WELDING RESEARCH A B tions happen could be investigated via visually observing the welding process. As shown in Fig. 3, a typical bubble evolution process was captured with the dysprosium lamp as the background light source, and the three images reflected three different typical moments, respectively. However, the droplet transfer and welding arc could not be observed through the bubble. Furthermore, continuous video of the bubbles was also captured, and the images at every 3 ms are shown in Fig. 4. The Metal Transfer Process Based on the image capture system with the laser as the background light source, the metal transfer process was visually sensed and observed. Figure 5 showed some typical images of the droplets during a period of about 0.25 s. At 0.03 s, the welding arc was short and that was the start of a metal transfer circle. At 0.029 s, the welding arc length had grown while the wire could be observed beginning to melt, and the liquid droplet had a diameter equal to that of the wire. When the time was 0.077 s, the droplet grew bigger, and the welding arc cathode spot continuously drifted on the substrate surface. At about 0.149 s, the droplet volume was larger and seemed to be deviating from the axis of the wire. It was thought that the compound repelling forces from the welding area were the key factor to push the droplet to lateral directions. At 0.167 and 0.171 s, the droplet gradually swayed back to the axis center due to gravity and bigger volume. At 0.179 s, the droplet grew to the peak volume, while the welding arc was clouded by the big droplet and nearly invisible. The oval liquid metal had a vertical diameter of about 6.78 mm and a horizontal diameter of 6.02 mm, which were about 4 times that of the wire. After that moment, the droplet was gradually transferred to the weld pool under the gravity, surface tension, and electromagnetic force. At 0.184 and 0.196 s, it can be seen the wire that had not been melted obviously 206-s WELDING JOURNAL / JUNE 2016, VOL. 95 extended downward, the oval droplet merged into the weld pool, and the arc length was shorter. At 0.228 s, the welding arc was completely invisible and a circle of the metal transfer had finished. At 0.223 s, the welding arc intensity increased, which meant a new circle had begun. The Underwater Welding Arc Behaviors During the monitored welding process, the welding arc was captured wandering on the substrate intensively and continuously. The experiments were conducted with direct current electrode positive mode, i.e., the substrate was connected to the negative electrode. That is to say the cathode spot on the substrate was not focused on the nearest point from the wire tip or molten droplet. Figure 6 shows the images of the wandering underwater welding arc every 0.5 ms. Even in this very short period, the cathode spot could be observed drifting around the wire axis. In most cases, the cathode spot was located in different places of the weld pool, and the switch from one place to another was very fast. It was calculated that the welding arc drifting frequency was higher than 2000 Hz, surpassing the image acquisition frequency of the high-speed camera. In addition, the linear moving speed of the cathode spot was calculated as about 4.1 102 cm/s. It is worth mentioning that watching the video with certain frames per second is a better way to observe the wandering arc behaviors. Weld Joint Appearance Figure 7 shows the obtained weld joint appearance and cross section. As shown in Fig. 7A, the welding torch moved along a straight line, but the weld joint seemed to have an uneven surface. To some extent, the bead deviated from the central line and was a little twisted. In addition, the cross section in Fig. 7B shows the oblique bead clearly. For the specific image, the melted metal on the left side was apparently less than that on the right. Subsequently, the reinforcement was also not symmetric and the HAZ had different widths on the left and right sides. Finally, the weld width and penetration were measured with values at about 9.21 and 2.76 mm, respectively. Discussion Typical Bubble Evolution Process Analysis The mechanism of the protections during wet FCAW can be described more specifically. First, the flux-cored wire is directly exposed in the water environment. Once the wire contacts the substrate, the current will produce heat and the temperature around that area will rise rapidly. Second, the flux in the wire will melt and the gas will be generated from the decomposed flux. Meanwhile, the surrounding water will be ionized into hydrogen and oxygen or be vaporized into steam. Subsequently, bubbles around the welding area will form to provide a protective atmosphere. Third, the welding arc is then ignited in the bubbles after the breakdown of the gas under the open circuit voltage. The arc could generate more energy and heat to produce bubbles continuously. Consequently, the droplet transfer and molten weld pool behaviors also proceed in the bubbles. The Fig. 7 — A — Weld seam appearance; B — cross section.
Welding Journal | June 2016
To see the actual publication please follow the link above