| Magnesium alloy has such characteristics as low density,high strength and easy machining,etc.High quality,high efficiency and low energy consumption connection of magnesium alloy is the necessary way to realize manufacturing lightweight.In consideration of its physical characteristics,low-power pulsed laser induced arc welding process is an effective potential way due to the advantages of strong welding penetration,fast welding speed and small deformation.However,low-power pulsed laser induced arc welding process has such problems as the growing number of parameters,the increasing level of selecting technology parameters and the process of complex regulatory.It is difficult to control the weld pool shape accurately through experimental control parameters.Especially in the process of magnesium alloy welding,due to its low melting point,fast thermal conductivity and complex interaction among multiple parameters,the difficulty of weld pool shape control is further increased,which greatly affects the forming quality of magnesium alloy welding joints.At the same time,it is easy for the magnesium alloy to create keyhole by using low-power pulsed laser induced arc welding process.The temperature,pressure,velocity of gas-liquid phase inside the keyhole and the parameters of keyhole morphology are the main factors to form the porosity and spatter defects.The induction and enhancement of arc by laser make the variation of keyhole more complicated.It is difficult to accurately analyze the mechanism of welding porosity and spatter defects by the traditional experimental method,and the welding defects cannot be eliminated fundamentally.Therefore,the realization of accurate shape control and defect control of welding pool,is the key problem to be urgently solved in the low-power pulsed laser induced arc welding of magnesium alloy.To sum up.the research object of this paper is low-power pulsed laser induced arc welding of magnesium alloy.Taken computer simulation technology as the research means,based on a hybrid heat source model with the characteristics of pulsed laser and the coupling effects of laser and arc,the temperature field and keyhole gas-liquid interface tracking model of welding process was established.Based on the temperature field model,the weight relation of core parameters of composite heat source on the weld pool shape was determined,and the database of the weld pool shape within the scope of the experimental parameters in this paper was constructed.The precise control of the weld pool shape was realized through the hierarchical regulation of the core parameters of the database.Based on the keyhole gas-liquid interface model,the correlation system between welding technology and keyhole behavior parameters was established to explore the influence mechanism of keyhole evolution behavior on the formation of welding pores and spatter defects,and the accurate process threshold for controlling welding defects was obtained.The development of this subject will lay a theoretical foundation and provide practical guidance for the realization of high quality and efficient laser induced arc welding technology.The main research contents and results are as follows:By fully considering the characteristics of pulsed laser and the coupling effects of laser and arc,a hybrid heat source model was established for the hybrid welding of magnesium alloys.In this model,the peak laser power is controlled by the excited current and pulse width,and the pulse output is characterized by the periodic square wave function.The arc action radius compressed by laser was introduced into the Gaussian surface heat source model to characterize the laser enhanced arc effects.Based on the heat source model,the temperature field model of the magnesium alloy pulsed laser induced arc welding process was established,and the thermal cycle curve and weld pool shape of the welding joint during a full pulse period were simulated,it’s worth noting that,the thermal cycle curves obtained by experiment were in good agreement with the simulation results.The error rates of the simulated fusion width and penetration depth were lower than those of the experimental results,indicating accurately the "1+1>2" effect of the pulsed laser induced arc welding.Based on the temperature field model of low-power pulsed laser induced arc welding of magnesium alloy,the influence of hybrid heat source parameters on the temperature and weld pool shape was studied.It is determined that arc current is the primary influencing factor of molten pool morphology,which could control the basic melting depth and width of the weld pool.Laser excited current is the secondary factor affecting the morphology of the weld pool,which could control the length of the nail head and the waist width of the weld pool.The laser pulse width has a slight influence on the width and waist width of molten pool,which is a micro influence factor on the morphology of weld pool.On this basis,the arc current used as the first data set,the laser excited current used as the second data set,and the laser pulse width used as the third data set,a simulation database of weld pool shape within the range of experimental parameters was constructed.A method for controlling the weld pool shape in laser induced arc welding of magnesium alloy was established by means of preliminary adjustment,accurate adjustment and fine adjustment called the three-level in the database system.By using this method,the accurate shape control of flat butt welding and T-shaped welding pool was achieved.Considering the keyhole behaviors had significantly effects on the welding quality during pulsed laser induced arc welding process,the COMSOL Multiphysics software and the Level-Set method in fluid mechanics were applied to establish a racking model of keyhole gas-liquid interface.The results showed that the evolution of the keyholes included four stages of formation,development,stabilization,and closure.The keyhole presented a "nail"shape with wide upper and narrow lower,and the airflow at the keyhole bottom was irregular and turbulent,which leaded to an asymmetric distribution of the keyhole front and back walls.A coupling effect of the evaporation recoil pressure of the liquid metal and the Marangoni effect caused a "wrinkle" behavior on the keyhole surface.There was a delay of the keyhole effect caused a "wrinkle" behavior on the keyhole surface.There was a delay of the keyhole closure after the pulsed laser radiation.The evolution of the keyhole width and depth was experimentally measured using a "sandwich method" and a high-speed camera.The simulations and the experimental results were agreed with each other,and the error rate was less than 10%.Based on the model of the gas-liquid interface of the keyhole during welding process,the influence of mechanical factors and process parameters on keyhole evolution behavior was studied,then the correlation system of keyhole behavior parameters was established,the influence mechanism of keyhole evolution behavior on welding porosity and spatter defects was revealed,finally,the accurate process threshold for controlling welding defects was obtained.It was found that the evaporation recoil force,generated by the liquid metal vaporization at the gas-liquid interface,was the main factor for the keyhole formation.The Marangoni effect enhanced the flow of the liquid metal at the upper and middle interfaces of the keyhole,and improved the smoothness of the keyhole interface.The surface tension inhibits the keyhole formation by tightening the keyhole interface.Laser excited current was the main factor that affected the maximum temperature and depth of keyhole.It was worth noting that,when the laser excited current was set below the critical value 125 A,liquid metal splash defects could be greatly reduced or even avoided.The arc current was the main factor that affected the delay behavior of the keyhole closure.The reduction of the arc current increased the Marangoni effect and hindered the backfill of liquid metal,increasing the temperature gradient at the middle and top of the keyhole,when the pulsed laser disappeared.When the arc current was set above the critical value 85 A,the feature of keyhole closure was changed from collapse backfill to order backfill,avoiding the defects caused by delayed keyhole closure. |
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