Requirements of the Schlieren Method to Analyze Welding Arcs Alongside the already described basic requirements such as the positioning of the mirrors, the quality of the Schlieren images of arc processes is above all determined by the light source and the slit (knife-edge) or alternatively colored filter pairs, which induce colored shadows and interferences — Fig. 3. The knife-edge or the filter affects the sensitivity of the Schlieren apparatus, whereas the magnitude of the deflected light can be assigned to different dye by using color filters. The applicability of apertures with horizontal or vertical slits, or an iris as well as two- and four-color filters was investigated. In all experiments, the open area of the slits was equal and the orientation of the illumination and the Schlieren slit was always identical. The two-color filters (blue/yellow and red/green) as well as a four-color filter, utilizing all four colors, were used. The best results were obtained using the twocolor filters, by which the turbulences could be visualized with very strong contrast — Fig. 3. In comparison, using the four-color filter, only marginal color nuances could be recognized. However, the light intensity was reduced when colored filters were used. Thus, the exposure time had to be extended whereby a strong cross-fading due to arc radiation resulted. Analyses of the influence of the geometry and the orientation of slits clarify that good results can be achieved with slits oriented perpendicular to the workpiece — Fig. 4. The hot gas above the workpiece was visualized using apertures with a slit, which were oriented parallel to the workpiece. The iris can be used to visualize gas flow in all directions, but the images are characterized by a lower brightness of the image. By reducing the slit width of the knife edge, less diffracted light, and consequently smaller differences in density, can be visualized (Ref. 6). At the same time the influence of the radiation of the arc decreases. However, less light from the light source passes the knife edge especially if the width of the knife edge is less than the focal diameter. The goal of the slit variation was to be able to visualize the turbulence and the density gradient of the shielding gas flow in the free jet of the process gas in close proximity to the arc individually. It was ascertained that in spite of a small slit width, the density variation produced by the arc dominated — Fig. 5. When using identical concave mirrors in the geometry described above, it is recommended that the shape of the light sources used be equivalent to that of the slit opening. Therefore, elongated rectangular light sources were used. Initially, the applicability of simple light bulbs was tested. Only by the use of high-luminosity light sources could the slit opening as well as the exposure time of the camera be reduced, so that: 1) The complete area of the gas flow was illuminated, 2) overexposure of the images due to the arc radiation could be avoided, and 3) minor differences in density could be visualized in the gas-free jet. Beside the power, the light source must generate a high light intensity on the knife edge. The gas flow in the boundary region of the process gas-free jet can be visualized well using halogen lamps. However, with the light sources used as described in Fig. 6, the area of the arc cannot be investigated in detail due to its strong brightness. Thus, further analyses employed alternative light sources such as a plasma arc and laser beam. The radiation energy of a plasma arc is approximately 10 to 20% of the total power. Thus, the radiation emission of a 250-A plasma arc with a voltage of 30 V is about 1000 W. Using this kind of arc is furthermore advantageous since the projection of the light source is rectangular, as the knife edge is. Considering the solid angle of emission, only 1% of the radiation reaches the mirror. Nevertheless, even this amount of light is sufficient to obtain a detailed flow WELDING JOURNAL 3-s WELDING RESEARCH Fig. 4 — Schlieren image of a 100-A gas tungsten arc with vertical (top) and horizontal (middle) apertures, and an iris (bottom). Fig. 5 — Images of Schlieren setups with a 3 × 6 mm focus slit and a Schlieren aperture slit of 2 × 6 mm (left), 3 × 6 mm (middle), and 5 × 6 mm (right). Fig. 6 — Schlieren images made by using 50-W automobile headlight (top, left), 150- W tungsten coiled filament lamp (top, right), 250-W tungsten coiled filament lamp (bottom, left), and 150-W halogen lamp (bottom, right).
Welding Journal | January 2014
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