Welding spot area fully stirred key-hole-free friction stir spot welding method and tool

The invention discloses a welding spot area fully stirred key-hole-free friction stir spot welding method and tool. The tool comprises a stirring needle, a shaft shoulder, and a clamp sleeve which are sequentially sleeved from inside to outside and are arranged in a relatively sliding manner, at least two stirring raised ribs are uniformly distributed peripherally on the outer wall of a stirring end of the stirring needle, a holding notch matched with the stirring raised rib is formed in the end face of the shaft shoulder, the stirring raised rib can stretch along the holding notch, and the end face of the stirring end of the stirring needle and the end face of the shaft shoulder and the clamp sleeve are on the same horizontal plane. According to the method and the tool provided by the invention, the defect of complicated distribution of a backfill friction stir spot welding joint is effectively eliminated, and meanwhile, the advantage that no key hole is left in the surface of the backfill friction stir spot welding joint is also kept.