Numerical Simulation Of The PAW Process

The plasma arc welding (PAW) possesses a higher productivity for its high energy density arc and a powerful arc force at the anode. This kind of welding technology can finish the welding process only with single pass, and even keyhole effect appears when the arc force is strong enough. But too many factors affect the stability of the PAW process, especially the status of the keyhole in keyhole PAW, which restricts its widely application in industry. How to keep the stability of PAW process has been concerned always. While, with the development of computer technology, numerical simulation is widely used in the welding technology. So simulation study on the PAW process is put forward here, to provide theoretical instruct for the PAW process.Based on the set of conservation equations for mass, momentum, energy and the Maxwell equations, a two-dimensional mathematical model has been developed. The model includes a part of nozzle and tungsten electrode to study the influence of nozzle configuration and electrode shape on the plasma arc characters. Setting proper boundary condition, some kind of CFD software based on finite volume method is used to solve the model with SIMPLEC algorithm. The temperature field, flow field and electromagnetic field of the plasma arc are gotten and analyzed from the model. And the influence of current, ionized gas flow rate, arc length etc on plasma arc character are also studied numerically, as well as the nozzle configuration and electrode shape. To verify the supposition that the plasma arc is in laminar flow state, another plasma arc model is built in which arc flow state is supposed to be turbulent. The results of the two kinds of arc model are compared, and the plasma arc is considered to be laminar judged by the ratio of effective viscosity and dynamic viscosity calculated in turbulent arc model. Also, the plasma arc is considered to be incompressible by the Mach number gotten form the laminar arc model.Analyzing the PAW process, three-dimensional weld pool models acted by static arc and moving arc respectively have been developed to predict the temperature and flow of it. The heat flux and current density distributions at the anode are taken from the results of plasma arc model. The solid part of workpiece is taken as liquid by setting a great viscosity to the workpiece when the temperature is above the melting point. Finally, the weld pool models are solved by ANSYS software. A three-dimensional non-axisymmetrical plasma arc model has been developed to study the plasma arc reflection phenomenon in the keyhole PAW, considering the existing of keyhole. The simulating results show that the arc reflection gets weaker with the increase of keyhole dimension, and a higher welding velocity to induce the offset between the arc axis and keyhole axis is necessary to the appearance of arc reflection. The theory of arc reflection detecting circuit is also simulated studied. The result shows that there is some certain relation between the keyhole dimension and voltage signal on the additional resistance, that is, the voltage signal on the additional resistance decreases with the increase of keyhole dimension. According to the simulated results, a new kind of arc reflection detecting circuit with two probes is designed which can detect the keyhole dimension and its changing.