Integrated Numerical Analysis Of Plasma Arc, Weld Pool And Keyhole Behaviors

In keyhole plasma arc welding (PAW), an open keyhole is formed within the penetrated weld pool, so that one-side welding with both sides formation can be achieved. PAW is one important joining technology for plates with medium thickness. However, the PAW process is sensitive to the welding process parameters, and the stability of the open keyhole and the adaptivity of PAW are weak. The thermal and pressure characteristics of plasma arc are the key factors influencing the formation of the open keyhole. The plasma arc, weld pool and keyhole interact with each other. Therefore, analysis of heat and pressure transfer mechanisms between the plasma arc, weld pool and keyhole during the PAW process is essential, and is of great significance for optimization of process parameters and expansion of the applicable range of PAW process.An axisymmetric model of the plasma arc is developed, includes the nozzle, tungsten cathode and arc plasma. The CFD commercial code Fluent and its user defined function are used to solve the conservation equations. The temperature, flow velocity and electromagnetic fields of the plasma arc are analyzed. The influences of nozzle diameter and plasma gas flow rate on the properties of the plasma arc are examined.The development of an integrated computational model of plasma arc-weld pool-keyhole for a stationary PAW is described. The ’Local thermodynamic equilibrium-diffusion approximation’ method is used to treat the sheath layer between the plasma arc and the weld pool. Based on this method, appropriate mesh size has to be employed on the anode surface, LTE (Local thermodynamic equilibrium) is assumed near the electrodes. In this way, the calculation of the complicated phenomenon in the sheath layer is avoided, and the problem of the equilibrium electrical conductivity near the electrodes is solved. A heat source term is added to the energy equation in meshes at the electrode surface to calculate the heat flux at the sheath layer. The physical phenomena in the whole welding process, including the weld pool surface depression, blind keyhole expansion, dynamic coupling between the arc plasma and keyhole, interaction of keyhole and weld pool, and the transient behaviors of temperature, fluid flow and electromagnetic fields in the whole domain, are numerically simulated. The dynamic variations of the current density, plasma arc pressure and heat flux at the weld pool surface with the evolution of keyhole boundary are quantitatively analyzed.The behavior of metal vapor is coupled with the axisymmetric model to predict the transient distributions of iron vapor. It is shown that the thermal plasma in PAW and the temperature.field in the weld pool are influenced by iron vapor from the molten pool surface.An integrated three-dimensional model with considering the relative movement between the electrode and work-piece is developed for PAW according to the technique of the moving grid system. As demonstrated by the numerical analysis results, with the moving of plasma arc along the welding direction the molten metal from the continuous melting of the metal in front of the weld pool flows around the keyhole to the back of the weld pool. The front keyhole wall is almost consistent with the molten-solid interface at the front of the weld pool, and here the molten metal layer is very thin. Most molten metal is mainly accumulated in the weld pool behind the keyhole. The axis of open keyhole channel is inclined backward with respect to the torch axis, and the open keyhole shape is asymmetric about the torch axis, which causes the unsymmetrical distribution of the heat intensity and pressure on the front wall and rear wall of the open keyhole.The model is validated by the pressure and current density measurements at the flat surface of the anode, dimensions of the keyhole exit and fusion line profiles. The predicted and measured results in these respects match with each other basically.