The Investigation Of Metal Transfer,Bubble Behavior And Bead Formation During Underwater Wet FCAW

As the demand of ocean resources raises,more and more platforms related with oil exploitation,oil and gas transport and naval vessels construction have been set up.The repair and maintenance of these platforms associated closely with underwater welding.Underwater wet flux-cored arc welding is easy to operate and is equipped with high efficient.Therefore,it is one of the most important development directions of underwater welding.Due to the impact of unique water surrounding,the droplet transfer during underwater wet FCAW is mainly composed with low frequency,non-axial globular repelled droplet transfer and surface tension transfer.The bubble behavior has significant effect on welding arc and droplet transfer however,the mechanism is still unclear.These have caused serious negative effect on welding stability and bead formation.This thesis investigated the droplet transfer,bubble behavior and bead formation through the visual examination of droplet transfer.Correspondingly improve method was proposed,as a result,the stability of welding process and bead formation were improved effectively.An image obtain system based on high speed camera and laser background light was specially designed to improve the quality of achieved droplet figures.Through analyzing obtained figures of droplets and synchronously collect electrical signals,the mutual effect between droplet,welding arc and droplet was investigated which provided a more comprehensive and clear understanding of droplet transfer during underwater wet FCAW.Due to the compression of underwater arc and fast cooling effect of water surrounding,the underwater bead has bigger depth to width ratio.Also since the droplet transfer during underwater wet FCAW was mainly composed with globular repelled droplet transfer mode,the bead formation is asymmetry and uneven.The droplet transfer mode during underwater welding will promote the generation of spatters.The spatter generation was easy to quantify due to the character the spatter during underwater welding cannot weld with the plate easily.Suddenly increased repulsive force and periodical behavior of bubbles promote the generation of repulsive spatters.The soared pressure inner rather small bubbles is the main reason of the explosive spatter.The periodical growth of bubble intended to drag the spatters inwards or annexed them directly which will cause the accumulation of spatters around the bead.This will cause overlap and indentation on the bead.Select appropriate welding parameters and higher current can reduce the generation of spatters effectively.The dynamic characteristics of bubbles during underwater wet FCAW have significant effects on the arc burning,droplet transfer and cooling of molten pool.Therefore,the shape of bubbles,the classification of bubbles and the relationship between bubbles and welding parameters were investigated.Under selected welding parameter,the maximum diameters of generated bubbles complied with approximately normal distribution.Higher pre-set welding parameters generated larger bubbles with gradually receding influences.To solve the common defects of wet beads i.e.the slag coverage of beads was poor and the bead appearance was not smooth enough,the authors proposed a novel gas-assisted underwater wet FCAW which significantly increased the volume of bubbles around the arc zone.As a result,the slag coverage and bead formation improved accordingly.Due to the special force condition inner the bubbles,underwater wet welding under constant voltage is difficult to achieve idea droplet transfer mode.A current waveform and corresponding control project was selected and a program based on Lab VIEW was compiled to fulfill the pulsed underwater wet FCAW.The electrical signals and visual images indicated that the droplet transfer mode during underwater wet FCAW was more uniform.The size of droplets reduced to some degree and the stability of arc burning also increased a lot.