WELDING RESEARCH weld pool. Since the arc is metal-vapor dominated (see section on high-speed spectroscopy), it might have a core temperature of approximately 7000–10,000 K as other research suggests (Ref. 20). In addition, between 10 and 20% of the electrical energy put into the process is converted into radiative energy in arc welding processes (Ref. 21). This radiative energy from the arc plus the heat from the weld pool, which is all trapped inside the cavern, leads to a heat accumulation. Therefore, a quick heating of the flux is to be expected. To verify these statements further, more precise investigations have to be conducted. High-speed images of the DCEN process with 600 A. In the direct current process with electrode negative polarity (DCEN), as shown in Fig. 8 and Ref. 16 (SOM6), the arc is much shorter than in the DCEP process and almost not visible. The droplet transfer happens beneath the surface of the base metal in a weld pool depression. Therefore, it is not visible. The flickering and fast drifting of regions, with high emissivity on the droplet, indicates a less stable arc behavior. This could be explained by the high amount of CaF2 in the flux, which is well known for destabilizing the arc. Cathode spots appeared all over the droplet, mainly in the upper region near the solid wire. This can be explained with lower temperatures and lower electrical resistance. The cavern showed less volume compared to the DCEP process, which is probably a result of lower internal pressure due to lower temperatures and a smaller amount of metal vapor. This is a result of a less concentrated arc on the wire, which is a typical feature of a DCEN process (Ref. 11). High-speed images of the DCEP process with 1000 A. While the cavern walls were largely covered with flux grains in the DCEP process with 600 A, more flux was molten in the DCEP process with 1000 A. This makes the observation more difficult due to high-viscosity slag moving in the visual field. This can be seen in Ref. 16 (SOM7) as a front view. Here the breakup of the cavern wall, consisting of high-viscosity molten slag, is clearly visible as well. The concentrated arc attachment on the droplet Fig. 8 — Front view of a DCEN process (Ref. 16, SOM6). Fig. 9 — Spectra from recording the positive phase (red line) and the negative phase (black line) corresponding to the left and right images in Fig. 10. shows a stable behavior with a higher emissivity compared to the lower current in DCEP. The material transfer mainly takes place in a streaming way. Due to the high amount of completely molten slag, the interior temperature of the cavern is supposed to be significantly higher than a DCEP process with 600 A. From this we can deduce that there was a higher amount of vaporized 496-s WELDING JOURNAL / DECEMBER 2016, VOL. 95 metal and elements from the flux in the cavern atmosphere. High-speed images of the AC process with 600 A. The alternate current process was conducted with a square wave and a frequency at 100 Hz. As can be seen in Ref. 16 (SOM8) (as a front view), the process is very stable despite the continuous changing of polarity. This is attributed to the fast rates of current changes due to the inverter power source technology. The positive and negative half-waves can be identified because the cathode spots appear on the wire during negative polarity. Compared to the DCEN process, the negative half-wave appears more stable with cathode spots running over the droplet. It is noticeable that the arc often moves to the upper part of the droplet and enlarges it by melting the solid wire in the negative half-wave. This can be explained with the preferred movement of the cathode spots into cooler regions on the wire due to less electrical resistance. It leads to an accelerated fusing of the wire and an increased deposition rate (Ref. 11). High-speed spectroscopy. In this part, the results of the spectroscopic measurements are presented. The aim of these measurements was to find a suitable spectral range to analyze the arc atmosphere and to identify significant changes within the process. This could help to enhance the understanding of the chemistry and mechanics in the cavern. Similar to the high-speed videos, the spectra were recorded from a low angle. The setup allowed a correlation between the spectra (Fig. 9) and the images (Fig. 10). The spectra were recorded along a vertical line across
Welding Journal | December 2016
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