## 1 Introduction  
In the automotive industry, the use of lightweight materials such as aluminum and magnesium alloys has been increasing steadily, with the aim of realizing weight reduction of vehicle bodies [1; 2].  
In the case of joining between aluminum alloys and steels by a conventional fusion welding process, the generation of Fe\({}_{\mathrm{x}}\)Al\({}_{\mathrm{y}}\) brittle intermetallic compounds at the joining interface is unavoidable, and thus the joining strength is greatly lowered [3; 4]. To minimize this problem, friction stir spot joining and mechanical fastening processes such as the self-pierce riveting method have been tried in automotive industries [5; 6; 7; 8; 9; 10]. One of the disadvantages of applying friction stir spot joining and mechanical fastening processes, compared with the resistance spot welding process, is low productivity and the consequent increase of manufacturing costs [11].  
The automated resistance spot welding process is very suitable for the high volume and high rate of production of car bodies. Thus, this welding process is widely used in automotive industries [12].  
In dissimilar metal welding which uses the resistance spot welding process, different material properties (e.g., thermal conductivity, electrical resistivity, and melting point) result in asymmetrical -weld nugget formation. In contrast to the conventional spot welding process, the defining feature of DeltaSpot is process tape that runs between the electrodes and the sheets being joined. The reason for using the process tape in a DeltaSpot welding machine is to transfer the welding current while protecting the contact surfaces of the electrodes from contamination by aluminum, zinc, or organic residues. This protection results in a significantly increased service life for the electrodes. By using process tape, not only can the service life of the electrode be improved, but the heat generation characteristics during spot welding can also be changed according to the type of process tape. The problem of nuggets being generated asymmetrically during welding between dissimilar metals by spot welding can be reduced by applying different types of process tape [13].  
On the other hand, it is well known that galvanic corrosion occurs in welded joints of dissimilar material. When we make dissimilar metal welds of Al alloys and steel, an aluminum alloy having a low electrochemical potential becomes an anode, and steel having a high potential becomes a cathode in the corrosive environment [14; 15; 16; 17]. In the case of dissimilar metal welding of steels and aluminum alloys, galvanic corrosion is more inhibited in galvanized steel and Al than in cold-rolled steel and Al [18].  
In this study, a 440 MPa-class galvanenealed steel sheet, one of the materials used in vehicle bodies, was dissimilarly welded to an AA6061-T6 alloy by applying a DeltaSpot process to evaluate the ultimate tensile failure load and corrosion characteristics. Dissimilar welding specimens were prepared under three types of welding conditions: a specimen with only DeltaSpot welding applied, a specimen with adhesive and DeltaSpot welding applied, and a specimen with adhesive and DeltaSpot welding applied after roughening the specimen surface. The spot welded specimens were subjected to a salt spray test, and a tensile shear test was performed on the specimens after the corrosion test ended to compare the apparent shear strength of a single-lap joint-welded specimen by tension loading.