Numerical Analysis of Dynamic Evolution of Weld Pool,Arc Bubble and Arc during Underwater Wet FCAW Process

Author:Zhao Bo

Supervisor:wu chuan song


Degree Year:2019





Underwater wet flux-cored arc welding(underwater wet FCAW)is a competitive process for underwater engineering structure manufacturing and repairing due to its remarkable advantages,such as simple operability,good adaptability,high production eficiency and low cost.However,during underwater wet FCAW process the arc,droplet,bubble,weld pool and surrounding water interact with each other in a narrow space within a scale of tens of millimeters,which involves complex physical and chemical phenomena,resulting in poor stability and difficult control of the welding process.At present,it is difficult for this process to guarantee the quality and reliability of weld bead formation,which limits its application in the manufacture of important offshore structures.It is meaningful to provide theoretical support for the controlling and optimization of the process,which is on the premise of in-depth understanding of the physical phenomena and mechanism involved in the underwater wet FCAW process.Hence,it is of great significance to quantitatively analyze the fluid flow and heat transfer behavior of the weld pool,the dynamic evolution of the arc bubble along with arc,and the interaction between arc and arc bubble during the underwater wet FCAW process.An experimental platform of underwater wet FCAW is set up to sensing the physical process.The arc voltage and welding current waveforms during the underwater wet FCAW process are collected in real-time.The images of droplet transfer,dynamic bubble change and arc profiles are captured by high-speed visual system.The thermocouples are used to detect the thermal cycles on the workpiece,and an improved infrared temperature measuring system is designed.And the weld bead formations under typical process conditions are obtained.Based on the above experimental data,the characteristics of underwater wet FCAW process and the influence of water environment on it are preliminarily analyzed.With consideration of the influence of surrounding water on arc contraction and heat dissipation of the workpiece surface,a finite element model for heat conduction in underwater wet FCAW process is established by using the commercial code SYS WELD.Based on this model,the temperature field and weld size&geometry of underwater wet FCAW and onshore FCAW are calculated.It is found that the calculated fusion line profiles at the transverse cross section of welds are in good agreement with the measured ones.The impact of water depth and water flow velocity on the thermal process in underwater wet FCAW is analyzed.The size of weld pool and temperature field under different levels of water depth and water flow velocity are predicted subsequently.Considering the characteristics of arc heat,arc pressure,heat loss conditions on workpieces。Marangoni force and impact of droplets in underwater wet FCAW a three-dimensional transient model of fluid flow and heat transfer in weld pool is developed.and the weld pool behavior and the forces acting on the weld pool are analyzed.The results show that the droplets in underwater wet FCAW have greater momentum than those in FCAW in air,which can push the molten metal to flow downward more strongly in weld pool.However,due to the longer period of droplet transfer,the influence of two successive droplets on fluid flow and heat transfer in weld pool is difficult to be superimposed.There is a vortex in the longitudinal cross section of weld pool,in which beneath the arc the molten metal flows downward to the bottom of weld pool,and then flows backward.This vortex plays a key role in maintaining and increasing the weld pool depth.At the same time,there is a vortex on the upper surface of weld pool,in which the molten metal flows outward from the arc center.and then flows backward.This kind of vertex has an important influence on maintaining and increasing the weld width.In underwater wet FCAW.the convective heat loss on the workpiece surface plays a dominant role in workpiece cooling and limits the weld width.The influence of the stronger longitudinal cross-section vortex in underwater wet FCAW process is offset by enhanced convective heat loss,so that the calculated weld penetration in underwater wet FCAW is not much different from that in FCAW in air.In the study cases,the effect of slag on the fluid flow and heat distribution in weld pool is hmited.A numerical model for dynamic evolution of the arc and arc bubble in underwater wet FCAW is established.The heat flux and current distribution of the arc plasma at different moment,as well as the transient behavior of the arc bubble in underwater wet FCAW process are studied.The interaction mechanism between the arc and arc bubble 1s quantitatively analyzed.The calculated results show that during a single period,arc bubble will undergo three stages of dynamic evolution:growing up-bottom contraction-necking and separation.The predicted bubble profiles at different stages are in good agreement with the images captured by the high-speed camera.The gas released by reaction of the flux core ionizes and dissociates in the arc column region to produce plasma,and flows to the workpiece surface at high speed under the action of the electric field of the arc.The strong gas flow 1s the main factor to maintain the bubble enveloping the arc.When the horizontal radius of the bubble contracts,the radius of the low temperature region around the arc decreases first,and then the higher temperature region is compressed gradually.The changes of arc bubble volume and morphology have little effect on the region with high current density in arc center.The morphology and volume of the arc bubble have some influence on the heat transfer from the arc to the cathode and the distribution of arc shear force,but have little influence on the distribution of arc pressure and the cathode heat loss due to electron escape.