Fatigue tests of Al-Mg alloy and low carbon steel joints by means of the spot friction stir welding, were carried out to investigate the basic fatigue properties. The influences of joining conditions on the fatigue behavior was also studied by using the joints which were joined under the different plunge depth conditions. From the experimental results, it was found that the fatigue life and strength were slightly different with difference in the plunge depth in case of tests under the higher applied load levels. There were three types of fatigue fracture modes which depend on both of plunge depth and applied cyclic load level. They were shearing type fracture due to detaching of the joining interface, fatigue fracture type in the aluminum alloy due to fatigue crack growth in perpendicular to the loading direction and the plug type fracture due to crack growth along the spot FSW region. Under the applied load which was the border level between two types of fracture mode, both fracture process propagated concurrently and the final fracture occurred when either process attained to the critical stage.
