WELDING RESEARCH Results and Discussion In the following subsection, some of the phenomenon observed with the high-speed camera is presented. In order to understand the still frames, the supporting online material is recommended (Ref. 16). This will help to enhance understanding on what each part in the frames represents, and it will facilitate identification of those parts. For a better understanding of the findings and explanations, Fig. 3 shows a snapshot of the clearly visible moment in the DCEP process. It is helpful to keep in mind from what perspective the process was observed. In this case, it was from a low angle just above the surface of the base material. The tunnel was the outer-limiting part of each image, and moving parts like the droplets, flux grains, weld pool surface, and the wire were visible. The only light source was the arc itself, except for the hot surfaces that represented a very small part of the total emission. Therefore, particles in front of the arc are seen as shadows. Particles next to or behind the arc are illuminated and can be seen as bright spots. The surface of the liquid metal has a low emissivity. Therefore, it has a high reflectivity. It was perceived as a reflecting surface similar to mercury at room temperature. High-speed images of the DCEP process with 600 A. The videos showed different effects. In Fig. 5, the DCEP process, with its SAW basic parameters, is shown in a front view, and in the following section the general findings are presented and discussed. Later on, the characteristics of the other parameter sets are discussed in comparison to the DCEP process. The front view of the DCEP process showed a stable arc behavior the entire recorded time. The droplet transfer was turbulent and changed randomly between short circuiting dropping, exploding, 494-s WELDING JOURNAL / DECEMBER 2016, VOL. 95 and repelling — Fig. 4 and Ref. 16, SOM1. Most of the time, flux grains, and small metal and slag droplets, were splattering through the cavern area. The analyzed videos show more or less turbulent processes inside the cavern, although the SAW process is known for a high grade of stability and smooth weld joints. These are probably a consequence of the slow solidification of the molten weld pool and the smoothing effect of the freezing slag. The reaction between flux and metal happens preferably at the contact point between molten droplet and cavern wall in the welding direction, where flux is continuously molten and absorbed by the droplet. This reaction is clearly visible in Ref. 16 (SOM2) as a front view, where the absorbed molten flux also leads to a change of emissivity in the metal droplet. This is probably the place and state in the welding process where the most intensive chemical slag metal reactions take place due to the spherical droplet shape (positive ratio of absorbing surface area to volume), the high reaction temperatures, and the constant flux supply. In the lower part of the frame, the base material with some flux and metal droplets can be seen. Obscured by the base material surface is a settling, which builds the weld pool, respectively the emerging weld joint — Figs. 5 and 6. It is created by the arc pressure onto the liquid weld pool. In the upper left corner of Fig. 5, parts of the melting tunnel are visible. Other effects take place behind the wire. One is the kinking of the unduloid, a mathematical term used to describe the geometry of the long molten metal droplet that is still attached to the upper electrode. Magnetic forces drive the kinking and throw molten metal to the back. This effect can also be seen in GMAW processes with high currents (Ref. 16, SOM3). The cavern was stable the entire observed time, with just a few flux grains falling from the walls. This means the minimum internal cavern pressure was equal to the pressure applied by the flux on top of the cavern. The cavern had a half-ovoloid shape with a minimum width of 12 mm based on the given scale and visible wire diameter. Figure 6 (SOM4) shows the rear part of the cavern where different effects appear compared to the front part. The view is mostly obscured by the debris coming from the falling flux. On the left side of this image, the sloping surface of the weld pool can be seen. In the center of the frame a part of the wire is visible, and left of the wire the molten cavern ceiling appears. It merges into the weld pool, visible on the left end of the frame. This part is where the cavern surface in this area is mostly molten and migrates toward the metal surface. As soon as the cavern surface gets in contact with the still molten weld joint, the cooling process starts because there is no heat input any more. Once the molten flux is cooled, it will peel off the weld joint as slag. This contributes to the high weld quality, since the cooling is slowed down and the atmospheric gases are held back during this process. In contrast to the smooth surface of the slag, once cooled, it can be said that most parts of the cavern wall are not as smooth. Fig. 5 — Front view of DCEP process with 600 A and flux melting into the droplet — see red marking (Ref. 16, SOM2). Fig. 6 — Side view behind the process showing the weld pool and the cavern ceiling (Ref. 16, SOM4).
Welding Journal | December 2016
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