Patent ID: 8281977

Claim:
A joining method comprising: a butting step in which metal members to be joined are butted against each other forming a butting portion, a first surface of the metal members are made flush with each other, and a back surface, opposite said first surface, of the metal members are made flush with each other; a preliminary joining step in which the butting portion between the metal members is preliminarily friction stir welded from the first surface of the metal members; a first primary welding step in which the butting portion between the metal members is friction stir welded from the first surface of the metal members by plasticizing and fluidizing again a plasticized region formed by the preliminary joining step; and a second primary welding step in which the butting portion is friction stir welded from the back surface of the metal members; wherein, before the first primary welding step, the metal members are fixedly locked on a platform of a friction stir welding machine, using a jig, whereby the first primary welding step is performed with the metal members held immovably on said platform; after the first primary welding step, burrs formed during friction stir welding are removed, and the metal members are then turned over so that the back surfaces thereof turn up; and the sum of the length of a stirring pin of a rotating tool used in the first primary welding process and the length of a stirring pin of a rotating tool used in the second primary welding process is at least equal to the thickness of the metal members at the butting portion.