<p>The lightweight structural applications of magnesium and aluminum alloys inevitably require welding and joining, particularly dissimilar welding between these alloys. The objective of this study was to investigate the feasibility of friction stir welding of AZ31B magnesium alloy and AA5059 aluminum alloy, with emphasis on the effect of welding speed on the mechanical properties and microstructure of the joints. The results showed that the diffusion layer at the interface consisted of three sub-layers from the Mg side to the Al side: a Mg + Al₁₂Mg₁₇ eutectic structure, a brittle Al₃Mg₂ + Al₁₂Mg₁₇ structure, and a Al₃Mg₂ + Al eutectic structure. When the welding speed increased from 120 to 200&#xa0;mm/min, the thickness of the diffusion layer decreased from ~ 7.31 to ~ 1.87&#xa0;μm. With increasing welding speed, the fraction of recrystallized and deformed grains on both the aluminum and magnesium sides increased, while the fraction of substructured grains decreased. The peak tensile strength of the welded joint first increased and then decreased with increasing welding speed, reaching a maximum value of 161.65&#xa0;MPa at 160&#xa0;mm/min, at which point the diffusion layer thickness was 3.07&#xa0;μm. The formation of the peak strength is mainly attributed to the dynamic balance between heat input, intermetallic compound growth, and plastic flow of the material at moderate welding speeds. This effectively suppresses excessive formation of brittle IMCs while avoiding insufficient material flow and defect formation, thereby optimizing the mechanical properties of the joint.</p> Graphical abstract <p></p>