Fig. 7 — Schlieren images made by using 250-A plasma arc (left) and 20-mW continuous wave laser (λ= 532 nm) (right). image in an area that was not recognizable before — Fig. 7. Using a 20-mW continuous wave laser of wavelength 532 nm in combination with a neutral gray filter with a transmittance of 1%, the radiation of the arc could be completely faded out — Fig. 7. However, by using a laser (point light source), a “digital” Schlieren image without intensity gradations results. Results and Discussion In order to analyze the gas flow even at the boundary region of the process gasfree jet, despite the intensive arc radiation, a GTAW arc is used as a light source. The orientation of the light source, as well as that of the Schlieren slit, is vertically aligned to the surface of the workpiece. The Schlieren technique was used to make high-speed images of the GTAW, PAW, and GMAW processes. GTAW GTAW with differing shielding gases, flow rates, and currents was analyzed — Fig. 8. The transition of the process gas-free jet to the atmosphere is especially good to visualize using argon with an appreciable helium percentage (50%) as shielding gas. However, it has to be assumed that helium has an essential influence on the arc geometry and, above all, on the gas flow. The arc current influences the temperature of the arc and the temperature of the effluent gas. From the Schlieren images, it can be clearly seen that the arc moves up farther on the tungsten cathode, that the core of the arc is brighter, and that there is a stronger flux of hot gas above the workpiece. Despite the brightness, the edges of the arc can be clearly detected. The Schlieren measurement method can be used to detect the turnover from a laminar to a turbulent gas flow of the process gas-free jet in GTAW. Turbulences surrounding the arc and turbulences in the effluent hot gas can be clearly distinguished at shielding gas flow rates of 30 L/min and more. PAW Investigating plasma arc keyhole welding was carried out by bead-on-plate welds (6-mm-thick, mild-steel plates). To ignite the main arc between the tungsten cathode and the workpiece, a pilot arc between the cathode and the copper nozzle (anode) must be initialized. The pilot arc serves as preionization of the arc gap between the electrode and the workpiece — Fig. 9. The Schlieren method is excellently suited to image the gas flow of the pilot arc. An advantage is the low radiation emission of this plasma jet. The Schlieren images of real keyhole welding trials were correlated with the respective welding results — Fig. 10. Clearly visible at low shielding gas flow rates is that the fluid flow above the hot weld joint (left of the torch) is dominated by thermal buoyancy. In contrast, above the cold steel sheet (right of the torch) an equal and laminar outflow can be seen. With higher shielding gas flow rates, the differences between the gas flows over the hot and the cold steel sheet are less pronounced. It is assumed that the high shielding gas flow counteracts the thermal buoyancy as well as causing the outflowing JANUARY 2014, VOL. 93 4-s WELDING RESEARCH Fig. 8 — Schlieren images of GTAW as a function of current, shielding gas, and flow rate. Fig. 9 — Schlieren image of a pilot arc (3 L/min plasma gas flow) where the hot plasma jet is clearly observable. The impinging hot gas on the workpiece and the effluent hot gas on the surface of the workpiece are visible by a dark plateau. Stalls in the periphery are detected by means of eddies.
Welding Journal | January 2014
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