Simulation And Verification Of Keyhole And Molten Pool Coupled Thermal-mechanical Transport Phenomena In Plasma Arc Welding

Welding is a very important basic technology in modern manufacturing industry. High-efficiency, high-quality and low-cost welding process is the eternal pursuit in the fabrication. Therefore, plasma arc welding (PAW) has become one of the most promising and efficient welding technology in the21st century because of its potential advantages. PAW is superior to the conventional arc welding at the energy concentration, larger arc stiffness, deep penetration in a single pass, narrow heat-affected zone, small welding deformation and high efficiency. Compared with laser welding and electron beam welding, PAW also has advantages in equipment cost, maintenance, less operation complexity and flexibility of torch movement. However, there are some unresolved key issues in the practical PAW application, such as the sensitive keyhole stability to process conditions, poor stability of weld formation, narrow access to quality welded joints and suitable parameter range. These problems restrict the broad application of this highly efficient welding technology. Hence, this paper conducted research on the dynamically coupled heat transfer characteristics between the weld pool and keyhole in order to provide theoretical guidance for the quality control of weld formation.Accoording to the literature review, a program, that the thermal-physical mechanism in PAW is revealed step by step, is conducted with the combination of theory and experiments. First, a welding experimental system was set up, and then a mathematical model was developed based on key parameters and physical phenomena during the welding process. PAW experiments were carried out on stainless steel plates, which provided validation for the simulation results.Based on macro heat transfer process, theoretical research was conducted by the development of an appropriate volumetric heat distribution to reflect the keyhole effect, which does not involve the real keyhole appearance, but to characterize the thermal-mechanical keyhole effect with proper volumetric heat source. It is called as the equivalent heat source model, which focuses on the prediction of temperature field similar to PAW experiment. Then a quasi-steady keyhole-tracking heat source model was developed to explore the energy propagation in the wake of keyhole evolution. The Volume of Fluid (VOF) method was applied to track the keyhole interface which was coupled with the dynamically developing keyhole-tracking heat source. The coupled thermal-mechanical mechanism between keyhole and the weld pool was investigated in details, as well as the heat transfer and fluid flow induced in the welding process. On the basis of quasi-steady heat source model, a moved keyhole-tracking heat source model was also developed to study the dynamic keyholing process and its remarkable effect on the energy propagation and fluid flow in the weld pool, concerning not only the keyhole evolution but also the welding speed.Up to now, domestic and foreign research on PAW generally ignores the thermal arc processing, but only focuses on heat transfer in the workpiece. However, the stable keyhole PAW process is strongly associated with the fierce thermo-mechanical effect of plasma arc. In order to more accurately understand the complete welding mechanism, it becomes quite important to investigate the interaction between the plasma arc and the weld pool. Hence, a unified model coupling plasma arc and weld pool has been developed to help gain access to the knowledge of energy conversion in the thermal plasma process and heat transfer to the weld pool with the consideration of keyhole effect. The model can calculate heat transfer from the plasma arc to the workpiece rather than just represent the heat with a source term. The model can more accurately describe the transport phenomena of coupled thermal field, electric field, magnetic field and flow field in plasma arc welding, as it is more closer to the actual physical welding process. Finally, a orthogonal test was carried out with7Factors and3Levels, and it is able to effectively predict the importance of various welding process parameters.