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2 mm low carbon steel plates were successfully welded by the fiat friction stir spot welding (FSSW) using double side adjustable tools,by which the keyhole formed in the conventional FSSW was eliminated and a flat surface on both the top and bottom sides of the welded joints was obtained.In addition,the hook shape usually generated in the conventional FSSW was eliminated by this technique,and the unbonded interface was parallel to the surface of the sheets.Owing to the enlarged bonded interface width by eliminating the keyhole and the intermixed interface by the adjustable probe,the plug fracture occurred under all the welding conditions in the present study.Due to the suppression of the thickness thinning and elimination of the hook shape,the joint performance was improved in the plug fracture mode.The shear tensile performance was considered to strongly depend on the microstructure in the tip area of the unbonded interface and the maximum shear fracture load of 23.0 kN was achieved in this study.