Abstract. While metal vapours have an important impact on the efficiency of the pulsed gas metal arc welding process, only a few papers are focused on this effect. In this paper, methods based on emission spectroscopy are performed to improve the understanding of the physical phenomena occurring during the high-current pulse. Boltzmann plots applied to iron lines, the Stark broadening of the 696.5 nm argon line and composition calculations assuming local thermodynamic equilibrium are used to determine characteristic parameters of the plasma. It is observed that the central part of the arc is mainly composed of iron. The percentage of iron increases quickly at the beginning of the high-current pulse, and slowly decreases, when the central part broadens. During the high-current phase the temperature profile has a minimum value of around 8000 K at the axis of the arc while the argon envelope of the central part reaches temperatures of approximately 13.000 K. High percentage of iron and high radiation of the plasma at the centre can explain the measured shape of the temperature profile.PACS number: 52.70.Kz 52.80Mg 81.20.Vj Keywords: Pulsed gas metal arc welding, GMAW, emission spectroscopy, Stark broadening Boltzmann plot IntroductionGas metal arc welding (GMAW) processes are widely used in industry to produce robust and high quality welded seams connecting metallic workpieces. GMAW processes use an electric arc in order to melt the welding wire so that metal droplets falling from the wire to the workpiece form the join. During the process the arc is enclosed in a shielding gas (typically argon) in order to control the current conduction path and protect the molten metal from oxidation by the atmospheric air. By using a pulsed current, it is possible to obtain a One Drop Per Pulse (ODPP) mode, which is characterized by a stable, periodical and controllable metal transfer to the workpiece [1]. The pulsed GMAW (GMAW-P) process is an effective way of achieving a controlled metal transfer to the workpiece over a wide range of heat and mass transfer rates [1]. Because of their good properties, ODPP GMAW-P processes are widely used in industry. They are intensively studied, in order to control, optimize and predict the quality of the welding seams.The conditions needed to obtain an ODPP are well studied in literature [1][2][3][4]. Parameters
The main aim of this investigation is to study the temperature and electrical resistance of the wire during gas metal arc welding (GMAW), because the wire heating contributes to a considerable extent to the power balance of the welding processes. As a reference, a typical pulsed GMAW process of mild steel with fixed wire feed speed of 4 m min−1 but varying free wire length and varying pulse current has been investigated. Three approaches have been applied for comparison: firstly, the surface temperature of the solid and molten wire parts has been measured with an infrared camera and a two-colour pyrometer with high-speed recording, respectively, and the wire resistance has been determined based on wire geometry and temperature dependent conductivity. Secondly, the resistance of the solid wire and the wire contact in the contact tube has been measured by an electrical probe. Thirdly, short circuit events together with surface temperature determination have been used to estimate the wire resistance. In addition, the temperature dependent conductivity of the corresponding wires has been measured systematically in a specific heating setup. Main results are a wire temperature exponentially decreasing with the distance from the wire tip and almost independent of the pulse current. The overall resistance of the wire part in the current path of the process including the contact tube scales mainly with the free wire length and reaches 13 m in our example for 15 mm free wire length.
In this paper, the voltage drop in pulsed gas metal arc welding (GMAW) is studied, focussing on the contributions of the different sections of wire and arc along the current path and their dependence on arc length and current. A typical pulsed GMAW process of mild steel in the one drop per pulse modus is considered as a reference. Voltage and current have been measured in experiments with fixed wire feed speed for varying arc length and free wire length. The temporally changing geometry of the arc and the molten wire parts has been studied carefully by highspeed imaging and image processing. In particular, the shielding gas and the metal vapour dominated parts of the arc have been captured simultaneously, and conclusions on the length of the current path in the arc column have been drawn. Comparing voltage measurements for the same current and different arc lengths, finally, the sum of cathode and anode sheath voltages as well as the mean electric field strength of the arc column have been determined. Sheath voltages increasing from 18 to 23 V for pulse currents from 350 to 650 A and a mean electric field of about 1.1 V mm −1 in the arc column, which is almost independent of the current, have been obtained. As a consequence, the Ohmic heating in the arc column contributes less than 20% to the overall power budget whereas the contribution of the electrode sheathes is larger than 60%.
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