The invention discloses a reverse backfill type friction stir
spot welding method, and belongs to the technical field of
welding. A reverse backfill type friction stir
spot welding tool is adopted, the end faces of a stirring needle, a sleeve and a pressing ring are located on the same horizontal plane and integrally pressed down to the upper surface of a welded plate, a welded workpiece is pressed, the stirring needle and the sleeve are rotated, the rotating speed of the sleeve is
omega1, the rotating speed of the stirring needle is
omega2, reverse rotation is conducted,
omega1 is smaller than omega2, and the surface of the welded workpiece is preheated. The sleeve rotates to downwards pierce into the welded plate on the upper side, meanwhile, the stirring needle reversely rotates to upwards draw back until the sleeve rotates to pierce into the surface of the welded plate on the lower side, and
metal is squeezed into a cavity of the sleeve; the sleeve and the stirring needle change directions simultaneously, and plastic
metal in a cavity of the sleeve is backfilled to the upper surface of the welded plate on the upper side; and the two return to the upper surface of the welded plate on the upper side and stop rotating,
welding is completed, and a weldment is obtained. According to the method, heat input in the
welding process is greatly reduced, interface metallurgical bonding of a stirring area and a
heat engine influence area is improved, and meanwhile
residual stress and deformation after test plate welding are reduced.