Abstract: The flow behavior of plastic deformation metal and surface condition of the welding work piece during the friction stir welding process have a great influence on the evolution of microstructure and welding defects of the welded joint. In this paper, the transfer behavior was studied in the friction stir welding joint by using ‘tracing copper foil’. In order to analyze and test the mechanism of the ‘weak-bonding’ defects for FSW joint, friction stir welding of anodized 2219 aluminum alloys was carried out. The results show that the oxide film forms ‘S’ line defects which begins with the retreated side of the joint, ends at the advanced side and extends to the nugget zone backward.