Simulation of metal transfer and weld pool development in gas metal arc welding of thin sheet metals.

Abstract

Gas metal arc welding (GMAW) is the most commonly used arc welding method in industry for joining steels and aluminum alloys. But due to the mathematical difficulties associated with the free surface motion of the molten droplet and the weld pool, the process is not well understood and the development of new welding procedures in the manufacturing industry highly depends on expensive, time-consuming and experience-based trial and error. In this dissertation, numerical methods are developed to overcome the difficulties and to simulate the metal transfer and weld pool development in the GMAW of sheet metals. The simulations are validated by experiments and used to study an industrial welding process. A numerical procedure is first developed to model the free surface motion in fusion welding processes. Thermal and electromagnetic models are integrated with the fluid models. Recommendations are made on the selection and improvement of publicly available numerical algorithms, while alternative methods are also reviewed. A model combining the enthalpy, effective-viscosity and volume-of-fluid methods is then developed to simulate the metal transfer process in globular, spray and short-circuiting transfer modes. The model not only describes the influence of gravity, electromagnetic force and surface tension on droplet profile and transfer frequency, but also models the nonisothermal phenomena such as heat transfer and phase change. The melting front motion, the droplet detachment and oscillation, the satellite formation and the fluid convection within the droplet are analyzed. It has been found that the taper formation in spray transfer is closely related to the heat input on the unmelted portion of the welding wire, and the taper formation affects the globular-spray transition by decelerating the transfer process. Experiments with a high-speed motion analyzer validate the simulation results. The model is then extended to simulate the initiation, development and solidification of the weld pool, with consideration of the droplet impingement on the pool surface. The characteristics of physical variables in the weld pool are analyzed. Burn-through of thin sheet metals and penetration of a multi-layered workpiece are also simulated. Experiments are used to verify the predicted weld penetration. Finally, numerical simulation is used to analyze an industrial welding process---the gas metal arc spot welding of multi-layered workpieces. The traditional spot welding and plug welding methods are simulated. It is shown that the plug method can ensure a stable short-circuiting transfer through a pre-made hole. Both the simulation and tensile-shear tests show that the plug method improves the penetration consistency by ensuring the effective joint diameter.