This study investigates the influence of tool rotational speed on microstructure and mechanical properties of friction stir welded joints between AA6351-T6 aluminium alloy and interstitial-free steel. The welds were produced at rotational speeds ranging from 200 to 1000 rpm at the processing interval of 200rpm for a constant traverse speed of 55 mm/min. Microstructural examination revealed Fe₃Al and Fe₂Al₅ intermetallic compounds at the Al/Fe interface, with thickness varying from 0.87 to 2.09 μm depending on rotational speed. Optimal joint strength of 184.42 MPa, approximately 75% of base aluminium alloy was achieved at 600 rpm, representing an ideal balance between effective material mixing and controlled intermetallic layer formation. Higher rotational speeds produced excessive heat input and thicker intermetallic layers, deteriorating joint strength. Fractography showed a transition from ductile to mixed-mode fracture with increasing rotational speed.