Abstract: A friction stir welding tool and weld metal structure with plural onion rings, in which the tool is formed on the tool head with three surfaces, which are connected to one another through three spaced surfaces. An eccentrical axis of the tool head deviates from a central axis of the tool handle. One of the three surfaces is a deviation cutting surface which is a flat surface formed by cutting off a wall thickness from one side of an outer peripheral surface of the tool head toward the central axis of the tool handle. Each of the three spaced surfaces is alternately formed with plural notches and plural protrusions. The above welding tool is used to perform a stir friction welding operation to two metals to form the weld metal structure with the same number of onion rings as the protrusions and notches of each of the spaced surfaces.