Theoretical model and experimental investigation of current density boundary condition for welding arc study

Abstract

Abstract. This paper presents results of theoretical and experimental investigation of the welding arc in Gas Tungsten Arc welding (GTAW) and Gas Metal Arc Welding (GMAW) processes. A theoretical model consisting in simultaneous resolution of the set of conservation equations for mass, momentum, energy and current, Ohm's law and Maxwell equation is used to predict temperatures and current density distribution in argon welding arcs. A current density profile had to be assumed over the surface of the cathode as a boundary condition in order to make the theoretical calculations possible. In stationary GTAW process, this assumption leads to fair agreement with experimental results reported in literature with maximum arc temperatures of ~ 21000 K. In contrast to the GTAW process, in GMAW process, the electrode is consumable and non-thermionic, and a realistic boundary condition of the current density is lacking. For establishing this crucial boundary condition which is the current density in the anode melting electrode, an original method is setup to enable the current density to be determined experimentally. High-speed camera (3000 images/sec) is used to get geometrical dimensions of the welding wire used as anode. The total area of the melting anode covered by the arc plasma being determined, the current density at the anode surface can be calculated. For a 330 A arc, the current density at the melting anode surface is found to be of 5×10 7 A.m -2 for a 1.2 mm diameter welding electrode. IntroductionIn arc welding, an electric arc is burning between a "workpiece" and an auxiliary electrode: -For gas metal arc welding (GMAW) process the auxiliary electrode is welding wire, which is usually the anode (reverse polarity). Heat transfer from the arc and Ohm's heating in the wire melt its tip forming droplets, transferred through the arc to the workpiece used as a cathode. -For gas tungsten arc welding (GTAW) process, the workpiece, which is usually the anode (straight polarity), locally melts due to heat transfer from the arc, forming a weld pool. Thermal behaviour of welding arcs and their electrodes can have significant effects on the subsequent weld quality and production rate. Hence, it is desirable to have a theoretical method which can predict the properties of both electrodes and arc plasma as functions of the arc operating conditions. The behaviour of an arc is governed by a coupled set of physical laws, i.e., Ohm's Law, Maxwell's equations and conservation equations of mass, momentum, energy and electrical charge [1,2]. The modelling of arc welding processes has been reported by a number of researchers [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. Most early numerical models have treated either only the arc plasma [2][3][4][5][6][7] or only the weld pool [8][9][10][11][12][13][14]. For the arc plasma, a plane surface of the solid anode has been set to be a bottom boundary for the electric potential and temperature which are given as the boundary conditions. The tungsten cathode has been assumed to be a...

Study on factors affecting the droplet temperature in plasma MIG welding process

“…This taper formation suggests that the MIG current could have dispersed from a higher position along the wire to the droplet. A large portion of the heat input into the wire functioned as an anode due the electron condensation by MIG current, and consequently melts the vicinity of the wire surface [32]. Therefore, in the plasma MIG welding, the presence of the plasma surrounding the wire could have expanded the arc to move upwards along the wire surface to disperse the MIG current.…”