Simulation Of Formation And Evolution Of Three-Dimensional Microstructure And Metallurgy Porosity In The Laser Beam Welding Of Aluminum Alloy

Welding porosity is one of the most common defects in laser welding of aluminum alloy.The porosities will not only reduce the effective bearing area of the welded joint,cause the stress concentration,but also reduce the strength,toughness and fatigue performance of the weld bead,and even lead to the welding crack.To explore the mechanism of porosity formation and evolution during the welding process,and to reproduce the dynamic process of competitive growth between dendrites and porosities during solidification of molten pool,the numerical simulation has become an important analysis method.Based on the cellular automaton method,the dendritic growth kinetics,and mechanism of nucleation and evolution of porosities,this dissertation has developed a three-dimensional dendrite growth and porosity evolution model during solidification process of molten pool.The dendrite growth and porosity evolution during solidification process of molten pool of Al-Cu alloy is simulated at the same time.Firstly,Al-Cu alloy laser beam welding experiments are performed.The welding parameters are selected by comparing the morphology and the penetration ratio of the weld bead.The temperature field,porosity number and porosity distribution are obtained by means of thermocouple temperature measurement and metallographic observation.Hardness test and tensile test are performed.Based on the dendrite tip growth kinetics theory,the three-dimensional dendrite growth model of solidification process is established by using the cellular automaton method.In order to reflect the physical essence of the solidification process of the weld pool,the model takes full account of the solute conservation at the dendrite tip,and introduces the dynamic undercooling related to dendrite tip growth velocity.To simulate the growth of grains with different crystallographic orientations,a "random orientation rule" is proposed for the growth of dendrite with different preferred orientation for both 2D and 3D system,which improves the cellular automaton evolvement rule.By using the model,solidification of laser welding of the Al-Cu alloy is calculated,equiaxed dendrites growth in the weld pool and columnar dendrites growth at the edge of the weld pool are simulated.The results reproduce the characteristics of dendrite morphology and solute concentration distribution during the solidification process in the molten pool at different moments.Meanwhile,the competitive growth of columnar dendrites with different nucleation numbers and different preferential orientations is simulated,and the effects of initial undercooling and cooling rate on the dendrite growth are studied.The results show that the increase of the initial undercooling can obviously raise the the dendrite growth velocity.The morphology of the dendrites will not change with the variation of undercooling when the initial undercooling is large enough.When there is no initial undercooling,the morphology of the dendrites will change from cellular dendrite to the dendrite with increasing cooling rate.When the preferential orientation of the dendrite growth is consistent with the direction of the maximum temperature gradient,the dendrites can grow continusely.Competitive growth occurs not only between columnar dendrites with different preferential orientations,but also between the columnar dendrites with the same preferential orientation.The geometry parameter of the porosity evolution is further extended into the three-dimensional system.Based on Sivets’ s law and the ideal gas equation,the model of porosity nucleation and evolution is developed.Combined with the dendrite growth model,the coupling model of dendrite growth and porosity evolution during solidification process is established.With the program designed and solved in the paper,three-dimensional equiaxed dendrites and columnar dendrites with porosities are simulated respectively.Dendrite morphology,porosity morphology,distribution of solute concentration,and distribution of hydrogen concentration during the solidification process are obtained.With the nucleation and growth of dendrites,the solute and hydrogen discharged from the front of the solid/liquid interface affect the distribution of solute field and hydrogen concentration field in the simulated region,which leads to the increase of solute concentration and hydrogen concentration near the dendrites.The hydrogen concentration in the liquid phase will rise with dendrite growth until it is higher than the supersaturation of the porosity nucleation,then the porosities begin to nucleate and grow,and the excess hydrogen will be absorbed.At the same time,the effects of initial hydrogen concentration and cooling rate on the simulation results of dendrite growth and porosity evolution are studied.The columnar dendrite growth and porosity evolution is simulated based on different nucleation numbers and different preferential orientations.The results show that the porosity nucleation occurs not only in the adjacent space of the columnar dendrites,but also in front of the columnar dendrites.The growth of dendrites will affect the porosities nucleation and growth,and the existence of porosities will hinder the growth of dendrite.The larger the initial hydrogen concentration,the easier the nucleation and growth of the porosities.The experimental results are compared with the simulated ones.With increasing the cooling rate,porosity number increases,the average porosity diameter decreases in the coupled dendrite growth and porosity evolution simulation.The dendrite and porosity simulation results are in good agreement with the laser beam welding experimental results.