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.
Magnesium, which is the lightest among the metals used as structural materials, has several advantages such as high strength-to-weight ratio and high recyclability. Recently, the demands of dissimilar metal joint of magnesium alloy to steel have arisen for weight reduction in transportation vehicle industry. However, it is well known that joining of magnesium alloy to steel is difficult because of differences in melting point and thermal conductivity between both metals. Therefore, new welding processes with high reliability and productivity for these dissimilar materials are demanded. Laser roll welding is one of the candidates, which is effective for joining of dissimilar metals. In the present work, laser roll welding of magnesium alloy to steel was tried to investigate the effects of the process parameters on the microstructure of the joint and the mechanical properties. As a result, existence of the interface layer consisting mainly of Fe and Al was confirmed, and increase in welding speed led to decrease in the layer thickness. In addition, increase of bonding area at the joint interface led to increase of the joint strength.
Computational model of submerged arc welding (SAW) was developed to clarify the arc phenomena and the heat source characteristics. Furthermore, the weld part during SAW was observed using an X-ray transmission system. To investigate the effect of the closed space on arc phenomena, the heat flux and current density distributions on a base metal surface during closed gas tungsten arc welding were measured by split anode method. Computational results showed that the metal vapor concentration in arc space during SAW became higher because the arc was generated in the closed space. However, its heat input to the base metal was almost the same as that of gas metal arc welding. Meanwhile, the welding current and arc voltage which largely affect characteristics of arc plasma agreed with the experiment results. The slag thickness obtained from the computation was also the same as that of the experiment, which supports the validity of this computational model. Moreover, comparative experiments with gas tungsten arc and closed gas tungsten arc showed that the closed space environment around the weld part in SAW did not affect heat characteristics of arc plasma. It was clarified that heat transfer and radiation energy of arc play different roles in SAW. Penetration of a base metal was formed by the heating from arc. Flux was melted by the radiative heating from arc, and it formed slag. It was suggested that the high heat efficiency from 90 to 99% during SAW was obtained since the radiation energy was given to a base metal indirectly through the slag.
It is well-known that diffusible hydrogen strongly influences cold cracking. However, it isn't understood well what kinds of factors in welding consumables affect diffusible hydrogen in weld metal. For example, it is thought that seamed flux cored wire has more amount of diffusible hydrogen than seamless flux cored wire. This study pointed out a misunderstanding of diffusible hydrogen content generated by FCAW. As a result of investigation, there was no correlation between wire structure and diffusible hydrogen content. Furthermore, welding-wire-related factors on GMAW and FCAW were classified clearly into the follows. (a) Surface lubricant, (b) Initial moisture of flux and (c) Moisture absorbed after production. Various welding wires were prepared and measured the diffusible hydrogen content in order to compare the effect of the each factor. As a result, the most influential factor is (b). The second is (c). The effect of (a) is the smallest.