Numerical Research On The Macrosegregation Of Large Scale Ingots Using A Volume-averaged Method

The large scale forging is the basic components of heavy machinery,while the ingot is the billet of forging.So the ingot fabrication is one of the important processings of heavy machinery industry.Unfortunately,some inevitable defects still occur during casting process of larg scale ingot including segregation,shrinkage cavity,heterogeneous morphorogy of crystals,etc.Among these defects,the macrosegregation is a kind of defect too difficult to alleviate during the entire fabrication.So it should be taken into account to be controlled during the casting process.The macrosegregation is caused from the relative motion between bulk melt and solid phase during the solidification.Experiment is a direct method to study the mechanism of macrosegregation.However,it is not a popluar method because of the costly,long laboratory cycle,and high energy consumption.Nowadays,as the development of computer science and engineering,the computational method becomes an efficient method to study the mechanism of macrosegregation by building macro transfer models with consideration of micro phenomena.Several multi-phase solidification models were introduced to study the macrosegregation of Fe-C binary alloy in current article.These models were integrated macro heat transfer,mass transfer,momentum transfer and component transfer with micro nucleation,growth of crystals to study the the mechanism of different macrosegregation.Finally,a novel process was introduced to alleviate the macrosegregation by controlling the solidification sequence based on the mechanism study.The main conclusions were summarized as follows:A two-phase dendritic structure solidification model with the consideration of secondary dendrite arm spacing(SDAS)was proposed in this study to predict the macrosegregation of industrial scale ingots.The model was validated by classic benchmark experiment and then used to predict the segregation of industrial scale steel ingot.The simulation results show that SDAS has little effect on solidification sequence,which governed mainly by heat transfer,but plays an important role on liquid flow which affects the final characteristic of segregation.Additionally,the flow channel,the remelting,and the sedimentation of equiaxed crystal are not the necessary conditions for the formation of quasi A-segregation.The quasi A-segregation is mainly caused by the instability of interdendritic flow.Finally,ingot size effect plays more important role than cooling conduction in the formation of the negative segregation zone under hot top.Based on previous dendritic two-phase model,a mixed columnar dendritic-equiaxed three-phase solidification model was applied to study the influence of the ingot size on the formation of macrosegregation.The simulation results show that the severity of the macrosegregation increased with the increasing ingot size.Slight segregation occurred when the ingot was small,i.e.,50 and 400 kg.The conic negative segregation zone and top positive segregation gradually appeared when the ingot weight became larger than 3300 kg.Additionally,the thermal-solutal buoyancy played a dominated role on the final segregation pattern in small ingots(i.e.,50 kg and 400 kg)after solidification.As the ingots became larger,this effect became weaker,and another effect,the sedimentation of dendritic equiaxed grains,became stronger.In the end,when the weight of the ingots exceeded 25000 kg,the sedimentation of the grains determined the characteristics of negative segregation in the bottom of the ingot.Finally,a limiting value in the negative segregation area was observed when the ingots exceeded 25000 kg.By tracking the formation history of the maximum segregation points in the 55000 kg case,we found the main reason was due to the region being fully occupied by dendritic equiaxed grains with a depleted solute.Meanwhile,the packing limit for the crystals was finite for the both experiments and simulations,so the extreme value of negative segregation for the ingots did not increase infinitely.Based on the previous three-phase solidification model,a dendritic structure 4-phase shrinkage model with the consideration of shrinkage and porosity criterion was proposed to simultaneously predict the formation of macrosegregation,shrinkage cavity,and porosity of industrial scale ingots.Based on the numerical and experimental results,the 4-phase model can be used to predict the macrosegregation and shrinkage cavity of large scale steel ingots.The numerical calculation results of macrosegregation and shrinkage cavity are in a good agreement with experimental results.Additionally,the shrinkage significantly affects the formation of macrosegragation in large scale steel ingot.In the bottom negative segregation region,the shrinkage not only increases the severity of negative segregation,but also increases the range.In the hot top region,the shrinkage reduces the range of positive segregation zone,and changes the location to a lower region.It is suggested that shrinkage should be taken into consideration when the model is developed to predict macrosegregation.The initialization of the A-segregation results from the instability of interdendritic flow,which could be driven only by thermal-solutal convection.The settling of equiaxed crystals increases the flow instability and leads to the growth of initiative A-segregation.Fiannly,the 3D lamellar structure A-segregation bands are the first time revealed in industrial scale steel ingot.Based on the mechanisms of macrosegregation,a novel solidification approach referred to as the gradual-cooling solidification progress was proposed to alleviate the macrosegregation in an industrial-scale ingot.The dendritic equiaxed-columnar mixed solidification model was verified using an experiment with a 55-ton steel ingot.Then,the model was used to simulate gradual-cooling solidification.Severe macrosegregation of the conventional ingot was observed,while alleviated macrosegregation was found in the novel case.The variation range of the segregation index was approximately 0.701(from-0.264 to 0.437)for the CSP case,yet only 0.260(from-0.158 to 0.102)for the novel case.The main reasons of the macrosegregation alleviation i were the changes to the solidification sequence and flow states during the solidification.