AbstractDissimilar AA5083 to GL D36 steel welds produced by Friction Stir Welding in lap joint configuration, with the aluminum plate placed on the advancing side, are studied regarding their mechanical, microstructural and interfacial properties for varying process parameters, i.e. welding and tool rotational speed. An increase of welding speed or decrease of the rotational speed causes the formation of tunnel defects, a decrease of the steel hook height and reduction of grain size in the aluminum stir zone. The maximum hardness is observed at the weld interface, due to the presence of intermetallic compound layers, identified as the Fe-rich phases as FeAl and Fe3Al. As the rotational speed increases, an increase of the IMCs thickness in the weld interface is found, which contributes to the degradation of the lap shear strength, due to the brittleness and high hardness of these phases. Overall, the maximum lap shear strength is obtained for welds showing macro (steel hook) and micro interlocks, as well as the formation of thin IMC layers at the weld interface.