Influencing factors on nugget formation during multipoint welding by single side resistance spot welding

Recently, weight reduction and improvement of crashworthiness of auto bodies have been important issues. At the same time, stiffness of auto bodies is also needed to ensure a smooth ride. Using hollow parts, such as bended pipes and hydroformed parts, is one of the solutions to the demand for both rigidity and light weight. To weld hollow parts and sheet panels together, welding methods which allow us to access from one side are required. Single side resistance spot welding (single side RSW) process is one of those, and has recently been attracting attention. However, because of the long current path and small electrode force, it is difficult to concentrate the electric current in the welding spot compared with conventional direct resistance spot welding (direct RSW). Furthermore, in multipoint welding, shunt current will occur easily, and the nugget formation will be inhibited. To obtain a guideline for making sound nuggets, influencing factors for shunt current were investigated. And a numerical study was carried out to discuss the difference between direct RSW and single side RSW. According to the CAE analysis, the shunt current of single-side RSW will be higher rate than direct RSW. The rate of shunt current was influenced by the electrical resistance of its current path. For this reason, with shorter distance between welding points, or with lower electrical resistance of material, it is difficult to get large nuggets. By enhancing the electrical resistance of shunt current path, shunt current could be reduced and a larger nugget would be obtained.