Investigation On The Integrated Control Mechanism Of Metal Transfer And Bubble Behavior In Underwater Wet FCAW

As an important technical means,underwater welding is increasing in offshore engineering projects such as offshore oil platforms,underwater mineral development,offshore pipeline repair,and construction and maintenance of nuclear power plants.The underwater wet welding based on self-shielded flux cored wire does not need additional drainage equipment.The equipment is simple,easy to operate,universal,and easier to realize automation and intelligence.It is an important trend of the development of underwater welding technology.However,the periodic upward floating bubbles will continue to interfere with the droplet transfer process,resulting in larger droplet diameter and lower transfer frequency,which seriously affects the stability of the welding process and weld formation.In this paper,the interaction mechanism of underwater droplet bubble is revealed by means of synchronous detection of visual signal and electrical signal,and the corresponding measures are put forward to improve the stability of welding process and improve the structure and performance of underwater weld.Through the reasonable design of optical path and the selection of background light source,a set of synchronous detection method of droplet,arc and bubble in the process of underwater wet welding is set up.Combined with the electrical signal synchronization technology,the dynamic behaviors of arc,droplet flying in the arc starting stage,bubble growth stage,bubble separation stage are studied.In the process of underwater arc starting,both electron emission and gas ionization are carried out in the bubble generated by resistance heat.Because the gas temperature in the bubble experiences a sudden rise and fall process,which leads to the instantaneous growth of the bubble volume and the subsequent collapse of the bubble.In the bubble growth stage,the fluctuation of arc and droplet is small due to the relatively stable gas flow in the bubble.In the stage of bubble separation,the high-speed gas flow in the necking area interferes with the arc and droplet greatly,which leads to the increase of the arc length and is not conducive to the stability of the welding process.When welding in the air,the droplet transfer process is relatively stable without the interference of external factors;while under the water,the periodic floating bubbles cause strong interference to the droplet process.In order to realize the integrated control of the droplet and bubble,it is very important to first reveal the mechanism of the bubble on the droplet transfer.The dynamic change rule of droplet and bubble under different arc voltage is studied,and the interaction relationship between droplet and bubble is described in detail.The reason of large droplet size and low transition frequency under low voltage is analyzed from the point of view of bubble hindering droplet transition.Under the same welding parameters,the underwater arc is significantly shorter than that in the air.The mechanism of the contraction of the underwater arc column is analyzed from the aspects of the rapid cooling effect of the water environment,the high content of hydrogen in the bubble,the interference of the air flow in the bubble,and the water pressure.In order to match the droplet transfer frequency with the pulse current frequency,the pulse current welding procedure based on LabVIEW was optimized,and the welding process was stable at the minimum pulse frequency of 8 Hz.By adding the rising edge with different slope before the peak current,it is found that when the rising edge is flush with the front phase of the peak current,the shape of the bubble is the most regular,and the stable droplet transfer process of "one pulse,one drop and two bubbles" is preliminarily realized under this waveform.In the mode of "multi pulse,one drop and multi bubble",there is not enough heat to transfer the droplet in one pulse period,and the interference of the bubble to the droplet is not obvious,the spatter is serious,which is not conducive to the stability of the welding process.In the mode of"one pulse,one drop and two bubbles",the frequency of pulse current is the same as that of droplet transfer,the heat input of droplet is more uniform,and the frequency of bubble is relatively fixed,which weakens the interference of bubble to droplet to a certain extent,and the welding process is more stable.The weld shape,microstructure,hardness and tensile properties were compared under the mode of large drop rejection,"multi pulse,one drop,multi bubble" and "one pulse,one drop,two bubbles".In the mode of large drop repelling transition and"multi pulse,one drop,multi bubble",the weld formation is poor and there are many splashes.The metallographic structure is mainly composed of massive ferrite.In the mode of "one pulse,one drop and two bubbles",the weld formation is more uniform and continuous,without obvious defects,closer to the weld morphology in the air.The metallographic structure is composed of massive ferrite and acicular ferrite,and the structure and composition are improved to some extent.The hardness of the weld joint in the three droplet transfer modes is equal to that of the base metal,and the most serious hardening is in the heat affected zone of the weld joint in the large droplet rejection transfer mode.In the mode of "one pulse,one drop and two bubbles",the hardness of the weld joint is slightly lower than that in the other two droplet transfer modes.In the mode of large droplet rejection transition,the fracture position appears in the weld or heat affected zone.In the mode of "one pulse,one drop and two bubbles",the fracture position appears in the weld,and the tensile strength value is not increased significantly.