Numerical Study On The Welding Pool Behaviour And Weld Formation Regularity During Electron Beam Welding Of Aluminium Alloy

Electron beam deep-penetration welding has been widely applied to the jointing and fabrication of large thickness aluminium alloy structural parts,in view of its a series of advantages such as high-energy density,large depth-to-width ratio of weld,small distortion and excellent properties of joint,etc.How to guarantee the formation quality of electron beam weld of large thickness aluminum alloy plate has become a focus.As is well-known,the heat transfer,fluid flow and keyhole evolution of welding pool is the key determining the formation quality of weld.However,the related theoretical studies on it are few and less in-depth.Therefore,the numerical simulation and experimental validation have been utilised to study the welding pool behaviours,based on which the weld formation mechanism of weld and regularities are obtained.These findings are significant to enrich the imperfect theoretical system and guarantee the formation quality of weld.Based on the deep understanding of the physical transport phenomena of welding pool in electron beam welding pool,the 3D mathematical models for the spot welding and continuous welding processes on 2219 aluminium alloy thick plate have been established separately,which couples thermal-fluid transportation of weld pool with keyhole evolution.According to electron beam divergence and energy deposition characteristics,combining VOF(volume of fluid)tracking algorithm with enthalpy-porosity technique,an adaptive heat source model which can automatically tracks the keyhole profile and determines the heating depth of electron beam has been proposed.It is composed of vapour supplementary heat source and Gaussian heat flux.Adopting the adaptive heat source model and combining various fluid driving forces such as vapour-induced recoil pressure,surface tension,Marangoni shear force and hydrostatic pressure,describing the heat transfer,fluid transportation with various flow states and keyhole evolution processes of transient weld pool under vacuum,such as physical phenomena of keyholing,stability,backfilling and keyhole collapse,etc.On this basis,elaborating the formation mechanism of weld for the spot welding and continous welding processes.The whole mathematical modelling is conducted by hydrodynamic software ANSYS FLUENT,by means of programming based on UDF(user-defined function),loading energy boundary conditions of heat source and heat dissipation,as well as momentum boundary conditions of various fluid driving forces in the form of source term.In addition,finite volume method is used to discretize the governing equation,while PISO(Pressure-Implicit with Splitting of Operators)algorithm is used to implement the numerical solution.Based on the established thermal-fluid coupled mathematical model,the formation mechanisms of spot weld under different beam currents,including the nail-head,nail-body,crater and spiking defect are systematically analysed.It shows that the formation of nail-head region is caused by the enhanced heat transfer derived from Marangoni shear stress and the thermal insulation of metal vapour for keyhole opening.The formation of nail-body region is caused by the keyholing effect of vapour-induced recoil pressure.And the formation of crater is caused by the falling back effect of bump during the inertial flow process of backfilled fluid.Meanwhile,as beam current increases,their dimensions are all increased.In addition,it is found that the formation of spiking defect is closely associated with the keyhole depth,keyhole collapse and duration time of beam at keyhole bottom.As keyhole depth increases,its instability is increased,which is inclined to collapse at weld root and capture the metal vapour hard to escape to form a large cavity.Additionally,due to the short duration time of beam at keyhole bottom,it is hard to feeding the cavity for the insufficient liquid metal amount,and thus promotes the formation of spiking defect.Based on the established thermal-fluid coupled mathematical model,the formation mechanisms of continuous weld under different beam currents,including weld penetration,weld width and reinforcement,are systematically analysed.Among the rest,the formation mechanism of weld penetration and weld width during continous welding process is similar to that of spot welding process.The formation of reinforcement is the comprehensive effects of "flow state Ⅲ","flow state Ⅵ" induced by recoil pressure and "flow state Ⅴ" induced by surface tension.The results indicate that as beam current increases,the metal amount with volume expansion transported to the top surface of posterosuperior part of welding pool exceeds the migrated metal amount of "flow state Ⅴ" and thus results in the increasement of weld reinforcement convexity.During the study of physical transportation phenomena in spot welding and continous welding processes,the evolution regularities of some characteristic quantities such as the maximum vapour pressure and the maximum keyhole wall temperature are also revealed.It shows that the attenuation distribution of heat flux of electron beam in depth direction is the intrinsic cause of this evolution tendency.The study also found that the maximum vapour pressure for all beam current conditions is all lower than 1 atm,and the corresponding maximum keyhole wall temperature is also lower than the boiling point of material.Based on these foundings,the differences between electron beam welding and laser welding,plasma arc welding are further analysed and elaborated.