WELDING RESEARCH JUNE 2016 / WELDING JOURNAL 207-s slag in the weld pool will float up to the surface to offer protection from the harmful gas and water environment, and also improve the microstructures of the solidifying metal through metallurgical reactions. Therefore, the bubbles play a very important role in protecting the welding area and can reflect the complex reactions between the water environment and welding process, i.e., arc burning, metal transfer, and weld pool solidification. The features of the typical bubbles in Fig. 3 at the three moments could be drawn. Figure 3A describes a new bubble’s generation and the previous floatation. Because of the increasing buoyancy caused by the continuously generated gas, the previous bubble began to float up and was divided into two parts. Part of the gas formed a new bubble covering the welding area with a diameter of about 10.95 mm (the smaller bubble with a 5.6 mm diameter was part of the new bubble). Meanwhile, the other part that contained most of the gas from the previous bubble floated up along with the fumes inside. After a short while, the bubble burst due to the torch disturbance, and the fumes were then dispersed in the water. Consequently, the fumes dropped slowly toward the workpiece due to gravity, which affected the visibility of the welding area. Figure 3B shows the bubble that had grown to its maximum volume, i.e., the threshold value before its breaking. The bubble seemed to be regular spherical with a diameter of about 15.6 mm. However, after careful observation, it could be seen that the surface was irregular and lumped. At the bottom of the bubble, a small part seemed to be “cut off” by the workpiece, and the interface (the diameter was about 10.8 mm) provided shielding atmosphere for the high-temperature zones, including the weld pool. Figure 3C shows a growing bubble, which appears irregular under compound forces such as buoyancy, exterior water pressure, and interior gas pressure. The central axis of the bubble slightly deviated from the torch. Its maximum diameter was about 14.6 mm, and the location was about 2.0 mm high from the substrate surface. By comparison, the bottom interface had a smaller diameter of 12.6 mm. Therefore, there was an acute curvature change between the two locations, which was relevant to the temperature gradient inside the bubble. It could be clearly seen that the bubbles periodically grew, broke away, and were generated again. A transient phenomenon was observed that the bubbles swelled explosively with large amplitude and immediately shrank with a smaller one. In other words, the bubbles grew in a pulsed way with varying speed rather than in a continuous smooth way. Furthermore, when the bubbles grew larger, their outer surface became more irregular and lumped. The reason may be that the gas in bubbles is mainly generated by the decomposed flux and vaporized and ionized water, which occurred intensively, and were also easily affected by the molten droplet, the welding arc, and the high-temperature weld pool. Each circle of the bubble’s evolution cost about 50–80 ms, and the frequency was about 12–20 Hz. It was thought that the wide range fluctuation of the frequency and amplitude was caused by the rapidly varied buoyancy, plus interior and exterior pressures of the bubbles. In addition, the interactions between the water environment and droplet transfer, welding arc, and weld pool behaviors influenced the gas generation at any time. Compared with air FCAW, underwater wet welding had intensively changed arc burning space, atmosphere, and pressure, which kept influencing the welding process stability. The Repelled Globular Transfer and Irregular Weld Joint According to the acquired images in Fig. 5, the droplet transfer mode during this underwater welding experiment was classified as repelled globular transfer with large droplets. The circle and frequency were about 0.225 s and 4.44 Hz, respectively. Meanwhile, the welding arc burning atmosphere was seriously influenced by the water environment. The phenomenon of the drifting cathode spots on the substrate was captured. In addition, the welding arc was compressed with a bell shape (images at 0.029 and 0.167 s) due to the three mechanisms, namely mechanical compression of the hydrostatic pressure, a thermal pinch- Fig. 8 — Droplet images. A — 0.167 s; B — 0.179 s. Fig. 9 — Deviation of the underwater welding arc. A B
Welding Journal | June 2016
To see the actual publication please follow the link above