Numerical Modeling Of Microstructure Evolution Involving Melting And Resolidification

In the preset thesis,the two-dimensional(2D)and three-dimensional(3D)cellular automaton(CA)models incorporating the solidification and melting mechanisms are proposed for the simulation of microstructure evolution of alloys in the mushy zone.The migration phenomena of liquid pools in the mushy zone due to temperature gradient zone melting(TGZM)are investigated using the proposed 2D and 3D CA models.The effects of temperature gradient and pulling velocity on the TGZM kinetics are studied through the CA simulations.The simulation results are compared with the analytical predictions.It is found that the liquid pool migrates through the liquidus into the bulk liquid when the pulling velocity is lower than the critical pulling velocity,or moves through the solidus into the entirely solid region when the pulling velocity is higher than the critical pulling velocity.For a given pulling velocity,the liquid pools located above the critical position move into the bulk liquid,while the liquid pools located below the critical position sink into the bulk solid.The average migration velocity of liquid pool caused by the TGZM effect increases with increasing the temperature gradient or decreasing the alloy composition.The results obtained from the CA simulations agree well with the predictions of the analytical model,demonstrating that the proposed CA model can reasonably describe the physical mechanism of TGZM.The microstructure evolution in the mushy zone of a SCN-ACE alloy in the presence of temperature gradient is investigated through the in-situ observation experiments and CA simulations.The phenomena of the secondary dendrite arms migrating toward the high temperature direction due to TGZM,dendrite arm coarsening and fragmentation have been observed from both the in-situ experiment and CA simulation.It is found that the average migration velocities of the secondary dendrite arms increase with the temperature gradient.The results obtained from the CA simulations and experiments are in good agreement with the predictions of the analytical model.For a fixed temperature gradient,dendrite arm migration becomes slower with time,when the pulling velocity is larger than the migration velocity of dendrite arms.Liquid inclusion migration in the mushy zone under a steep temperature gradient is simulated for an initially equiaxed grained Al-7 wt.%Cu alloy.It is found that the liquid inclusions are preferentially formed at grain boundaries and migrate towards the high temperature direction.After 300 s of temperature gradient holding,the mean Cu concentrations decrease and increase significantly in the hot and cold regions of the mushy zone,respectively.The simulated concentration profile agrees reasonably well with the experimental measurements.Three mechanisms of dendrite arm coarsening are identified,which includes melting of small dendrite arms to the benefit of the adjacent bigger arms,dendrite arm coalescence starting from the tips or the roots.The simulation results show that remelting is one of the dominant mechanisms for dendrite arm migration and coarsening of bigger dendrite arms by melting small dendrite arms.In addition,remelting also promotes the other mechanisms of dendrite arm coarsening.