Dissimilar lap joints of low carbon steel and Al-Mg alloy were obtained by friction stir spot welding. Mechanically mixed layer between top and bottom plates was not formed at the weld nugget due to the limited tool penetration and the pin height of welding tool lower than the thickness of Al plate laid in top side. These welding conditions made it possible to weld steel plate using welding tools made out of a general tool steel. With increasing tool penetration depth (TPD), tensile shear force of joint increased and maximum value of 3?0 kN was obtained at the TPD of 0?5 mm, but excessive tool penetration beyond 0?5 mm was caused in a deformation of Al plate of top side. In the result of interface observation, interaction layer was formed between Fe and Al alloy, which was constituted by various intermetallic compounds (IMCs). Consequently, the size of strongly bonded area containing Fe 3 Al and Fe 4 Al 13 IMCs increased with TPD, which resulted in the increase in joint strength under the limited TPD.

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