Research On Underwater Wet Welding Stability And Metal Transfer Control

Underwater wet flux-cored wire self-shielded arc welding(FCAW) is attracting increasingly widespread attention for its advantages such as high welding efficiency and applicability to automated welding.Compared with welding on land,underwater wet welding has poorer welding stability and increased spatter quantity.Developing special welding equipment is an effective approach to further improving welding quality.However,due to the absorption of light by water in the underwater welding environment and the interference of bubbles,the optical observation of the underwater wet droplet transfer process is difficult,and the research on underwater wet welding droplet transfer control technology is difficult to carry out smoothly.This paper acquired clear dynamic images of wet welding process by using X-ray based high speeding imaging technology and breaking the restriction of water environment on observing the droplet transition of wet welding.Based on that,this study systematically studied the droplet transition process of underwater wet FCAW,and formulates targeted droplet transition control method for the purpose of enhancing the stability of welding process.With an aim to accurately and efficiently the stability of welding process,it screened the stability evaluation indicators for various welding processes,and then determined the welding process stability evaluation indicators cored by arc voltage variable coefficient,achieving quantitative analysis over the welding process stability.The droplet transition process of underwater wet FCAW was studied and divided into repelled metal transfer,surface tension metal transfer and short circuit metal transfer.The impact of welding criterion on the droplet metal transfer process of wet welding was also studied: Increasing welding speed and arc voltage or decreasing wire feeding speed will increase the probability of repelled metal transfer;short-circuit metal transfer is easy to occur if wire feeding speed is increased or arc voltage is reduced.According to analysis over the welding process,continuous short circuit process may easily induce droplet explosion,which is unfavorable to the welding process stability;in case of repelled metal transfer,the droplets hang on the wire end for a long time,which makes it easy for arc breaking to happen to the disadvantage of the welding process stability.From the perspective of welding process stability,the proper welding criterion should be arc voltage: 30-34 V,welding current: 180-220 A and welding speed: 1.5 mm/s.This paper summarized common spatter types and occurrence mechanism thereof in the process of wet welding.There are three major spatter types: repulsive spatter,gaseous escape spatter and explosive spatter.Besides,the relation between droplet transition and spatter type was studied.Under the condition of normal arc voltage,the frequency of spatter in the repelled metal transfer process is the highest,reaching above 4 Hz,and the amount of spatter increases with the rise of the repelled metal transfer ratio.This paper determined the strategy of enhancing welding process stability through controlling the ratio and duration of repelled metal transfer.The law of arc environmental difference influencing the welding process was comparatively analyzed from three perspectives: protective bubble,wire and arc: the volume of protective bubble increases as the welding criterion upgrades and the zone protected by the bubble expands accordingly.Quick loss of arc heat in water has big impact on the melting process of wires,so the wire end has low temperature.The underwater arc shrinks greatly because of the influence of water environment.The arc electric resistance increases,and the melting curve of underwater wet FCAW is offset to the upper left compared with that in air.The mechanism of repelled metal transfer of underwater wet welding was analyzed by comparing the influence of ambient difference of arc on droplet force.Due to the introduction of gas pressure and gas flow drag force in the water environment,the resistance to the metal transfer process of underwater wet flat welding droplet,and the increased surface tension affected by the underwater chilling environment,the hindering force on the droplet increases,making it is difficult for the droplet to transfer into the pool;Due to the location of the cathode spot and the generated area of the bubble drifting,the external force on the horizontal direction of the droplet pushed it towards the direction of the resultant external force until the droplet broke away from the wire.The above reasons lead to a large proportion of repelled metal transfer in underwater wet welding.In order to suppress the repelled metal transfer ratio,the application of pulse current technology in underwater welding was studied.The pulse current promotes droplet transition and curbs the repelled metal transfer so as to improve the wet welding process mainly through periodically changing the electromagnetic pinch force.Although the area of arc wrapped droplet zone decreases due to arc contraction,the electromagnetic pinch force promotes droplet transition on the whole due to the increase of droplet size and arc wrapping angle in underwater welding.Based on that,this paper put forward the indicator “degree of periodic action of electromagnetic pinch force” to comprehensively evaluate pulse current parameters,and used the reinforcement variable coefficient of weld joint to evaluate the welding quality under different conditions.When the pulse frequency is 15-20 Hz,the peak current is 350 A;when the pulse frequency is about 20 Hz,the reinforcement variable coefficient is less than 10%,and the welding effect is good.In order to further suppress the repelled metal transfer ratio,and improve the stability of welding process,a controlled wire feeding system was designed and studied.Research on pulsed wire feeding process shows that at the moment when wire feeding is paused,the inertial force received by the droplet is the reason for accelerating the droplet transition.The higher the pulse frequency is and the longer the pause time is,the more prominent the effect of the inertial force is.The pulse frequency of 30 Hz and the duty ratio of 50% are more suitable for underwater wet FCAW.The application of wire feed-retract method in underwater wet welding was studied,and it was found that using external mechanical force to withdraw wire can accelerate the droplet transition process,reduce the repelled angle,and better realized the purpose of inhibiting the repelled metal transfer.However,due to the forced extension of arc in the process of wire withdrawal,the fluctuation degree of molten pool is severe,and the stability of welding process is worse than that of pulse wire feeding method.The welding process under different droplet transfer control modes was compared and analyzed.According to the results,the pulse current method is more suitable for wet welding.Compared with the traditional method,the pulse current mode can,under the premise that the heat input remains unchanged,reduce the repelled metal transfer ratio by 50%,and optimize the weld microstructure and mechanical properties,so it is suitable for underwater wet FCAW.