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Feed forward control of back bead for root gap variation in V-groove welding without backing plate

A stable back bead in the first layer of one-sided multilayer reinforced welding is important to obtain goodquality welds. For this reason, the method of back bead welding with the back root retained in place with a ceramic or other type of backing plate is generally used. During fabrication of box-shaped structures, however, it is in many instances difficult to remove the backing plate after welding. Manual backing plate removal is also incompatible with the needs of automatic welding procedures, so that, for reasons of fabrication costs, it is desirable to weld such structures without the use of a backing plate. In this situation, it is imperative to control the welding conditions, such as the welding current, wire feedrate and welding speed as well as the torch position, including weaving, in response to any external disturbances, such as groove shape or root gap variations. The purpose of this study is to establish a welding procedure with a high degree of adaptability to such external disturbances. This article describes an investigation of the welding conditions being sequentially set in response to the root gap regarded as strongly affecting back bead formation and further outlines a back bead feed forward control system based on root gap detection.