Research On Interaction Mechanism Between Droplet Transfer Process Of Underwater Wet FCAW And Water Environment

With the advantages of low cost,easy operability and high adaptability,underwater wet welding technology is widely used in offshore structure construction and maintenance fields,such as offshore oil drilling platform,nuclear power plant,oil pipeline,et al.Among them,flux cored wire welding technology has a broader application prospect in deep water environment due to its high welding efficiency and high degree of automation.Because the underwater wet welding process is directly exposed to water,the complex water environment will have an interactive effect on the process of metal transfer and heat transfer,significantly reduce the welding arc stability,change the type of droplet transfer,and then directly affect the quality of welded joints.However,the research on the interaction mechanism between water environment medium and welding process is still blank in the world.Therefore,this paper explores the main problems encountered in underwater wet welding,reveals the influence mechanism of water environment on the whole wet welding process,and puts forward measures and methods to improve the quality of welding process.This paper has a very important reference value for promoting the development of underwater wet welding technology.In this paper,the difference between wet welding process in shallow water and onshore welding was compared,and the influence of water environment on welding process is analyzed.With the aid of X-ray high-speed imaging system,the special phenomenon of arc bubble formed in the process of underwater wet welding is captured.The influence mechanism of the shape and behavior of arc bubble at different stages on arc stability,droplet transfer behavior and welding spatter was revealed.The effects of salinity and water temperature on droplet transfer,molten pool morphology and flow behavior were studied.It is observed that a large amount of gas precipitates from the molten pool and tries to escape from the molten pool in the underwater wet welding process.Taking this as a breakthrough point,the reasons of high porosity and diffusible hydrogen content in underwater wet welded joints were analyzed.The differences of weld quality,microstructure evolution and mechanical properties between underwater welding and onshore welding were compared.The main problems existing in the underwater wet welding process were summarized.It is pointed out that the increasing number of hydrogen induced cracks resulting from high diffusion hydrogen content is one of the main reasons for the performance deterioration of underwater wet welded joint.The effects of flow velocity and flow direction on droplet transfer process,molten pool shape and flow behavior were studied.It is found that the increase of flow rate would worsen the welding arc stability,which is mainly manifested by the significant increase of variation coefficient of welding electrical signal.With the increase of flow rate,the ratio of repelled transfer mode increases and the ratio of surface tension transfer mode decreases.With increasing flow rate,the proportion of welding spatter caused by droplet that can't transfer into molten pool increased.Taking downstream for example,the proportion of welding spatter increases from4.23% to 56% with increasing water flow rate to 2.5m/s.The shape and position of molten pool are changed by water thrust,which has a negative effect on weld appearances.At the same flow rate,the weld width decreases significantly and the depth-to-width ratio increases significantly,which is closely related to the further contraction of the arc and the increase of current density caused by the impact of water flow.By properly tilting the torch angle and using the horizontal component of arc force to over the horizontal thrust produced by part of the water flow on the droplet,the droplet repulsive angle can be significantly reduced,the droplet transfer process can be improved and the weld forming quality can be improved.The evolution of microstructure and properties of weld metal obtained by underwater wet welding of E40 steel at different flow rates was studied.It is found that due tounder the same cooling conditions,there are more hard and brittle structures such as lower bainite and martensite in the weld.At the same time,the one with higher carbon equivalent is more sensitive to cold cracking,which makes the selection range of underwater wet welding process parameters narrow.Based on the research on the influence of water flow on droplet transfer process and weld microstructure and properties,this paper proposes that the underwater wet welding process should be carried out under the condition that the flow rate does not exceed 1.5m/s.For high carbon equivalent steel,the critical value of maximum flow rate for welding operation should be further reduced.Based on the X-ray high-speed imaging system,the visualization of wet welding process in water depth of 0-150 m is carried out for the first time.The influence mechanism of water depth on the shape of arc bubble,droplet transfer process,gas evolution behavior in molten pool and welded joint quality are analyzed.It is found that the stability of arc combustion is significantly reduced by the increase of ambient pressure,and the probability of short circuit and arc breaking phenomenon increases obviously.Both the size of arc bubble and droplet decreases with the increase of pressure.Although the decrease of the size of arc bubble reduces the gas drag force on the droplet to a certain extent,the extent of droplet swing around the wire tip during the growing process is reduced.However,the smaller the droplet size,the lower the gravity which makes the droplet transfer to the molten pool,which leads to the larger deviation angle of droplet transfer,and increases the proportion of large angle repulsive transfer and repulsive spatter.However,with the increase of ambient pressure,the gas cannot escape from the molten pool promptly,which makes the porosity and diffusible hydrogen content of weld increase significantly.Taking the water depth of 150 m as an example,the porosity increases from 0.67% to 8.67%,and the diffusible hydrogen content increases from 23.3 ml / 100 g to 66.3 ml / 100 g.The performance of the welded joint decreases with the increase of the water depth.The welding porosity and crack are the main reasons for the performance and reliability of the deep water welded joint.Based on the above research on the influence of water environment on welding process quality,a new underwater wet welding process is developed in this paper.The gas drag force on the droplet is reduced by constraining the arc bubble behavior,and the droplet transfer behavior is improved.In addition,it can reduce the impact on the molten pool caused by the unstable rupture of the arc airbag,and improve the forming quality of the weld.At the same time,it can reduce the heat loss caused by the free floating of arc bubble,increase the effective heat input and reduce the proportion of hard and brittle phases.By introducing ultrasound into the water environment,the droplet transfer process is actively controlled by using the ultrasonic excited jet.As a result,the ratio of the unstable droplet repellent transfer and repulsive spatter is reduced by 23% and 4% respectively.By introducing ultrasonic energy into the molten pool,a large number of negative pressure cavitation bubbles are generated in the molten pool by ultrasonic cavitation effect,which can absorb free hydrogen in the molten metal,and effectively reduce the porosity and diffusible hydrogen content.The ultrasonic cavitation effect can refine the weld microstructure by breaking the dendrite,and improve the mechanical properties of the weld metal to a certain extent.