Coupling Simulation Of Macrosegregation And Inclusions In Large Ingots

Large castings and forgings are the core components of major equipment in the manufacturing field,while ingots are the master billet of forgings,which directly affect the selection of processing and the quality and service life of the product.Due to the intrinsic characteristics of alloy solidification,different types and scales of defects such as component segregation,shrinkage porosity,shrinkage cavity,inclusions,etc.will inevitably appear after solidification of large ingots.In the context of the rapid development of computer technology and the multiplication of computing power and storage capacity,building a multi-component and multi-phase evolution model during the solidification of ingots to simulate the solidification process of large ingots is an important method in this field.Most of the published models can only predict single defect such as macrosegregation.The ignorance of inclusion flow and solidification shrinkage affects the accuracy of model prediction.In addition,there is still a lack of elucidation of the microscopic behaviors such as precipitation and growth of different types of inclusions during solidification and the interaction mechanism of macroscopic behaviors such as flow and macrosegregation.Aiming at the above shortcomings,in this paper,based on the multiphase flow and volume average method,on the basis of the solidification segregation model of writer’s current research group,macrosegregation and inclusion coupled solidification models with Mn S inclusions and Al₂O₃ inclusions as the target inclusions have been established.In this way,it reveals the formation mechanism of macrosegregation and other defects as well as their interaction mechanism,and provides theoretical foundation support for green,low-cost,and efficient research methods and tools to design large-scale ingot manufacturing process window.For the inclusion,Mn S,a Fe-C-Mn ternary and multi-phase solidification model was established.The model considered the macroscopic phenomena such as heat transfer,mass transfer,component transmission and flow during the solidification process,as well as microscopic phenomena such as the nucleation and growth of grains,and also coupled with Mn S inclusion precipitation and growth behavior.In order to adapt to the formation of Mn S inclusions under different solidification conditions,the solidification of columnar crystals and the mixed solidification of columnar crystals and equiaxed crystals were modeled separately.The columnar solidification model includes three phases:liquid phase,columnar phase and inclusions（the phase defined here is the hydrodynamic phase,the same below）.It is found that the precipitation of Mn S mainly affects C and Mn by changing the flow field and consuming solute Mn solute.For the mixed solidification of columnar and equiaxed,the modeling includes four phases:liquid phase,equiaxed phase,columnar phases and inclusions,wherein the modeled equiaxed behaviours include nucleation,sedimentation,dendritic morphology and competitive growth with columnar phase.The reliability of the model was verified by comparing the particle size,volume fraction,critical precipitation temperature,number density and other aspects of inclusions with experimental data.The main factors affecting the precipitation of Mn S inclusions and the critical conditions for precipitation of Mn S under different S initial components were studied.The model was applied to 2.45 tons of steel ingots,effectively predicting the macrosegregation,the distribution of Mn S inclusions,and the CET.Aiming at the Al₂O₃ particles,the model employed the Lagrangian framework for tracking simulation and used fractal theory to describe the cluster-like morphology.The model considered four phases,i.e.,liquid phase,columnar,equiaxed dendrites,and Al₂O₃ particles,and was applied to a 55 tons of large steel ingots to study the influence of inclusion flow on macrosegregation.Combining the previous experimental results and the simulation results of this paper,the formation mechanism of the negative segregation zone at the bottom of the ingot and the positive segregation zone at the top were analyzed and discussed.It was found that for spherical Al₂O₃inclusions,the"effective"diameter of inclusion particles that affect macrosegregation is about 15-20μm.Relevant studies on inclusion clusters shown that the inclusion clusters formed by the aggregation of small particles are not only difficult to float and remove,but also aggravate macr-segregation.Finally,based on the above-mentioned Euler-Lagrangian framework,the process of equiaxed crystal spherical to dendritic transformation was further considered,and gas phase was introduced to balance the reduction in volume produced by solidification shrinkage,and a five-phase model was constructed to comprehensively predict macrosegregation,shrinkage and multi-size inclusion defects.The shape of the top shrinkage cavity obtained by simulation was consistent with the experimental results.In addition,the obvious typical A segregation was observed in the simulation results,and the angle was about 83°with the horizontal.The typical A segregation angle observed by the dissection experiment was 82.5°,which was very close to the simulation.And the solute segregation index on the central line of the ingot was also consistent with the experimentally measured value,which confirmed the reliability of the model.The flow and distribution evolution of multi-size spherical inclusions and cluster inclusions were studied.The simulation found that inclusions with large size were distributed at the upper part of the ingot（later solidifying zone）,while the ones with small size were often located in the lower part of the ingot（early solidifing zone）.Besides,for both spherical and clustered inclusions,there were a certain amount of inclusions existing in the mushy zone during the solidification process,which draged with the liquid phase in the solidification front to change the distribution and degree of macrosegregation.