Patent ID: 7810700

Claim:
A method of joining aluminum alloy members, comprising the steps of: contacting aluminum alloy members each made of a 6000 series aluminum alloy with T4 temper and performing a friction stir welding operation on a contact portion of the aluminum alloy members at which they are held in contact with each other, so as to obtain a joined body including (i) a stir zone formed by a frictional heat generated by a stirring action of a pin which is inserted into the contact portion of the aluminum alloy members, (ii) a heat-affected zone also softened by the heat generated by the friction stir welding and located outwardly of the stir zone, and (iii) a base material portion of the aluminum alloy members other than the stir zone and the heat-affected zone; performing, on the joined body within 24 hours after completion of the friction stir welding operation, an underaging treatment which is an artificial aging treatment; and completing the underaging treatment before hardness of the stir zone reaches maximum hardness, wherein the underaging treatment is performed before GP zones are formed at the stir zone of the joined body, and is a heat treatment in which the joined body is heated at a temperature of 160-200° C. for not longer than two hours, whereby the hardness of the stir zone is the highest, with a hardness of the heat-affected zone being the second highest, and a hardness of the base material portion being the lowest.