Abstract: Friction stir lap welding (FSLW) is a solid-state joining technique that effectively mitigates the formation of porosity, hot cracking, and compositional segregation associated with the high temperatures in fusion welding. It enables efficient and high-quality bonding of dissimilar materials such as aluminum and steel. In this study, FSLW experiments were performed on 6061-T6 aluminum alloy and QP980 steel to investigate the influence of varying welding speeds (at a constant tool rotational speed of 600 rpm) on joint formation and the welding process.The results indicate that, within the range of parameters examined, increasing the welding speed leads to improved surface smoothness of the weld. Due to the combined effects of tool stirring and heat input, the joint microstructure is characterized by four distinct zones: the weld nugget zone (WNZ), thermomechanical affected zone (TMAZ), heat-affected zone (HAZ), and base metal (BM), each exhibiting unique metallographic features.Mechanical testing reveals that the lap-shear strength of the joint initially increases and subsequently decreases with increasing welding speed, reaching a maximum at 100 mm/min. Microhardness profiles show a “W”-shaped distribution on the aluminum alloy side and an inverted “V”-shaped distribution on the steel side. Furthermore, as the welding speed increases, the corresponding reduction in heat input impairs the plastic flow of the material during welding, resulting in an increase in the load experienced by the tool.