Study On The Fusion Forming And Plasma Dynamic Characteristics In Ultrasonic-assisted Narrow-gap Laser Welding With Filler Wire

The welding of thick plate structures has been widely used in more and more industrial fields such as nuclear power engineering,shipbuilding,oil and gas pipelines.The traditional arc welding method not only needs to process large angle grooves,but also brings a series of problems such as high heat input,large deformation after welding and stress concentration,low welding efficiency and large consumption of welding consumables.The narrow-gap laser welding with filler wire only needs to process the4⁸°groove,the heat input is low,as well as the deformation,and the filling of the wire can reduce the sensitivity of the assembly gap,adjust the weld metal composition.However,due to the small volume of the molten pool and the large aspect ratio of the weld,the narrow gap laser filler wire is easy to form defects such as pores and side wall fusion.In this paper,ultrasonic waves were applied on the basis of narrow-gap laser welding with filler wire,and the cavitation effect and sound flow effect of ultrasonic waves were used to adjust the plasma and weld formation in the groove to improve the joint quality of narrow-gap laser welding with filler wire.Firstly,the test platform for ultrasonic-assisted narrow-gap laser welding with filler wire was built,and the welding process test was carried out with different parameters.Through the analysis of the weld formation,the optimized welding process parameters were determined,thus achieving 12mm thick Q345E.Narrow-gap laser welding with filler wire under ultrasonic assisted conditions,and the microstructure and properties of the joint were analyzed after welding.The results showed that ultrasonic can effectively improve the porosity defects of narrow-gap laser welding with filler wire.Compared with conventional narrow gap laser welding with filler wire,the porosity of ultrasonic assisted laser filler wire weld was reduced by 50%and the grain in the weld zone was obviously refined,and the microstructure of the heat affected zone was not significantly different.In the tensile test of the joint,the tensile specimens were broken in the base metal,and the impact property of weld zone were improved by about50%compared with the laser filler wire due to grain refinement.The hardness of the weld zone was also generally higher than that of the weld zone.The dynamic characteristics of plasma and the transition behavior of fusion during ultrasonic assisted laser welding with filler wire were studied by high-speed imaging and spectral acquisition.The synergistic mechanism of ultrasound and laser was revealed.As the ultrasonic loading position and the weld distance increased,the plasma area gradually decreased.Compared with no ultrasound,the electron density and temperature of the plasma were higher with the ultrasonic assist,the electron density was about 10¹⁶cm^-310¹⁷cm^-3,and the plasma temperature was about 4000⁶000K.The effect of ultrasound enhances the eruption of metal vapor and plasma in the keyhole,so the trend of plasma electron density and temperature at different locations from the weld surface was more pronounced than when no ultrasound was applied.Through the analysis of the characteristics of the droplet transfer,it was found that in the case of loading ultrasonic,the plasma and metal vapor eruption was more intense,the generated steam ejection force was stronger,the droplet was blown directly above the keyhole,and the droplet was mainly at the welding wire.The density and temperature were also higher,and the heat storage in the molten pool was more,which prolonged the escape time of the pores,and could improve the pore defects to a certain extent.As the distance between the ultrasonic loading position and the weld increased,the effect of the ultrasonic on the molten pool became weaker.When the distance is30mm,the droplet transfer had no significant difference with no ultrasound.At this time,the steam ejection force was weak during the welding process,and the droplet grows in front of the end of the welding wire.Only after the droplet transition ends until the current droplet had not grown,the molten pool could receive the direct irradiation of the laser,so the penetration was smaller,and the electron density and temperature of the plasma were lower.