Numerical Simulation Of Macrosegregation Defects In Steel Castings Based On Continuum Model

During the casting solidification process, the macrosegregation defects are usuallygenerated, which lead to different microstructure and mechanical properties in products.Macrosegregation is difficult to eliminate in the subsequent processing and always remainin final products, which affects the products working lifetime and performance, evencauses the security problem. However, the transport phenomena in macrosegregationformation process are difficult to investigate by experimental research; moreover, there aresome enduring disadvantages in the experimental researches, such as long developmentcycle, large consumptions of resources and so on, which make the goal of casting qualitycontrolling hardly to achieve. Therefore, by taking the numerical simulation technology toinvestigate the macrosegregation formation in steel castings, and explore the formationmechanism of macrosegregation defects and seek the optional process to control thedefects of macrosegregation, has a very significant value in both theory and practice toimprove the quality of steel casting products．Firstly, the formation mechanism and main influence factors of macrosegregationformation are studied. Based on the continuum model of classical mixture theory, a threedimensional coupled mathematical model is established for describing the thermo-solutaltransfer behavior of multicomponent alloy solidification process. The Finite VolumeMethod and Modified Projection Method and other technologies are used to develop thenumerical simulation system for the prediction of macrosegregation. And then the modeland solution method are validated by a benchmark experiment.Secondly, focusing on the2D and3D steel castings case study, the systematicanalysis of different types of macrosegregation defects (Positive segregation, Negativesegregation and A-type segregation) formation under the natural convection are discussed.Subsequently, the influences of grid size, cooling rate, casting size and shape and thesecondary dendrite arm spacing on the formation of macrosegregation defects are furtherstudied. What’s more, the comparison of3.3t large ingot experimental data and numericalsimulation results are performed to further demonstrate the practicality of developed steel casting simulation system.Thirdly, based on the theory of dendrite lapping and by applying the conversionfunction of viscosity and permeability, a unified mathematical model which considers thefree equiaxed grains movement for the description of multicomponent alloy solidificationprocess is established. The2D and3D steel casting solidification process are simulatedand the influence of the free and negative solute equiaxed grains movement on theformation of macrosegregation is analyzed.Finally, a mathematical model which considers the electric potential is established fordescribing the transfer behavior of multicomponent alloy solidification process understatic magnetic fields. The solidification process of2D and3D steel casting underdifferent magnetic fields is calculated to analyze the influence of magnetic intensity. Theresearch results indicate that a suitable magnetic field is beneficial to reduce the type anddegree of macrosegregation. And the numerical simulation technology provides a reliableguidance to choose the suitable magnetic fields.