WELDING RESEARCH Visual Sensing of the Physical Process The typical behaviors of bubbles, droplets, and welding arc during KEYWORDS • Underwater Welding • Droplet Transfer • Arc Behavior • Visual Sensing • Bubbles Introduction During Underwater Wet FCAW underwater wet FCAW were visually sensed and analyzed BY C. B. JIA, Y. ZHANG, B. ZHAO, J. K. HU, AND C. S. WU More and more metal structures have been serving in the sea for exploring natural resources and conducting many other activities. Due to corrosion, storm loads, damage made by vessels, dropped objects, and fatigue, maintenance and repair of the metal offshore structures have necessitated research and development on underwater welding technologies (Ref. 1). The arc welding methods applied under water mainly include hyperbaric and wet welding (including local-dry underwater welding). Generally, the hyperbaric underwater welding methods are considered as a technology that can obtain high-quality weld joints via avoiding the harmful impact from the water environment (Ref. 2). However, according to Liu and Olson, the cost of hyperbaric welding is much higher than that of wet welding, and the wet welds have already been made on carbon steel structures at depths as low as 200 m (Ref. 3). In addition to underwater arc welding, 202-s WELDING JOURNAL / JUNE 2016, VOL. 95 it is worth mentioning that underwater friction welding and laser beam welding have also been developed. Friction welding can be utilized to avoid many problems associated with the arc welding process under water (Ref. 4), but the joint geometry is limited (Ref. 5). Also, underwater laser beam welding has a critical requirement of the shielding condition (Ref. 6). Consequently, wet welding methods have been more widely utilized because of two main significant advantages, i.e., low cost and convenience. As a typical wet welding method, underwater flux cored arc welding (FCAW) has been developed for automatic and semiautomatic processes particularly in deep water, and it has great potential to be applied to repair or even construct oceanographic structures based on remote-operated vehicles or autonomous underwater vehicles (Ref. 7). The greatest challenges are the extreme conditions around the welding area caused by the wet water environment. Direct contact with water of flux-cored wire makes arc maintainance, metal transfer, and solidification of the weld pool totally different from conventional FCAW in air. The comprehensive, theoretical, and experimental research on the complicated physic-chemical and metallurgical process has been strongly required (Ref. 8). For consumable arc welding methods, e.g., gas metal arc welding, metal transfer and arc behaviors are two of the most important factors to determine welding process stability and final weld joint quality (Ref. 9). Similarly, the metal transfer process (from solid wire to liquid weld pool) during underwater FCAW has significant influence on fluid flow in weld pool, weld formation, and weld joint quality. In addition, the underwater arc acts as the main heat source to generate sufficient energy to ABSTRACT The physical process of underwater wet flux cored arc welding (FCAW) was visually sensed and investigated. An image capture system was developed to monitor the bubbles, droplets, and arc behaviors via avoiding the serious disturbances from the harsh water environment. The complex dynamic behaviors of the bubbles’ evolution were described and explained according to their transient images. The acquired clear images of droplets showed the typical metal transfer mode, i.e., the repelled globular transfer with large droplet size and low transfer frequency. Subsequently, the welding arc characteristics were examined, revealing it was compressed by the water environment and had special intensive drifting and deviation behaviors on the cathode spot. High electric potential gradient and current density of the arc were disclosed and possible reasons were given. Finally, the uneven and asymmetric weld joint appearance was probably caused by the special welding process under water, especially the unstable droplet transfer. C. B. JIA (jiachuanbao@sdu.edu.cn), Y. ZHANG, B. ZHAO, J. K. HU, and C.S. WU are with the Key Laboratory for LiquidSolid Structural Evolution and Processing of Materials Ministry of Education, Shandong University, Jinan, P.R. China.
Welding Journal | June 2016
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