Hai-Lung Tsai (tsai@mst.edu, 573-341-4945), Jun Zhou, Wenhai Zhang, Yun Wang, Fanlin Zhu, P.C. Wang, R. Menassa, Donn Glander

This project is focused on investigating the combination of laser welding and gas metal arc welding (GMAW). Science-based studies have been conducted to fundamentally understand the relationships between various process variables and the resulting weld quality. In the modeling, the interactions of shielding gas from laser welding and from GMA welding and their possible unification, the interplay among various GMAW parameters and laser parameters, the recoil pressure and other possible mechanisms contributing to the depth and shape of the keyhole, the generation and flow of arc plasma in the keyhole and their effects on keyhole dynamics; and the laser-plasma interaction will be studied. Several benchmark experiments will be conducted, including spot welding, bead-on-plate, and lap-joint, and the resulting welds will be compared against the modeling predictions. The weld will be thoroughly examined and tested, including the weld bead shape and penetration, micro-hardness testing, porosity testing, mechanical strength testing and fatigue testing. In addition, extensive parametric studies will be conducted and the Taguchi method will be used to design the experiments and to identify critical process parameters to achieve quality welds. The goal of this project is to seek the best possible process window to ultimately achieve quality welds.

Web link for this project: http://web.mst.edu/~laimp/LJP1.html#

1. “Modeling the transport phenomena during hybrid laser-MIG welding process,�? J. Zhou, W.H. Zhang, H.L. Tsai, S.P. Marin, P.C. Wang and R. Menassa, IMECE’03, Washington, D.C., November 16-21, 2003.

Abstract: Hybrid laser-MIG welding technology has several advantages over laser welding alone or MIG welding alone. These include the possibility of modifying weld bead shape including the elimination of undercut, the change of weld compositions, and the reduction of porosity formation in the weld. Although the hybrid laser-MIG welding method is becoming popular in industry, its development has been based on the trial-and-error procedure. In this paper, mathematical models and the associated numerical techniques were developed to calculate the heat and mass transfer and fluid flow during the laser-MIG welding process. The continuum formulation was used to handle solid phase, liquid phase, and mushy zone during the melting and solidification processes. The volume-of-fluid (VOF) method was employed to handle free surfaces, and the enthalpy method was used for latent heat. The absorption (Inverse Bremsstrahlung and Fresnel absorption) and the thermal radiation by the plasma in the keyhole, and multiple reflections at the keyhole wall were all considered in the models. The transient keyhole dynamics, interactions between droplets and weld pool, and the shape and composition of the solidified weld bead were all predicted for both the pulsed laser-MIG welding and three-dimensional moving laser-MIG welding. Computer animations showing the fluid flow, weld pool dynamics, and the interaction between droplets and weld pool will be shown in the presentation.

2. “Numerical modeling of keyhole dynamics in laser welding,�? W.H. Zhang, J. Zhou and H.L. Tsai, Proceeding of the SPIE, Vol. 4831, 2003, pp. 180-185.

Abstract: Mathematical models and the associated numerical techniques have been developed to study the following cases: 1) the formation and collapse of a keyhole, 2) the formation of porosity and its control strategies, 3) laser welding with filler metals, and 4) the escape of zinc vapor in laser welding of galvanized steel. The simulation results show that the formation of porosity in the weld is caused by two competing mechanisms: one is the solidification rate of molten metal and the other is the speed that molten metal backfills the keyhole after laser energy is terminated. The models have demonstrated that porosity can be reduced or eliminated by adding filler metals, controlling laser tailing power, or applying an electromagnetic force during keyhole collapse process. It is found that a uniform composition of weld pool is difficult to achieve by filler metals due to very rapid solidification of the weld pool in laser welding, as compared to that in gas metal arc welding.

3. “Modeling of hybrid laser-MIG keyhole welding process,�? J. Zhou, H.L. Tsai, P.C. Wang, R.J. Menassa and S.P. Marin, ICALEO’03, Jacksonville, Florida, and October 13-16, 2003, Sec. A, pp. 135-141.

Abstract: Recently, hybrid laser-MIG welding technology has increasingly attracted interest in both industry and academia. By combining the two welding processes, it can modify the weld bead shape including the elimination of undercut, change the weld compositions, reduce the porosity, improve welding bridge-ability, decrease the susceptibility of hot cracking, and increase welding speed. So far, the development of laser-MIG welding technology has been based on the trial-and-error procedure. In this paper, mathematical models and the associated numerical techniques have been developed to simulate the laser-MIG welding process. The transient keyhole dynamics, interaction between droplets and weld pool, and the shape and composition of the solidified weld were predicted for a three-dimensional moving laser-MIG welding. The heat and mass transfer and fluid flow in molten metal and temperature distribution inside the keyhole were studied. In the model, the volume-of-fluid (VOF) method was employed to track free surfaces. The Inverse Bremsstrahlung absorption of laser energy inside plasma, Fresnel absorption and the multiple reflections at the keyhole wall, and the thermal radiation by the plasma in the keyhole were all considered. Computer animations showing the fluid flow, weld pool dynamics, and the interaction between droplets and weld pool will be shown in the presentation.

4. “Impingement of filler droplets and weld pool dynamics during gas metal arc welding process,�? Y. Wang and H.L. Tsai, Int. J. Heat Mass Transfer 44 (2001), pp. 2067-2080.

Abstract: A mathematical model and the associated numerical technique have been developed to simulate, for the first time, the dynamic impinging process of filler droplets onto the weld pool in spot gas metal arc welding (GMAW). Filler droplets driven by gravity, electromagnetic force, and plasma arc drag force, carrying mass, momentum, and thermal energy periodically impinge onto the base metal, leading to a liquid metal puddle. Transient weld pool shape and the complicated fluid flow in the weld pool caused mainly by the combined effect of droplet impinging momentum and surface tension are calculated.

5. “A comprehensive model on the transport phenomena during gas metal arc welding process,�? F.L. Zhu, H.L. Tsai, S.P. Marin and P.C. Wang, Progress in Computational Fluid Dynamics, an Int. J. (in press).

Abstract: In gas metal arc welding (GMAW), current is the most important factor affecting the mode of metal transfer and subsequently the weld quality. A table, one droplet per pulse (ODPP) metal transfer mode was found experimentally to be able to reduce spatter and improve weld quality. Using the previously developed comprehensive mathematical model, the influences of different current profiles on the droplet formation, metal transfer, and weld pool dynamics were studied. Four types of welding currents were studied, including two constant currents and two waveform currents. The results show that welding current and the resulting electromagnetic force are the key factors affecting droplet size and droplet frequency, and determining the metal transfer mode. The model has demonstrated that a stable ODPP metal transfer mode can be achieved by choosing a current with proper waveform for given other welding conditions.