| With the development and the utilization of the marine resources,the assembly,constructions,repairs and maintenances of the structures touching water urgently require the mature underwater wet flux-cored arc welding(FCAW)technology,but currently the wet welding process and the quality are not stable and often out of control,unable to meet the growing technological demand.Welding metallurgical behavior exists through the entire welding process and plays a decisive role in welding process stability,droplet transfer characteristics,molten pool behaviors,weld formations and the welding quality.Therefore,exploring the root cause of the poor stability of the wet welding process from the metallurgical perspective and the corresponding improvement measures are the best ways to improve the stability of the welding process and the quality of the joint.This article starts from exploring the characteristics of the metallurgical reactions of the wet welding,establishing the connection between the droplet transfer process and the dynamic behavior of the molten pool to clarify the strategy of improving the stability of the welding process.Developing a kind of self-protected flux-cored wire performing well in the whole process of the wet welding is the final step.Based on the principle of the thermodynamic reaction,this paper explores the metallurgical reactions of the flux core reaction zone,the droplet reaction zone and the molten pool reaction zone in separate conditions,onshore and in the water.Combined with the analysis of the environment before and after the experiment,the phase structures of droplets and the slag made during different welding stages are analyzed,clarifying the main characteristics of the wet welding metallurgy: during the underwater welding process,a large amount of gas participates in and forms,resulting in a larger change of the droplet structure and the slag micromorphology.Based on this,the potential effects of metallurgical characteristics on the droplet transfer process and the dynamic behavior of the molten pool are discussed,which provides a basis for establishing the relationship between the welding metallurgy and the welding process.The droplet transfer characteristics of the wet welding were explored.The X-ray high-speed imaging system was used to observe the droplet transfer process of the wet welding and the characteristics of wet welding droplets are obtained: the droplets are always inflated and exhausted during the formation,transition and falling periods.During the process,there are three common droplet transfer modes: the repulsive transition mode,the surface tension transition mode and the spatter-like transition mode.By analyzing the characteristics of the existing droplet and the related force model,a more accurate force model is established,revealing the great importance of the gas power during the process of the droplet transfer in the underwater welding.The influence of the welding metallurgy on the welding process was aware.Based on the electrical signal synchronous acquisition system,the arc characteristics of onshore welding and underwater welding were analyzed.By establishing the relationship between the droplet transfer and the arc stability,the roots of the unavoidable short circuit and open circuit were figured out.Four common spatter formation modes were obtained through the observation of the process of the wet welding spatter formation,and the mechanism of the formation of each mode was analyzed by setting up the force models.Through the comprehensive analysis of the droplet transfer process,the arc stability and the welding spatter,the working mechanism of the Welding metallurgy on the stability of the welding process was determined,and the cause of the poor stability of the wet welding process was clarified: the gas within the droplets is charged and discharged at a high frequency,making the arc unstable and the droplets repelling out and splashing easily.Furthermore,from the perspective of the surrounding environment medium and metallurgical reaction,the formation mechanism of the gas inflation-exhaust process of the gas in the droplet is revealed.The verification test is carried out by changing the main metallurgical components in the core,and the main source of the gas in the droplet is clarified.For the first time,the internal shape of the weld pool in wet welding was observed.Using the X-ray high-speed imaging system,the onshore and underwater welding pool images were collected and analyzed,revealing the main characteristics of the wet welding pool: the interior of the pool is mainly composed of bubbles,and the liquid metal exists in a form of gas wall.The dynamic behavior of the molten pool was explored.By analyzing the influence of the droplet impact and the evolution of bubbles inside the molten pool on the movement of the molten pool,the cause of the severe fluctuation inside the molten pool in the wet welding was exposed.In order to qualitatively and quantitatively evaluate the stability of the molten pool fluctuations,a fixed point height waveform on the molten pool was used to evaluate the molten pool fluctuations on onshore and underwater welding.The standard deviation of the underwarer molten pool waveform(1.7168)was about twice that of the onshore molten pool(0.9038).By comparing and analyzing the dynamic behavior of the molten pool and the evaluation results with different welding parameters,the rationality of this evaluation method was confirmed.The formation process and the impact of the slag were discussed.By observing the whole growing process of the slag,from the melting to the solidification,the effect of the exhaust process in the arc bladder and its molten pool on the dynamic behavior of the slag is explored,clarifying the role of the liquid slag in the welding process:the liquid slag can promote the transition of the droplet,reduce the spatter,improve the stability of the droplet transition and the fluctuation of the molten pool by suppressing the content of gas in the droplets and the pools.The conclusion lays the foundation for the subsequent development of the new metallurgical system.A new type of underwater wet welding flux cored wire was developed.Based on the above results,a new kind of CaTiO 3 slag-based flux-cored wire with high melting point was designed.CaF 2 and TiO 2 were the main components in the flux core,and the ratio was designed to be 1:1.Through the observation and analysis of the welding process of the new flux cored wire,the features of the welding process was clarified:the liquid slag covers the entire welding area such as the molten pool,the arc and the droplet,acting closely to the submerged arc welding.The stability of the welding process was evaluated and the results showed that the welding process was very stable,no arc interruption during the welding process and the short circuit time accounting for only 0.1%.The transition of the droplet was completed when the size was small.There were no obvious bubbles in the molten pool and no obvious spatter during the entire welding process.Furthermore,the quality of the welded joint and the phase structure of the slag were investigated,and it was confirmed that the slag was mainly composed of CaTiO 3.The welding seam was well shaped,dominated by the mixed area of the cell crystal and the cell dendrite And the content of diffusible hydrogen in the weld is low,which is about 25% less than the existing welding wire,reaching to 3ml/100 g. |
PDF Full Text Request