Electron Beam Welding Dynamic Heat Source Model Based On Micro-area Electron Interaction And Keyhole Behavior

Electron beam welding(EBW)plays an important role in the fields of national defense,nuclear energy,aerospace and nation-level large-scale scientific programs due to its advantages,such as high energy density,high penetration,small welding deformation zone,high controllability and adaptability on dissimilar metals.The dynamic behaviors such as heat transfer,fluid flow and keyhole evolution in EBW are the key factors to determine the bead quality of the weld.In the research of electron beam welding molten pool and keyhole evolution,numerical method is an effective means,and the establishment of electron beam heat source model is the basis and important determinant of numerical calculation and simulations.Most of the existing theoretical models are analytical models,which can obtain effective calculation results,but it is difficult to realize the coupling with the keyhole wall when using the heat source model.Most studies have not considered the effect of metal vapor on the electron beam and the interaction between the electron beam and the keyhole wall.In this paper,the physical nature of electron beam is fully considered without assuming the effective boundary of heat source in space,and a dynamic electron beam heat source is established based on the micro-area electron interaction and applied to the simulation of electron beam welding process.First,the heat source model established in this paper is aimed at the micro-area electron interaction in metal vapor.Monte-Carlo method was used to theoretically investigate the scattering effect of 60 k V electron beam in gaseous target with various thickness,density and material considering both elastic and inelastic scattering effects.The broadening effect of beam radius caused by gas scattering is calculated quantitatively,and a mathematical formula is given and applied in the new heat source model.In order to calculate the micro-area electron interaction on keyhole surface,an optimized random electron position method is used for beam energy discretization.The actual collision position of electron on keyhole surface is calculated with progressive searching tracking method along with VOF free interface method,and the coupling of heat source and dynamic keyhole surface is achieved.The energy distribution of electron beam and the secondary energy distribution caused by backscattering are accurately described,and the mathematical expression of the three-dimensional electron beam dynamic heat source model is obtained.The dynamic heat source model is used to simulate the deep penetration welding process of thick plate,and the depth range of heat source penetration in 2219 aluminum alloy under different welding parameters is accurately simulated,which is in good agreement with the actual welding process.The early stage of deep penetration welding is the period of rapid drilling,in which liquid metal is sprayed rapidly to form spatter;after that,the volume of molten pool metal gradually increases with the welding process,resulting in less spatter.In the process of deep penetration welding,keyhole and molten pool fluctuate rapidly.The opening and closing of keyhole determine the distribution of electron beam energy in depth direction.The driving force of keyhole opening and closing is the local vertex current at the back of molten pool and the fluctuation of electron beam concentrated heating area.The total thermal efficiency of the electron beam is mainly affected by the backscattering effect;in the narrow keyhole,the electron beam is easy to form large angle backscattering on the keyhole wall,and the energy of the electron beam can be distributed in the deeper region of the keyhole.The dynamic heat source model was used to simulate the molten pool behavior of scanning horizontal welding on thin plates.It is found that the driving force of halfpenetration molten pool is mainly the recoil pressure caused by liquid metal evaporation;The flow of liquid metal in full penetration molten pool is more complex,and Marangoni flow is dominant on the upper surface of molten pool,and the surface tension and recoil pressure are the driving forces of molten pool flow;The combined action of gravity and scanning makes the mass distribution and flow field distribution asymmetric on the two sides of the weld pool,which leads to the asymmetry of the fusion lines.The simulation results accurately reproduce the experimental results in the aspects of penetration,width and fusion line angle.