Multimaterial fabrication such as joining of steel and aluminum is currently prominent in a variety of industries. Friction stir welding is a novel solid-state welding process that causes good joint strength between steel and aluminum. However, the phenomenon contributing significant strength at the interface is not yet clear. In the present study, the interface of the friction stir lap-welded aluminum and coated steel sheet having joint strength maximum (71.4 pct of steel base metal) and minimum, respectively, under two parameter combinations, i.e. , 1000 rpm 50 mm min −1 and 500 rpm 100 mm min −1 , was exclusively characterized by X-ray diffraction, transmission electron microscopy (TEM), concentration profile, and elemental mapping by electron-probe microanalysis. A TEM-assisted EDS study identifies the morphologies of large size Al 13 Fe 4 and small size Fe 3 Al-type intermetallic compounds at the interface. The diffusion-induced intermetallic growth (thickness) measured from a backscattered image and concentration profile agreed well with the numerically calculated one. The growth of these two phases at 1000 rpm 50 mm min −1 is attributed to the slower cooling rate (~3.5 K/s) with higher diffusion time (44 seconds) along the interface in comparison to the same for 500 rpm 100 mm min −1 with faster cooling rate (~10 K/s) and less diffusion time (13.6 seconds). The formation of thermodynamically stable and hard intermetallic phase Al 13 Fe 4 at 1000 rpm and travel speed 50 mm min −1 in amounts higher than 500 rpm and a travel speed of 100 mm min −1 results in better joint strength, i.e. , 71.4 pct, of the steel base metal.