Backfill type friction stir welding method and device

The invention relates to a backfill type friction stir welding method and device. According to the technical scheme, a novel friction stir spot welding device is used for achieving backfill type keyhole-free welding. The method comprises that a stir head is pushed down to press a part to be machined, the position of an outer sleeve of the stir head is invariant, a shaft shoulder rotates at a high speed to stab into a welding bead, a stir needle moves far away from the welding bead direction, the shaft shoulder begins to perform welding after stabbing for certain depth into the welding bead, softened metal flows into space formed by the welding bead, the stir needle and the shaft shoulder, the shaft shoulder is withdrawn from the part to be welded, the stir needle pushes the metal in the space to press the part to be welded, and accordingly, a key hole is filled, welding is completed, and the stir head is unloaded and withdrawn. Compared with the prior art, the backfill type friction stir welding method and device have the advantages that uneven welding beads caused by key holes are eliminated, and the welding penetration depth evenness is guaranteed during discontinuous welding beads.