Research On Molten Pool Behavior And Metal Transfer During Electron Beam Welding Under Different Gravity Level

The development of space technology requires space construction,repairment and maintenance techniques.Space welding is one of the most critical methods in space construction,repairment and maintenance.Among the particular environmental factors in outer-space,the micro-gravity has the most significant influence on fusion welding.Electron beam welding has been proved to be the most suitable welding method in the space.In this thesis,the flow behavior of electron beam welding pool and the metal transfer under gravity level of 1G and 0G were studied by numerical simulation based on space welding.A multi-physics numerical simulation model of electron beam welding was developed firstly.The whole process model including the simulation of electron beam emission and transmission process,the interaction between electron beam and the workpiece,the thermo-fluid coupling in the electron beam welding molten pool.Electron beam welding experiment was conducted in 1G environment to verify model reliability.The accuracy of the numerical model was evaluated.The conduction mode and deep penetration mode of electron beam welding molten pool flow behavior,as well as the metal transfer behavior of electro n beam welding with filler wire were simulated under the gravity magnitudes of 1G and 0G.The simulation results were analyzed to reveal the action mechanism of 0G gravity on electron beam welding.The multiphysics numerical model of electron beam welding consists of two parts: the dynamic heat source model and molten pool flow model of the electron beam.The radial energy distribution function of the electron beam was obtained through the electron optical simulation of the electron gun.The energy utilization efficiency and penetration depth of the electron beam in the workpiece were obtained through the calculation of the interaction between the electron beam and the workpiece.The dynamically distributed surface heat source model of electron beam was established by tracking the free surface of the molten pool and projecting the radially distributed electron beam energy to the surface layer of the molten pool.The solidification and shrinkage model,cavitation and bubble model,3D recoil force model of metal vapor and the oxide film model were established and included in the molten pool flow model of electron beam welding.The ceiling temperature of pool model,substitution of cavitation model,was further developed to solving the computational stability problem.The molten pool behavior of pure aluminum,304 stainless steel and QCr0.8 copper by electron beam welding under 1G gravity were calculated using the established model and compared to electron beam weld experiment of the same condition.The results showed that the proposed molten pool flow model of electron beam has high accuracy in weld appearance simulation under 1G gravity.The experimental and numerical calculated topside weld width of aluminum heated by stationary electron beam for 0.5s were 5.5 mm and 5.6 mm respectively.The corresponding backside width was 4.7 mm and 4.5 mm respectively.Compared to gravity level of 1G,the depression of conduction mode electron beam welded molten pool and the risk of burned thorough decreased under 0G gravity,which is better for the welding process.The effect of 0G gravity on the appearance of the partial penetration weld was insignificant.The action mechanism of 0G gravit y on the molten pool was the disappearance of natural convection and depression induced by hydrostatic pressure.The natural convection strength increased with the molten pool depth and the relative sag of molten pool caused by hydrostatic pressure increased with the increase of the molten pool radius under 1G gravity.The maximum flow rate induced by natural convection in the aluminum molten pool with a depth of 2mm and radius of 3mm was 8.68% of that driven by Marangoni convection under 1G gravity.The relative sagging depth of molten pool under 1G gravity was 3.5%.According to the test standard of low bond number experiment,t he low bond number experiment of electron beam welding can be conducted in gound environment to reflect the welding condition in space when the aluminum thickness was less than 2mm and maximum molten pool radius was less than 3mm.The recoil force of metal vapor expelled the molten metal from the center to the outside region and a keyhole formed in the molten pool in deep penetration mode electron beam welding.The molten metal at the front of keyhole was driven to upside and backside of the keyhole,while the bulge at the keyhole wall was pushed to the bottom by recoil force of metal vapor.The fluctuations of the keyhole and convergence of upward and downward flow promoted the collision of keyho le wall to form bubbles.The bubble formed by the collision of keyhole will prevent the electron beam from heating the bottom of the keyhole.Thus defects such as sawtooth shape weld,spiking and cold shut were tended to form at the bottom of the weld.The bubble will expand when heated by electron beam and compel liquid metal from bottom to the upside and outside of the molten pool.Thus the bottom of the molten pool will be heated directly by electron beam once the bubble breakup.Hence the welding penetration will increase rapidly.0G gravity decreased the difficulty of discharging the liquid metal at the bottom of the molten pool by metal vapor recoil force or bubble expansion due to the disappearance of hydrostatic pressure.0G gravity will promote welding penetration which is beneficial to deep penetration mode electron beam welding.The Al-4at%Mg alloy with a thickness of 3mm heated by electron beam for 7.5ms with the power of 3.6k W under 0G gravity,while only 75.7% was penetrated under 1G gravity with other conditions keep constant.The filler wire could not be sent to the position that the wire can be heated directly by electron beam under 0G gravity.The steady metal transfer of the filler wire could be realized by the beam deflection process and the insertion transfer mode during electron beam welding with filler wire under 0G gravity.