Sponsored by the American Welding Society and the Welding Research Council Visualization of Gas Flows in Welding Arcs by the Schlieren Measuring Technique The influence of typical welding parameters on the gas flow for the GTAW, GMAW, and PAW processes is demonstrated using the high-speed Schlieren technique Introduction SUPPLEMENT TO THE WELDING JOURNAL, JANUARY 2014 BY E. SIEWERT, G. WILHELM, M. HÄSSLER, J. SCHEIN, T. HANSON, In gas tungsten arc welding (GTAW) the arc and the weld pool are protected against the influence of atmospheric gases by a shielding gas. Contamination of the shielding gas leads, among other things, to arc instability, oxidation, porosity, and spatter. Furthermore, atmospheric gases such as oxygen, carbon dioxide, or nitrogen affect the characteristics of the plasma and influence the arc spots at the cathode and anode. Therefore, one important goal of welding torch development is to generate an optimal gas flow through the welding torch in order to guarantee a stable and protective shielding gas coverage. To achieve this, it is most important to avoid flow separation and turbulence in the shielding gas nozzle. In order to minimize the experimental effort by performing numerous welding experiments, computational fluid dynamics and gas flow diagnostics can be used. In prior work, attempts were made to follow this route. As described in Refs. 5 and 13, the computational fluid dynamics were used to optimize the welding fume exhaustion. However, in these simulations, the arc was either neglected or significantly simplified by being modeled as a source of thermal energy with a preset momentum. In Refs. 1 and 8, the commercial software ANSYS CFX was used with a contained arc module to calculate the shielding gas flow and the diffusion. However, the models used were based on assumptions and many simplifications. Moreover, the torch geometry was often simplified in order to reduce the numerical mesh size. Thus, verified experimental findings are needed for proofing and calibrating of these models. To analyze gas flow fields, particlebased methods such as the laser doppler anemometry (LDA) and the particle image velocimetry (PIV) can be used. By Zschetzsche (Ref. 2) the applicability of both methods for the measurement of gas flow in arc welding was tested and the PIV method was adapted to measure different welding processes. The method enabled a nonintrusive and temporally resolved detection of a two-dimensional gas flow field in GTAW and gas metal arc welding (GMAW). However, LDA and PIV measurements are extremely cost-intensive and require a high measuring technique effort. An easier way to visualize gas flows is the Schlieren technique, which has been known since the 17th century (Refs. 3, 7, 14–16). Typical applications where the Schlieren measuring method was previously used are airplane aerodynamics, ballistics, and ventilation technology (Ref. 6). Schlieren studies of electrical discharges (arcs) were first carried out by Toepler (Ref. 3). In the field of cutting technology, oxygen cutting analyses were carried out by the Schlieren technique in the 1930s (Ref. 9). Gas flow studies of arcs by the Schlieren technique are especially used in plasma cutting processes and thermal spraying (Ref. 6). Gas flow visualization of plasma cutting arcs and the interaction of the arc with the workpiece are known from investigations by Settles (Ref. 10). These investigations can be extended to image the gas flow and turbulences below the workpiece as well. In order to detect instabilities in the plasmacutting process, Heberlein (Ref. 11) used the Schlieren technique in combination with current and potential measurements as well as acoustic recordings. An explanation of the relationship between nozzle design and cutting quality was derived based on Schlieren images. In contrast, Schlieren measurements of welding processes are not so common. For plasma arc welding with alternating current, McClure and Garcia (Ref. 4) de- M. SCHNICK, AND U. FÜSSEL KEYWORDS Shielding Gas Gas Contamination Gas Flow Dynamics Gas Tungsten Arc Gas Metal Arc Plasma Arc Dipl.-Ing. E. SIEWERT, Dr.-Ing. G. WILHELM, M. HÄSSLER, Prof. Dr.-Ing. J. SCHEIN, and Dr. T. HANSON are with Center of Excellence AAP (advanced arc processes), a coop of Linde AG Co. and the Lab of Plasma Technology, University of the German Federal Armed Forces, Munich, Germany. Dipl.-Ing. M. SCHNICK and Prof. Dr.-Ing. U. FÜSSEL are with the Department of Joining Engineering and Assembly Technology, University of Technology, Dresden, Germany. ABSTRACT Gas flows in and around welding arcs have a strong influence on the welding process. Atmospheric gases reach the arc due to turbulences and diffusion mechanisms and this affects the arc and the weld pool. Using optical analysis of the gas flow during welding with and without the arc present reveals possible mixing and thus the causes of contamination can be determined. The Schlieren method offers a simple way to do this. In this paper, the setup of a Schlieren measuring system and the influence of the most relevant setting parameters are described as well as their influence on the Schlieren images. WELDING JOURNAL 1-s WELDING RESEARCH
Welding Journal | January 2014
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