Heat treating friction stir welded aluminum alloy members and joined product

1. 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.

2. The method according to claim 1 , wherein the underaging treatment is performed using a heating means selected from a salt bath, an oil bath, an air-heating furnace, an iron, infrared heating and induction heating.

3. The method according to claim 1 , wherein the underaging treatment is sequentially performed on a portion of the aluminum alloy members that has been formed into the joined body by formation of the stir zone, following the formation of the stir zone at the contact portion of the aluminum alloy members by performing the friction stir welding operation on the contact portion.

4. The method according to claim 1 , wherein the aluminum alloy members to be joined have the same thickness.

5. The method according to claim 1 , wherein a rod-like rotary jig having at its leading end a pin provided coaxially is used to perform the friction stir welding operation.

6. The method according to claim 1 , wherein the friction stir welding operation is performed on a butted portion of the aluminum alloy members.

7. A joined product for press forming constituted by a joined body, the joined body being obtained by integrally joining aluminum alloy members each made of a 6000 series aluminum alloy with T4 temper, at a contact portion thereof at which the aluminum alloy members are held in contact with each other, such that a stir zone is formed at the contact portion with a heat-affected zone formed outwardly of the stir zone, wherein:

the stir zone and the heat-affected zone are formed by performing a friction stir welding operation on the contact portion of the aluminum alloy members and are softened by heat generated by the friction stir welding operation; and

the joined body is subjected, before GP zones are formed at the stir zone, to an underaging treatment which is an artificial aging treatment and the underaging treatment is completed before hardness of the stir zone reaches maximum hardness, 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.