In the field of vehicles, in view of improving fuel consumption, there is a need for lightweighting of various components constituting automobiles. Particularly, a wire harness used in automobiles is a component having a second heaviest weight next to an engine in an automobile and thus, for lightweighting, there have been efforts to change a material of a conductor (core wire) of an electric wire used in the wire harness from copper to one of aluminum and an aluminum alloy. Normally, a base material made of one of copper and a copper alloy is used for a terminal connected to a leading end portion of an aluminum or aluminum alloy wire. Accordingly, since there is a possibility that exposed aluminum produces dissimilar metal corrosion and the conductor becomes defective at a connecting portion between the conductor and the terminal that are made of the aforementioned materials, it is necessary to take measures such as to shield the aluminum conductor from the outside world.
To this end, a method in which a crimp portion as a whole is formed as a closed tubular body and an electric wire conductor is covered with such a tubular body is being considered. However, it is not easy to form a closed tubular body from a flat plate material. Although it is conceivable to obtain a tubular body by processing a flat plate material into a C-shape and welding open end faces with each other, copper and a copper alloy used as a base material of the terminal has light absorptivity of less than 10% in a laser light wavelength region. In other words, since copper and a copper alloy have bad laser weldability, it was not easy to form a closed tubular body by welding. Although laser welding is possible by increasing a power of the laser light, welding defects such as blowholes are likely to occur. In addition, due to high conductivities of copper and a copper alloy per se, a heat-affected zone (HAZ, Heat-Affected Zone) is enlarged by laser welding, and thus even if laser welding is possible, cracking by stress concentration or the like is likely to occur.
Patent Document 1 proposes a method of increasing a laser light absorptivity by creating a concave light collecting wall in a laser irradiation surface (front surface). Patent Document 2 proposes a method of increasing laser light absorptivity of a material made of copper by a process such as Sn plating onto a plate material. Such a method includes placing a tab terminal portion, which is formed as a separate body, against a pattern portion, each being made of a copper alloy and constituting a bus bar, and a laser light absorptivity of the copper alloy is increased by applying a treatment such as tin plating on the base material of the tab terminal portion in advance. However, neither Japanese Laid-Open Patent Publication No. H11-144774 nor Japanese Laid-Open Patent Publication No. H10-334962 discloses improving absorptivity in a laser light wavelength region while decreasing HAZ.
The present inventors have studied the methods disclosed in Patent Documents 1 and 2 and found that, although it is possible to simply improve laser weldability, there is a problem that a region of HAZ becomes larger at the same time due to an improvement in heat transfer. When a region of HAZ is large, cracks are likely to occur due to stress concentration. Accordingly, when laser welding is performed by the methods described in Patent Documents 1 and 2, it can be said that a material property is low.