WELDING RESEARCH tions, the droplet behavior and material transfer were described as to their dependence on welding parameters and filler materials. Basic statements about arc behavior and electromagnetic blowing effects in SAW could be described. A more recent publication, which adapted this method, is from Mendez (Ref. 3). He used a tube made out of rolled steel sheets open at both ends. He found a droplet detachment frequency of approximately 9 Hz at 500-A DCEP, and 13 Hz at 500-A AC. At 1000-A DCEP, he found that a tapering electrode tip with a buried arc and a molten tail was ejected through a mechanism resembling an electromagnetic kink instability. Also, no obvious signs of external gas entrainment were detected. Nevertheless, the flow of gas has to be chosen within narrow limits. Otherwise, it can affect the natural atmosphere inside the cavern. In other papers, the process was observed with x-rays. The first investigators were Ostapenko and Grebelnik (Refs. 4, 5), who recorded single frames. The images had a poor resolution and could not reflect the dynamics of the process. Eichhorn (Ref. 6) managed to create a 500 fps x-ray film that could resolve spatter with a diameter of 0.16 mm. The advantages of this method were the lack of influence on the process, the long possible observation 492-s WELDING JOURNAL / DECEMBER 2016, VOL. 95 time, and the great level of freedom in observation perspective. On the other hand, the two disadvantages were that the shadow image did not show details of the surfaces (being perpendicular to the incident ray) and the necessary safety precautions due to the harmful radiation. A detailed observation of the cavern structure, the molten droplet, or even the arc behavior was greatly restricted. X-ray observation in SAW was also performed in Refs. 8–11. The main outcome of these investigations was the additional knowledge in the basic droplet behavior as it depends on welding parameters, polarity, and used welding fluxes. Experimental Apparatus and Procedure Two major improvements have been achieved, supplementing previous experiments. First, a thin-gauge metal foil was used as tunnel material to keep the disturbance of the process as small as possible. It can be seen in Fig. 1 ending under the flux. The material was steel foil with a very low amount of alloying elements (Table 1), and it had a thickness of 25 μm, which reduced the effect of additional material to a negligible amount. This tunnel was placed in two different ways. One way is shown in Fig. 1, which gives a side view of the process. The second way was to put the tunnel along the welding direction. In this way, a front view of the process could be achieved. Similar to the setup in Franz (Ref. 2), a shielding gas was introduced into the tunnel. This helped to keep the cavern from collapsing due to the open channel, and to keep atmospheric gases outside. The additional tube brazed to the tunnel served as a gas inlet. Second, a spatially resolved highspeed spectrometer system was added to the setup. It had to monitor the process from the same direction as the high-speed camera since the tunnel had a narrow angle of aperture. This was achieved by a 90-deg mirror that was placed in front of the lens at its blind spot — Fig. 1. This blind spot design was related to the internal mirror positions of the lens. The mirror lens was a long-distance microscope from Questar Corp. called QM 1. In combination with a highspeed camera (HSC; MotionPro Y4- monochrome from Integrated Design Tools, Inc.) and an infrared filter, the images could be recorded at 5000 fps with only a slight disturbance caused by the arc. This was sufficient to play back the fast processes inside the cavern and give a visual overview of the processes concerning the metal transfer and flux behavior. The acquisition was synchronized with the second camera (MotionPro Y4-monochrome), recording the high-speed spectra from the 0.5-m monochromator (Princeton Instruments Acton SP2500). By doing so, it was possible to find the connection between high-speed camera images and the spectra. The spectrometer was chosen due to several advantages. It has a high spectral resolution to determine which species are present inside the arc. Preliminary trials showed acquisitions with a mini spectrometer do not provide sufficient resolution to determine, and distinguish between, species present in the cavern. The spectrometer was equipped with three gratings with different groove densi- Fig. 1 — Setup with highspeed camera and spectrometer. Fig. 3 — Snapshot of the SAWDCEP process. Fig. 2 — Chemical compositions of SAWslags.
Welding Journal | December 2016
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