Fig. 1 — The law of refraction as foundation of the Schlieren optic. scribed the necessity for a gas flow analysis. However, their work contained no corresponding results or Schlieren images. Allemand and Schroeder (Ref. 12) used the Shadowgraph method (Ref. 6) in order to visualize the drop transfer during gas metal arc welding. For illumination, a He- Ne laser was used. The photographs are, however, overexposed due to the presence of the arc and the drop transfer was difficult to observe. This paper describes an attempt to use the Schlieren technique to visualize the shielding gas flow in different arc welding processes. The principle of operation and the experimental setup of the Schlieren technique are described. The most important settings and their influence on the quality of the Schlieren images of GTA are described so that the range of application and the limit of the Schlieren technique can be specified. The results of the gas flow analysis for GTA, GMA, and plasma arc welding (PAW) are presented, where the influences of typical welding parameters on the gas flow are displayed. Experimental Procedure Physical Principle and Measuring System By the Schlieren technique, differences in density that cause changes in the refraction index n, in the propagation velocity c and in the direction of light propagation direction, can be visualized in transparent media. The angle of refrac-tion relates itself to the incident angle α (1) Thus each change in density of the medium causes a change in the direction of light propagation — Fig. 1. The differences in density that are observed during the welding process are caused, according to the ideal gas equation, by differences in pressure, temperature, and concentration. In order to make differences in density in transparent media visible, the interference and the shadowgraph methods can also be used alongside the Schlieren technique. In the interference method, two light waves are superimposed so that an interference pattern is generated. The interference image allows the reconstruction of the location and the intensity of the light refraction as well as the speed of the gas flow, the density, and the temperature. However, this measurement method requires high precision in the adjustment of the measuring equipment. By the shadowgraph method, deflection of the light can be made visible by means of the generated intensity of illumination dispersion E, which is proportional to the second derivation of the density along path y (Equation 2). (2) This method enables conclusions to be drawn about the density gradient, but not about the direction. Compared to the interference method, a lower resolution and sensitivity can be reached (Ref. 6). Due to a marginal overhead (the integration of a knife edge), it is possible to separate the deflected from the uninfluenced light, in order to increase the resolution and sensitivity. Furthermore, with the so-called Schlieren technique, it is possible to determine the direction of the measured density gradient. The change in intensity of illumination caused by the light deflection is proportional to the first derivation of density according to the position (Equation 3). (3) In contrast to the interference method, the Schlieren technique is a simple and robust measuring system. However, an exact identification of gas flow characteristics is not possible. The experimental setup is carried out as a Toepler’s Z-Schlieren assembly with two concave mirrors — Fig. 2. This assembly is compact and avoids errors due to chromatic aberration caused by the optical lenses. The concave mirrors are axially parabolic mirrors with a diameter of 150 mm and a focal length of 1200 mm. The diameter lies in the recommended area from D = f/6 to f/12 (Ref. 6). In the region between both mirrors, parallel light is generated. In this optical path, different welding arcs (Schliere) are inserted, influencing the propagation of the parallel light. In the focus of the first mirror, an aperture is placed to produce a point light source enabling the production of parallel light by mirror 1. The knife edge is placed in the focus of mirror 2. The knife edge is used to improve the contrast by blocking the deflected light. Images of the Schlieren are generated by a high-speed camera with a 200-mm objective with a macrolens. The exact position in which the Schliere is arranged between the two mirrors has no influence on the measurement outcome. The deflection level of the light a in the Schlieren aperture depends only upon the angle of deflection and the focal length f of the mirror. Δa = ε﹒f (4) α β = = sin sin c c n n 2 1 2 1 ρ Δ ∂ ∂ E y 2 ∼ 2 ρ Δ ∂ ∂ E y ∼ JANUARY 2014, VOL. 93 2-s WELDING RESEARCH Fig. 2 — Toeplersche Z-Schlieren assembly. Fig. 3 — Schlieren images, used filter pairs: blue/yellow (left) and red/green (right) with a shielding gas flow of 30 L/min of argon.
Welding Journal | January 2014
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