gas to deviate from laminar flow. Using low shielding gas flows, a considerable formation of oxides can be determined, which is due to contamination of the protective cover. It must be concluded that the formation of a turbulent gas flow (15 L/min shielding gas) does not always lead to bad gas protection cover of the weld pool. A sufficient gas flow is necessary in order to counteract the thermal buoyancy above the hot workpiece. GMAW Gas metal arc welding is characterized by a high radiation emission of the metal vapor plasma. Schlieren images of gas metal arc welding processes are therefore especially difficult to create at high currents. As part of the investigations, Schlieren images were taken of a short arc — Fig. 11. In the images, gas flow separations at the shielding gas nozzle and the contact tip are, in contrast to GTAW, clearly visible. A reason for that is the high, very hot contact tip located inside the shielding gas nozzle caused heating of the shielding gas. For the analysis of a pulsed arc or a spray arc, it is necessary to use powerful light sources or to mitigate wavelengths with special intensive radiation emission of the arc by filters. Conclusions The Schlieren method was used to visualize gas flows in welding processes. The main conclusions are as follows: 1) The Topler Z-Schlieren configuration enables cost-efficient and time-resolved gas flow analysis. 2) It was ascertained that a powerful tungsten filament lamp and arcs were especially appropriate as light sources. In contrast, inferior images were obtained with widened laser beams. 3) It is possible to detect the transition from a laminar to a turbulent gas flow in a process gas-free jet in GTAW by increasing the shielding gas flow from 10 to 30 L/ min. 4) Through the Schlieren method, the gas flow of a nontransfer pilot arc can be excellently visualized. During studies on a plasma arc keyhole welding process, it was shown that high shielding flow rates, despite intensive turbulences, provide a better protection of the process and counteract diffusions effects. 5) First investigation on GMAW processes showed that high torch temperature principally abets the Schlieren analysis of the process gas-free jets. Due to the high radiation emission of the arc, powerful illuminants in combination with optical filters are necessary, especially in the analysis of spray and pulsed arcs. References 1. Schnick, M., Füssel, U., and Zschetzsche, J. 2006. Simulation and measurement of plasma and gas flows in plasma arc welding and cutting. 8th International Seminar — Numerical Analysis of Weldability, Graz, Austria. 2. Zschetzsche, J. 2007. Diagnostics of gas shielded arc welding processes. Dresdner Fugetechnische Berichte. Band 14. 3. Toepler, A. 1906. Observations according to a new optical method. Ostwalds Klassiker der Exakten Wissenschaften Nr. 158. Leipzig, Germany. 4. Garcia, G., McClure, J. C., Hou, H. and Nunes, A. C. Gas flow observation during VPPA welding using a shadowgraph technique. NASA-CR-204347. 5. Cooper, P., Godbole, A., and Norrish, J. 2007. Modelling and simulation of gas flows in arc welding. Implications for shielding efficiency and fume extraction. Proc. on the 60th Annual Assembly of the International Institute of Welding, Dubrovnik, Croatia. 6. Settles, G. S. 2001. Schlieren and Shadowgraph Techniques. Springer; Berlin, Germany, ISBN 3-540-66155-7. 7. Schardin, H. 1934. Toepler’s Schlieren method: Basic principles for its use and quantitative evaluation. Forschungsheft 367 –— Beilage zu Forschung auf dem Gebiet des Ingenieurwesens Ausgabe B Band. July/August. 8. Speiseder, M., and Lang, A. 2006. Optimization of the MIG-welding process by the use of numerical simulation and PIV measurement. The electric arc — A technology with a non-exhausted potential. Dresdner Fugetechnisches Kolloquium, TU Dresden, Germany. 9. Zobel, T. W. 1936. Increase of the cutting speed while flame cutting by the use of a new nozzle geometry. VDI-Verlag GmbH, Berlin, Germany 10. Settles, G. S. 1998. Visualization of liquid metal, arc, and jet interactions in plasma cutting of steel sheet. 8th International Symposium on Flow Visualization. 11. Kim, S. J. 2009. Fluid dynamic instabilities in plasma arc cutting. PhD dissertation. Minnesota, Faculty of the graduate school, University of Minnesota. 12. Allemand, C. D., Schoeder, R., Ries, D. E., and Eagar, T. W. 1985. A method of filming metal transfer in welding arcs. Welding Journal 64(1): 45–47 13. Ebert, L. 2007. Optimization of fume extraction of torch integrated fume extraction devices. TU Chemnitz. Abschlussbericht AiF-Vorhaben 14:436 BR. 14. Foucault, J. B. 1859. Annales de l‘Observatoire Impérial de Paris. 15. Mach, E. 1889. Further ballistic-photographic experiments. Sitzungsband Akad. Wiss. Wien. 98: 1303–1309. 16. Rheinberg, J. H. 1896. On an addition to the methods of microscopical research, by a new way of optically producing coulour-contrast between an object and its background, or between definite parts of the object itself. J. Roy. Microsc. Soc., Ser. 2, 16(8):373–388. WELDING JOURNAL 5-s WELDING RESEARCH Fig. 11 — Schlieren adaptor of a short arc (3 m/min wire feed). Fig. 10 — Schlieren images of plasma arc welding (S235, 6 mm; welding speed, 20 cm/min; PG-flow, 3 L/min; plasma gas three-hole-nozzle, 3 mm; torch distance, 5 mm; shielding gas flow 5 L/min (top); 15 L/min (bottom).
Welding Journal | January 2014
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