Simulation Of Dendritic Grain Growth In Weld Pool Of Ni-Cr Binary Alloy

The microstructure of the weld seam has profound influence on the mechanical properties of welded joints. The primary solidification structure, which directly influences the grain morphologies, macrosegregation and microsegregation and the formation of solidification crack, have crucial effect on the mechanical properties of the joints. However, it is hard to get the dynamic evolution of grain morphologies in weld pool with conventional experimental test methods because the welding solidification is a non-equilibrium process with the characters of elevated temperature, and instantaneity. Therefore, the study of microstructure evolution during weld solidification process is a complicated and important field. The current study focuses mainly on the mesostructure and microstructure in HAZ (heat-affected zone) and the solid-state phase changes in weld seam, in which many notable progresses have been made. However, the simulation of microstructure evolution during weld solidification process remains on exploration stage and there is not series of achievements published in the field.In this paper, the Cellular Automaton method is coupled with the Finite Difference technique to constitute the macro-micro (CA-FD) model, which is used to simulate dendrite grain growth in weld pool of nickel-chromium binary alloy. Firstly, the thermal model of flat plate TIG (Tungsten Inert-Gas arc welding) is established. The thermal field of welding process is solved by finite difference technique and the computation results are verified by experiments. Moreover, the macro temperature field in a definite zone is interpolated to acquire the micro temperature field. According to this micro temperature field and its interaction with solute diffusion field and microstructure evolution process, the grain growth processes in weld pool are simulated.Based on the solute conservation in the front of the dendrite tip, the dendrite grain growth model in the weld pool is established according to the grain growth kinetics theory and by using the cellular automaton method. To reflect the physical principle of weld solidification as much as possible, kinetic undercooling is applied and the non-equilibrium solute partition coefficient which changes with the growth velocity is considered in the grain growth model. The liquidus slope which changes with the solute concentration is used too in the model. In order to simulate the growth process of the grains with different crystallographic orientations, a method with"diagonal simulating angle"is presented, which is an effective complement for the cellular automaton evolvement rule.The dendrite grain morphology evolution processes in the different area of the weld pool are simulated using the CA-FD model for the approximated binary alloy of GH3030. The growth of the equiaxed grain in the center of the pool, the columnar grain in the edge of the pool and even the competitive growth between them are simulated. The simulated results present the grain morphologies, the solute distribution during the weld solidification process and the competitive growth between different dendrites as well. Based on the simulated results, the characters of the grain growth in the weld pool are analyzed. The simulated results indicated that the competitive growth in the weld pool of this Ni-Cr binary alloy is intense, the dendrite grain morphologies are complicated and grain boundary segregation is severe for the welding method and alloy used in the paper. At the same time, the grain boundary segregation is more severe in the zones where the equiaxed grains react with columnar grains.The paper further studies the influences of the difference of welding conditions, especially the weld velocity and nucleation condition, on primary dendrite spacing. The simulated results indicated that the average primary dendrite spacing decreases with the increase of weld velocity when the nucleation conditions is given and the average primary dendrite spacing has the tendency of converging to a constant value when the welding parameters are given.Furthermore, an analytic formula about secondary dendrite spacing in the weld seam is deduced, which establishes the relationship between the welding parameters and the secondary dendrite spacing. When the growth process of secondary dendrite arms between two neighboring primary dendrite trunks in the weld pool is considered, the precondition for the growth of secondary dendrite arms is profoundly discussed according the constitutional undercooling theory.Simultaneously, the metallographs test is conducted and the photos of grain morphologies in weld pool are obtained. The maximum and minimum of secondary dendrite spacing are measured. The comparison of simulated and experimental results indicates that not only the simulated grain morphologies agree with the metallographs but also the secondary dendrite spacing is accordant to a certain extent, which confirms the reasonability and feasibility of the simulation model and simulating process in this paper.Primary software for simulating grain growth in weld pool is developed based on the current study. The system consists of the temperature field computation, the grain growth simulation and the post-data treatment. The module of temperature field computation helps users to define initial conditions and boundary conditions and to implement the computation of macro thermal field of the weld. The module of grain growth simulation is used to achieve the simulation of grain growth in different zones in the weld pool. The post-data treatment module presents users with the curves and graphs about simulated results. The system provides a foundation for development of the software platform of weld microstructure simulation.