Modeling Of Heat And Mass Transfer During Gas Metal Arc Welding

Gas metal arc welding (GMAW) has been applied for jointing of metal in broad industries due to its high productivity, simplicity and ease of use, such as automotive, aerospace, shipbuilding, defense, oil, MEMS, electronics and many other industries. With the increase of mechanization and automation, selection of optimum operating parameters of GMAW becomes increasingly essential for higher weld quality, productivity and safety. This study presents a numerical investigation on the heat and mass transfer occurring in GMAW under various welding conditions. A 2D comprehensive mathematical model employing the volume of fluid (VOF) technique and the continuum formulation is developed, coupling the transient processes of arc plasma's generation and change; droplet formation, detachment, and impingement onto the workpiece; and welding pool dynamics. This model is used to study the electrode melting phenomena and the determination of wire-feed-speed (WFS), and the roles of shielding gas compositions in arc plasma and metal transfer. Furthermore, by considering the combined effects of droplet impingement, gravity, electromagnetic force, plasma arc force and surface tension force, a 3D moving GMAW model is developed to predict the complex transport phenomena and their effect on the formation of ripples on the bead surface.In GMAW, the consumable electrode wire must be continuously fed in such a speed that it balances the electrode melting rate in order to achieve a stable welding. Due to the strong interplay with the other welding variables and periodic oscillations induced by the formation and detachment of droplet, the determination of WFS is very complex. The 2D model is used to examine the thermal process and melting phenomena in moving electrode, and the influence of WFS on stability of welding process. Taking into account the effects of welding current and electrode diameter, the equilibrium WFS for stable welding processes under various conditions are presented. It is found that the electrode extension length fluctuates as a function of time in a narrow range in which the electrode melting can "self-adjust" itself leading to an equilibrium WFS for a stable welding; otherwise may lead to either the electrode burn-back or the electrode stick-onto the weld pool. The predicted equilibrium WFS are in good agreements with the published experimental data that were obtained through the trial-and-error procedure.Based on the 2D model, the effects of shielding gas compositions on the transient transport phenomena occurring in GMAW of mild steel at a continuously constant electric energy or constant current were studied. The behaviors of plasma arc and metal transfer for GMAW operating in arcs of pure argon,75% Ar+25% He,50% Ar+50% He and 25%Ar+ 75% He are obtained. The results indicate that the arcs in various shielding gases behave very differently due to the significant difference in thermophysical properties, especially the ionization potential. With the more addition of helium content, results in 1) the change of plasma arc shape from bell-like to cone-like and 2) the change of arc pressure distribution along the workpiece surface from Gaussian-like to flat-top with decreasing peak value. In high helium arc, the arc becomes increasingly contracted and produces a more significant upward electromagnetic force near the cathode, leading to the dramatically distorted distributions of arc parameters. The increase of helium content and the resulting arc contraction induce an upward electromagnetic force at the bottom of the droplet that sustains the droplet at the electrode tip. Thus, the more oblate droplet and the longer droplet formation time are produced. The behaviors of the predicted droplet shape and detachment frequency are confirmed by the published results.The 3D model is employed to predict the transient distributions of velocity, temperature and species, weld pool dynamics and surface rippling on the solidified weld bead during a GMAW process. It is found that the surface ripples are formed by the interplay between the up-and-down weld pool dynamics, caused mainly by the periodic droplet impingements, and the rate of weld pool solidification. The effects of various welding parameters, including the welding current, droplet size, droplet frequency, droplet impinging velocity, and travel speed on the pitch (distance between two ripples) and height of the ripple are investigated. At high weld current, the impingement of small droplets with very high frequency produces the strong weld pool convection and slow solidification rate, forming fine and dense ripples. This study will help to provide a reliable and productive welding technology for a wide range of industrial applications. It will have a much broader impact and benefit as follows:1) the improved fundamental understanding of arc plasma and metal transfer in GMAW; 2) the significant advance in GMAW process with right selection of operating parameters, such as WFS, shielding gas composition and so on, and hence the innovation in metal-joining technique; 3) the advance the industrial productivity of producing high quality welding components, thereby reducing overall production costs.