Won-Ik Cho

Influence of driving forces on weld pool dynamics in GTA and laser welding

IntroductionThere are several driving forces such as surface tension, buoyancy force, arc pressure, electromagnetic force, recoil pressure, drag force, etc. in gas tungsten arc (GTA) welding and laser welding, which have different influences on flow dynamics in the weld pool.Mathematical modelsThis paper investigates the influence of respective driving forces on weld pool dynamics by using mathematical models and numerical simulations. In numerical simulations, the flow patterns in the weld pool and the maximum fluid velocity caused by respective driving forces can be observed, since driving forces can be applied separately.Results and discussionAs all driving forces are applied under experimental conditions, the results of experiments and numerical simulations were compared to validate the numerical simulations and mathematical models used in this paper. In GTA welding, Marangoni flow can be considered as the most dominant force in the radial direction, while the velocity magnitudes of the z-axis direction of all respective forces except buoyancy and drag force are almost the same. The influence of buoyancy force is negligible. In keyhole laser welding, however, recoil pressure can be considered the dominant force, while the other driving forces have only a negligible influence on fluid dynamics in deep keyhole welding. In laser–GTA hybrid welding, recoil pressure can be considered the dominant force.

A Study on the Process of Hybrid Welding Using Pulsed Nd:YAG Laser and Dip-transfer DC GMA Heat Sources

Until now, many researches on laser-arc hybrid welding processes have been conducted mainly for high power CW laser and high direct current arc to weld the thick steel plates for shipbuilding. Recently, however the usage of thin steel plates, which tend to be deformed easily by thermal energy, is been increasing because of demand of light structure such as car body in the automobile industry. Accordingly, heat sources having relatively low heat input such as pulsed laser, dip-transfer DC GMA and pulsed GMA seem to be applied more increasingly and the study about those heat sources is needed more intensively. Any heat source mentioned above can not stand alone without weld defects at a relatively high welding speed for increasing the welding productivity. This is main reason to apply the hybrid welding process which uses pulsed laser and low-heat-input GMA heat sources simultaneously to weld the thin steel plate. In this study, parameters of pulsed laser and dip-transfer DC GMA welding are studied firstly through preliminary experiments, and then analyzed in the viewpoint of their physical phenomena. Before conducting the hybrid welding, a pulse control technique is developed based on the parallel port communication and Visual C++ 6.0. Owing to development of this technique, interactions of laser and arc pulses can be controlled consistently. Using the pulse control technique, the hybrid welding is conducted and then its interactive welding phenomenon is analyzed.

Modeling of keyhole formation in laser and laser ARC hybrid welding based on CFD simulations

Volume-of-Fluid (VOF) method is used to calculate the free surface shape of the keyhole using a unique ray-tracing algorithm which can take into account the effects of multiple reflections, laser heat source shape (Gaussian, etc.), laser beam profile, metal vapor shear stress, metal vapor heat source, sulfur contents, etc. on the formation of keyhole and subsequently their effects on molten pool behavior, fusion zone shape, etc. The model can be used alone for laser welding and it can also be combined with arc heat source models such as gas metal arc welding (GMAW) to simulate the laser-GMA hybrid welding. Salient features of the model like ray tracing, search of free surface cells, multiple reflections and laser material interaction using simplified Fresnel’s reflection model by the Hagen– Rubens relation are discussed in little detail. Other than keyhole formation, factors considered in the simulations include buoyancy force, Marangoni force, recoil pressure and especially pore generation is simulated b...

Influence of the Groove Angle on Arc Characteristics in Pulsed GMA Weaving Welding

In this paper, arc characteristics of V groove joints using pulsed GMA welding were found out. The bevel angles of 22.5˚ and 30.0˚ were chose to make the V groove configuration with the groove angles of 45˚ and 60.0˚, respectively. In the experiment, the arc current waveform measurement and the high speed photography were taken to investigate the arc characteristics for a single-beveled asymmetric workpiece. Consequently, the welding current was changed abnormally around the edge of groove. As the arc moved close to the groove face, the welding current was increased rapidly because the welding arc was affected by the inclined surface. Also the welding current waveforms were measured for the double-sided symmetric workpiece to verify the previous measurements for the single-beveled workpiece, and similar current waveforms were found.

Numerical Simulation of Bubble and Pore Generations by Molten Metal Flow in Laser-GMA Hybrid Welding

Three-dimensional transient simulation of laser-GMA hybrid welding involving multiple physical phenomena is conducted neglecting the interaction effect of laser and arc heat sources. To reproduce the bubble and pore formations in welding process, a new bubble model is suggested and added to the established laser and arc welding models comprehending VOF, Gaussian laser and arc heat source, recoil pressure, arc pressure, electromagnetic force, surface tension, multiple reflection and Fresnel reflection models. Based on the models mentioned above, simulations of laser-GMA hybrid butt welding are carried out and besides the molten pool flow, top and back bead formations could be observed. In addition, the laser induced keyhole formation and bubble generation duo to keyhole collapse are investigated. The bubbles are ejected from the molten pool through its top and bottom regions. However, some of those are entrapped by solid-liquid interface and remained as pores. Those bubbles and pores are intensively generated when the absorption of laser power is largely reduced and consequently the full penetration changes to the partial penetration.