Our previous work has demonstrated that, when welding a large diameter steel stud in the horizontal position, it is necessary to prevent molten metal from dripping from the weld pool. The formation of blowholes in the weld metal must also be suppressed to maintain the soundness of the joint. In addition, it was found that a joint can be considered suitable if its defect ratio does not exceed a threshold level of 10%, and that this can be achieved by optimizing various welding parameters, such as the arc holding time and current. In the present study, two processes were used to further improve the soundness of joints stud-welded in the horizontal position. First, the ferrule was inverted in order to reduce the interstices between it and the stud, and to eliminate gas through-grooves. Both factors are deleterious because they can allow molten metal to hang down from the weld pool. Inversion of the ferrule was found to be effective at supporting the molten metal and ensuring a suitable degree of fusion. Second, the welding atmosphere was controlled by employing aluminum as a deoxidant in conjunction with argon gas to shield the welded region from ambient air, thus reducing the formation of blowholes in the weld metal. The formation of blowholes was significantly reduced by the combined effects of these two methods.
Resistance spot welding of three steel sheets is used in automotive manufacturing. The determination of welding conditions for three steel sheets welding is more difficult than that for two steel sheets welding because in the former case, two interfaces are simultaneously welded. The welding condition to improve the weld strength of thin and thick sheets' interface is required for thin, thick, and thick sheets welding. In addition, avoiding weld spatter is an important technical concern for the three sheets welding. This paper discusses the triply coupled effects of elasto-plastic large deformation contact, electric current, and thermal conduction for the three sheets welding via triply coupled finite element analyses. The triply coupled effects of thin and thick sheets' interface and those of thick and thick sheets' interface are compared to discuss the special characteristics of the triply coupled effects of three sheets welding. The nugget diameter of the thick and thick sheets' interface is larger than that of the thin and thick sheets' interface because the electrical contact resistance and the base electrical resistance of high-strength steels are higher than those of general-strength steels. Furthermore, the chain of the coupled effects of electrical resistances, Joule heat generation, temperatures, and electrical resistances is strong. As the effect of welding current on temperature by the coupling effect for the thick and thick sheets' interface is stronger than that for the thin and thick sheets' interface, the possibility of spatter increases. Increasing the nugget diameter solely for the thin and thick sheets' interface using electrode force is difficult because the effect of the electrode force on the nugget diameter is approximately the same for both interfaces. On the contrary, material characteristics can affect the nugget diameter solely for either of the interfaces. The possibility of spatter increases for the welding condition with a sheet gap because the temperature of the contact edge for the sheets' interface increases.
Consumption of tungsten electrode during TIG welding process is one of unavoidable problems and many studies have been progressed to improve the consumption resistance. In general, some kinds of oxides (ThO2, La2O3, Ce2O3) are added to the tungsten electrode in order to make the thermionic emission easy and control the electrode temperature below the melting point of tungsten. However, the lifetime of electrode is still limited within a few hours because the additives evaporate and a lack of additives is caused on the electrode surface. In this study, numerical simulations which focus on the evaporation and diffusion phenomena of additives in the electrode were performed in order to clarify a consumption mechanism and identify effective factors for the lifetime of electrode. As time passed, the mass concentration of additive decreased due to an evaporation phenomenon whereas the additive was supplied from inside to outside of electrode by a diffusion phenomenon. When the degree of coverage of additive decreased, the electrode temperature quickly increased and it reached the melting point of tungsten. The lifetime of electrode was strongly depended on the physical properties of additives such as diffusion constant and melting point of their oxides.