Numerical Investigation of Heat Source Characteristics in Arc Spot Welding Using Constricted Nozzle

In modern manufacturing where products are becoming smaller and lighter, parts of which those products consist also become smaller and thinner. Therefore, a joining of thin sheet metals is one of essential techniques, and developments of the joining and welding processes for the sheet metals, which are based on laser welding, friction stir welding and so on, have been conducted. However, they have some problems: e.g., equipment for the process is too large and maintenance of it is complicate. On the other hand, an arc spot welding using a special nozzle named constricted nozzle was developed and was practically used by Murata et al. Because this welding process is based on a tungsten inert gas (TIG) welding, the equipment is small and easy to maintain compared with above welding processes. During this welding process, the tip of a copper nozzle named outer nozzle which is an anode is pressed against a base metal, and welding is performed. Using low-welding current and short arc length, this welding process can weld thin sheet metals. In order to weld even thinner plates, it is necessary to control this welding process to achieve lower heat input. However, heat source characteristics of this process have not been investigated, and it is difficult to clarify this welding phenomena by experimental observations because the weld part is covered by the outer nozzle. Therefore, in this study, a two-dimensional axisymmetric simulation of an arc spot welding using the constricted nozzle was carried out to verify the heat source characteristics of this welding process. Computational model used in this study assumed two-dimensional axisymmetric and steady state. Governing equations were discretized by the finite volume method and solved by the SIMPLE (Semi-Implicit Method for Pressure-Linked Equations) algorithm. The mass in a computational domain was conserved by the mass conservation equation, and momentums in the radial and axial directions were determined by momentum conservation equations. The current density was calculated using the current conservation equation and the Ohm’s law, and the magnetic flux density was obtained by the vector potential. The temperature at each grid point was determined by the energy conservation equation considered additional heat transfers on anode and cathode surfaces as heat generation rates. A base metal was a water-cooled copper and the cathode was a tungsten electrode. Welding current, arc length and flow rate of argon gas which was used as an inner gas were set to 100 A, 0.3 mm and 1.5 L/min, respectively. In this study, heat source characteristics of conventional TIG welding were also simulated under same welding conditions for comparison. As a result, in the arc spot welding with the constricted nozzle, it was clarified that a part of the outer nozzle was in contact with the arc plasma region where the temperature was higher than 7,000 K. The computational result showed that the current flowing into a base metal surface was 72 A. Since the welding current was set to 100 A, it was suggested that a current of 28 A flowed directly from the outer nozzle to the tungsten cathode through the contact face of arc and outer nozzle. Moreover, the region of the tungsten cathode where the temperature was higher than the melting point of lanthanum oxide added as the electron emitter was narrower than that in the conventional TIG welding. This was because the current flowing to the tip of the cathode was smaller than the welding current, causing decrease of the Joule heating in the cathode tip. By the outer nozzle in contact with the base metal, the heat input range in the arc spot welding was narrower than that in the conventional TIG welding. In addition, it was also clarified that the peak of the heat input density in the arc spot welding was about 66 W/mm^{<span style="font-size:10.8333px">2</span>} lower than that in the TIG welding.