A Study On Molten Pool And Keyhole Moving Behaviors Of Invar Alloy By Laser-MIG Hybrid Welding

With high specific strength,good fatigue properties and other characteristics,composite material plays a more and more important role in aircraft manufacturing industry.Invar alloy is applied in composite material mould because its thermal expansion coefficient is similar to composite material.Large-scale composite material parts of aircraft require large-scale forming mould,while Invar alloy large mould can't form at one time so that welding structure of production is adopted usually.In this paper,the method of combining experiment with simulation was used,the molten pool and keyhole moving behaviors of Invar alloy by laser-MIG hybrid welding was studied.Firstly,the numerical analysis model of the fluid flow was established on the basis of some basic assumptions,considering the physical process of laser-MIG hybrid welding.The VOF method was used to solve the free surface.5mm-thick Invar steel was welded by laser weld and laser-MIG hybrid welding respectively to observe the macro morphology of weld joints.Meanwhile 19.05mm-thick Invar steel was welded by laser-MIG hybrid welding to analyze the weld depth-to-width ratios.It was found that the backing weld showed “straight nail heads” and the filling weld was “goblet-shaped”.Secondly,two-dimensional model of 5mm-thick Invar alloy laser-MIG hybrid welding was established to research the relationship between the physical parameters and keyhole formation.The results showed that keyhole formed under the combined action of recoil pressure,thermal capillary force and surface tension in the welding process.Further,the three-dimensional hybrid welding model was established to research the keyhole behavior,molten pool temperature distribution and the characteristics of molten pool flow.The results showed that the keyhole remained open in the process of welding.The experimental and calculated results conformed to the theoretical simulation.Thirdly,5mm-thick Invar alloy single laser welding process flow field model was established.The comparison between the experiment results and the simulated results has also validated the applicability of the keyhole model and gauss heat source model.Compared with laser-MIG hybrid welding simulation results,the results showed that the keyholes wave growth regularity of laser welding and hybrid welding were consistent and could be divided into three phases obviously,but the fluctuation range was small.In the process of laser-MIG welding,molten drop could effectively inhibit the "backward" phenomenon of keyhole.Finally,based on the established model of 5mm-thick Invar alloy laser–MIG hybrid welding,the influence of laser power,welding speed and current on keyhole formation and molten pool flow were revealed.The results showed that with the increase of laser power,the intensity of keyhole back-end vortex increased and the depth of the keyhole increased.With the increase of welding speed,the vibration range of the keyhole depth reduced and the depth of the hole and the flow speed of the pool reduced at the same time.With the increase of current intensity,the depth of the keyhole and the velocity of the molten pool increased effectively.Through the research above,the keyhole forming process and molten pool flow behavior were studied in the process of Invar alloy laser-MIG hybrid welding,which provided theoretical basis for the optimization of the laser-MIG hybrid welding parameters.