Method of joining heat-treatable aluminum alloy members by friction stir welding and joined product obtained by the method and used for press forming

Abstract

A method of joining heat-treatable aluminum alloy members by friction stir welding, including the steps of: a T4-treatment-performing step of performing a T4 treatment on heat-treatable aluminum alloy members so as to impart T4 temper to the heat-treatable aluminum alloy members; a joining step of joining the heat-treatable aluminum alloy members with T4 temper by friction stir welding to provide a joined product; and a reversion-treatment-performing step of performing a reversion treatment, the reversion-treatment-performing step being carried out prior to or after the joining step.

Description

[0001] The present application is based on Japanese Patent Application Nos. 2003-163367 filed on Jun. 9, 2003, 2003-371055 filed on Oct. 30, 2003, and 2003-373408 filed on Oct. 31, 2003, the contents of which are incorporated herein by reference. BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] The present invention relates in general to a method of joining heat-treatable aluminum alloy members by friction stir wielding and a joined product for press forming obtained by the method. More particularly, the invention relates to such a method capable of advantageously providing a joined product with good ductility, accordingly with good press formability while effectively preventing occurrence of fracture at a stir zone and heat-affected zones of the joined product obtained as a result of joining the heat-treatable aluminum alloy members by friction stir welding, and such a joined product for press forming advantageously produced by the method [0004] 2. Discussion of ...

AI Technical Summary

Benefits of technology

[0015] In the light of the aforementioned situations, the inventors of the present invention have made an extensive study to solve the problems related to the press formability of the joined product obtained by joining together the heat-treatable aluminum alloy members. As a result of the study, it has been found that the concentration of stress on the stir zone and the heat-affected zones during the press forming operation can be avoided by optimizing relationship in strength among the stir zone, the heat-affected zones and the base material, more specifically, by making hardness of the stir zone and the heat-affected zones larger than that of the base material.

[0016] For optimizing the relationship in strength among the stir zone, the heat-affected zones and the base material, the inventors have made a study from the metallurgical viewpoint on respective metal structures of the stir zone, heat-affected zones and base material portions after friction stir welding. The study has revealed that the heat-affected zones located in the vicinity of the stir zone and having the lowest hardness are in a state in which the metal structure is rehardened or reverted (in a reversion state). It has been further revealed that the strength or hardness of the base material is effectively reduced as low as or lower than that of the heat-affected zones (a) where a reversion treatment for temporarily extinguishing GP zones (clusters) existing in the metal structure is performed on T4-treated heat-treatable aluminum alloy members in advance and subsequently the aluminum alloy members are joined together by friction stir welding after the reversion treatment or (b) where T4-treated heat-treatable aluminum alloy members are joined together by friction stir welding to provide a joined product and a reversion treatment for temporarily extinguishing GP zones (clusters) is subsequently performed on the joined product for reverting the metal structure of the base material. Consequently, the fracture due to the stress concentration on the heat-affected zones in the press forming can be advantageously avoided and the stress can be effectively distributed over the base material portion with a large area, so that the amount of deformation of the joined product as a whole is advantageously increased. Therefore, the ductility and the breaking elongation of the joined product can be effectively enhanced, leading to an improvement in the press formability.

[0017] The present invention has been accomplished based on the findings described above. It is therefore an object of the invention to provide a method of joining heat-treatable aluminum alloy members by friction stir welding, which method is capable of providing a joined product having excellent ductility and accordingly excellent press formability without suffering from fracture at a stir zone and heat-affected zones by optimizing relationship in strength among the stir zone, the heat-affected zones and a base material. Another object of the invention is to provide a joined product for press forming with enhanced press formability advantageously produced by such a method.

Problems solved by technology

However, even where the T4-treated members are butted and joined together according to such a friction stir welding process to provide the blank, softening inevitably occurs at heat-affected zones of the blank which are located adjacent to a stir zone at which the T4-treated members are welded and which are affected by a heat generated in the friction stir welding process.

If such a blank is used for press forming in the as-welded condition without undergoing any post-treatment, stress concentrates locally on the heat-affected zones at which the strength (hardness) is the lowest due to the heat generated upon welding, so that the heat-affected zones tend to deform faster than the other portion of the blank and often break or fracture.

The fracture or breakage at the heat-affected zones raises a problem of limitation in a configuration of the press product to be obtained from the blank and a degree of elongation or ductility (a deformation amount) of the blank.

Those Patent Documents, however, are silent about press formability and ductility of the joined product obtained by joining the aluminum alloy members.

Accordingly, the strength of the heat-affected zones is lower than those of the welded region and the base material, so that the stress concentrates on the heat-affected zones, causing fracture at the heat-affected zones.

Therefore, there still exists the problem of limitation in the configuration of the press product to be obtained from the joined product and the elongation degree (the formable amount) of the joined product.

None of these methods aim at refining the aluminum alloy members.

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