WELDING RESEARCH bubbles were difficult to observe because of the serious disturbances from the harsh environment. To conclude, the exact shapes and behaviors of welding arc, droplets, and bubbles are still not clear enough according to current research results. Research on the mechanisms of the interactions under the impact of the water environment strongly requires the support of the visual images to further understand and improve the technology. In this paper, a high-speed camera was employed to construct a visual sensing system to monitor the welding arc, droplets, and bubbles. Clear images were acquired to provide evidence of some given inferences about the complex physic-chemical phenomenon. The bubbles’ evolution, metal transfer, arc behaviors, and weld joint appearance were carefully described and analyzed. Furthermore, the relevant interaction mechanisms were preliminarily discussed by the authors. Experimental Procedures Welding System Bead-on-plate welding experiments were designed and conducted in shallow water (0.4 m) in an oblong tank. It is worth mentioning that the wet welding process, even at very shallow water, is already very different from the air. The weld appearance was acquired much differently from that obtained in air, indicating the seriously compressed welding arc (Ref. 13). Therefore, the chosen low depth could maximize the influences of the water environment and minimize the influences of other variables that could be affected by pressure (Ref. 24). A welding power source, LET 500, was employed to keep the arc voltage constant during the welding process. Meanwhile, an ordinary wire feeder driven by a DC motor was controlled to feed the wire with different speeds via adjusting A B C Fig. 3 — Typical bubbles during underwater wet FCAW. A — Bubble floatation and new bubble generation; B — bubble with maximum volume; C — a growing bubble. Fig. 4 — Dynamic behaviors of the bubbles during underwater welding. the armature voltage using an extra DC voltage regulator. As such, the welding current was then automatically changed by the power source to maintain a certain 204-s WELDING JOURNAL / JUNE 2016, VOL. 95 preset arc voltage and melt the continuously fed fire. According to many experiments conducted by the authors, the current value 205 A and the arc voltage 30 V were typical and common for underwater FCAW to acquire the typical physical process. One of the most commonly utilized metals, Q235, was chosen as the base metal in this paper. As for the welding consumables, one lime-titania-type flux-cored wire (PPS series, 1.6 mm diameter) was applied, which was developed by E. O. Paton Electric Welding Institute especially for underwater wet welding. Some other parameters are listed in Table 1. A long copper welding torch (0.7 m) was designed and inserted in the water tank with two glass lateral walls. When welding, the torch was kept stationary, and the substrate was controlled to move uniformly and in a straight line. Through the transparent glasses, a high-speed camera and spectrometer could be used to sense the welding process. Image Capture System There are very complicated reactions and interactions during underwater wet FCAW. Many factors such as the intense variation of an arc burning atmosphere cause complex light route distortions and other disturbances. The main problems in capturing clear images are as follows: First, the influences from the water environments on the high-speed camera should be avoided via designing waterproof protectors or isolating the camera by laying it outside a water tank. Second, periodically changed and irregular bubbles cause optical reflec- Table 1 — Welding Technical Parameters Parameters Material of Base Workpiece Welding Welding Arc Wire Metal Dimension Speed Current Voltage Extension mm mm/min A (average) V mm Values Q235 8 300 100 200 205 30 16
Welding Journal | June 2016
To see the actual publication please follow the link above