appearance and clear surface defects. On the other hand, rotation rates of 200 and 300 rpm with different welding speeds and vertical forces resulted in some sound joints that were further investigated for microstructure and mechanical properties. The sound lap joints were examined via optical microstructure, SEM, and EDS investigations. For the mechanical properties, both tensile shear testing and hardness testing were used. The transverse macrographs showed intermixing between the two dissimilar materials with an almost irregular interface. The hardness profile in both materials showed a significant increase across the different regions from the Base Material (BM) to the nugget zone, with a maximum value of 260 Hv in the stainless steel and 245 Hv in the carbon steel. This increase is mainly attributed to the grain refining in the weld region due to the dynamic recrystallization and transformations upon the thermomechanical cycle. The tensile shear load of the joints varied from 20 to 27 KN for the FSWed joints, with the highest joint tensile shear load of 27 KN for that produced at 300 rpm tool rotation and 25 mm/min welding speed.
