Trends and new applications of spot welding for aluminium alloy sheets

With recent transport machinery and appliances including vehicles, measures to reduce energy consumption and to cope with environmental issues are important tasks to consider whilst the enhancement of functions and comfort is also sought. Technologies to reduce weight are an effective means to meet this demand and its requirements and, therefore, a variety of methods have been proposed. Representative methods include (1) those using thin-sheet structures of high-strength materials, (2) those using lighter materials such as aluminium alloys and magnesium alloys, and (3) those using multi-materials in which macromolecular materials, lighter material, highstrength steels, etc. are combined to make use of individual properties of materials. Among these methods, those which use high-strength materials can be employed with conventional welding methods and designing methods for steels, and so, helped by a steady supply of high tension steels and the development of ultra-high tension steels, they have already been applied to many service production sites. The methods using lighter materials and/or using multi-materials are the areas expected to see their range of application grow but they will require similar or dissimilar material welding processes of lighter materials such as aluminium alloys. Although welding technology and design methods for aluminium have nearly been established, there are still problems remaining to be solved in service production sites which are hindering the expansion of their application range. In different material welding between aluminium alloys and steel materials, a large difference between them in their thermal and physical properties affects their weldability, and hence various devices and measures have been tried out when usual welding methods are employed. Those welding methods tried include MIG welding, diffusion welding, laser welding, ultrasonic welding, friction welding and friction stir welding. At present, however, there have not been sufficient studies which take account of the service application of the welding methods. On the other hand, resistance spot welding, which has been employed for the assembly processes of vehicles and carriages, has advantages such as that it enables the use of existing equipment and apparatus, and that it can readily be automated for mass-production. Hence, it has been hoped that resistance spot welding can be applied to different material welding. Some methods have been proposed to make this possible. One of them is a method of using cladding materials. This method has problems caused by the insertion of the cladding materials, such as a reduction in productivity and in the life of electrodes owing to high current. Hence, its applicable range is restricted. Another similar method proposed is the method of using pure aluminium as the insert material to prevent a reaction phase from being created at the weld interface between different materials. To deal with these problems, the authors proposed a new resistance spot welding method of using a cover plate for the similar or dissimilar material welding of aluminium alloy sheet materials. This method is to enhance the heating and keep the temperature of the fused area in the spot welding of aluminium alloys by executing welding with the cover plates inserted between the materials to be jointed and the electrodes. In this report, this method was applied to the welding between aluminium alloys as well as the different material welding of aluminium alloys to mild steel and to stainless steel, to evaluate the effectiveness of this welding method from the aspects of the mechanical properties of the welded joints and the microstructure at the weld interface. The study will be described in the following sections.