Simulation On Solidification Microstructure Of Fe-C Binary Alloy In Weld Pool Using Cellular Automaton Method

The microstructure of weld pool after solidification is non-equilibrium solidification structure for the characteristics of high temperature,dynamic and instantaneity of weld process,which has important influence on the mechanical properties of weld joint.In order to get high quality weld joint,it is very important to control the solidification process of weld pool.With the development of computer and continuous improvement of solidification theory,using numerical calculation method to solve the solidification of weld pool will help to further understand the complicated physical process of weld,and provide theory basis for optirmizing weld technology and improving the quality of weld joint.In this paper,a simulation model of dendritic growth and solute distribution is developed,which is on the base of physical mechanism of nucleation and dynamics theory of dendrite growth,takes into account the anisotropy of interfacial energy,interface perturbation,solute diffusion,constituent undercooling,curvature undercooling and weld pool shape,etc,and uses cellular automaton method with Moore type neighborhood.This model simulates single equiaxial crystal,multi-equiaxed crystals,columnar crystals,and the competition growth between columnar and equiaxed crystals of Fe-C binary alloy,discusses the effect of undercooling,disturbance amplitude and cooling rate on dendrite growth,quantitatively researches the growth velocity of dendrite tip and secondary dendrite arm spacing.The results indicate that solidification structure with complicated branching structure is formed by competitive growth between dendrites and the control of thermal and solute diffusion;the solute mainly concentrates among dendrite arms and its concentration increases with the extension of solidification time;there is always dendrite segregation during dendrite growth,which is mainly distributes in the secondary and tertiary dendrites;undercooling affects crystal nucleation,growth morphology and solute concentration,the greater the undercooling,the more complex the crystal morphology,and the greater the solute concentration;disturbance controls the crystal branch,when the disturbance increases,the cellular and dendrite crystal will appear in the process of crystal growth,but the solute concentration basically has no change;cooling rate have an impact on the velocity of dendrite growth and solute concentration,improving cooling rate can get faster velocity of dendrite growth and higher solute concentration;the study of tip velocity can confirm the stability of model;the curve of secondary dendrite arm spacing is capable of determining the cooling rate range of welding solidification.The model of welding temperature field is established by finite difference method,and the macro temperature field model and the micro dendrite growth model are coupled together via the method of dividing two sizes of mesh.The two models simulate respectively TIG welding temperature field and microstructure evolution process of Fe-C binary alloy after coupling temperature.The results show that the temperature field in weld pool is the key factors that determine crystal nucleation and preferential growth direction,and distributes like ladder;the temperature gradient increases gradually from the center of weld pool to the edge of weld pool;after coupling temperature,the nuclei of columnar crystals appear before dendrite crystals,the phenomenon accords with the theory of solidification,and the rules of dendrite growth and solute concentration are constant.