The invention discloses a refill friction stir
spot welding method which is characterized in that a circular convex block is arranged at a
welding spot of an upper plate of a workpiece; during
welding, a hold-down ring of a stirring head is contacted with the upper plate and surrounds the convex block, and a stirring needle and a sleeve are contacted with the convex block; with the application of
ultrasonic vibration to positions, on the outer side of the hold-down ring, of the upper plate, a material in a
welding zone is plastified through the rotation of the sleeve and the stirring needle, then the sleeve pricks downwards to penetrate through the upper plate and pricks into a lower plate by a certain depth, the stirring needle is drawn back with the downward pricking of the stirring needle, after the sleeve reaches the designed downward pricking depth, the sleeve and the stirring needle return to initial positions, and continue to rotate and press down in the initial positions to allow the surface of the welding spot to be smooth, and after the stirring head stops rotating, an operator continues to apply the
ultrasonic vibration for a certain time. According to the refill friction stir
spot welding method, by employing the convex block for compensation, a pit of the welding spot produced after the welding can be eliminated; through the
ultrasonic vibration, material flowing and material exchange between the upper plate and the lower plate can be improved, and the joint quality is improved; and the operator continues to apply the ultrasonic vibration after the welding, so that the
residual stress of the welding spot can be reduced.