Study On The Evolution Mechanism And Numerical Simulation Of Forming Defects During Cast-filling Processes

Defects prediction and process optimization are the ultimate goals of numerical simulation technology of casting macroscopic process.But the forming defects(such as gas entrapment,misrun,cold shut and oxide inclusion),which may be generated during cast-filling processes,often only can be analyzed indirectly with the filling simulation results.It fails to take full advantage of the numerical analysis benefits of numerical simulation.With the practical requirements of casting simulation technology gradually increasing,the related defects analysis cannot meet the demand of actual production.Therefore,it has significance for optimizing the casting process and improving the overall performance of casting,to study the forming defects during cast-filling processes by the numerical simulation method.In this paper,all aspects involved in the numerical simulation of the forming defects during cast-filling processes were deeply studied and discussed,including the multiphase flow mathematical modeling in the cast-filling process,the evolution mechanism and prediction of cold shut defect,the evolution mechanism and prediction of oxide inclusion defect,and so on.Meanwhile,the related solver development works here were based on CFD(computational fluid dynamics)open source code,which can help the majority of researchers in-depth study of numerical simulation technology in cast-filling processes.Firstly,considering that liquid-solid conversion has a great influence on the cast-filling process,a calculation model of mushy region flow behavior through measurement of solid-fraction was developed,which could effectively investigate the flow behavior of mushy region in different stages.Generally,the critical solid-fraction method was adopted for mushy region with high solid-fraction,the variable viscosity method was applied for mushy region with low solid-fraction,and the porous medium drag-based model was used for mushy region with middle solid-fraction.The mathematical model of gas-liquid-solid multiphase flow was established,which was used to calculate the coupling effect of “air-liquid metal-solidified metal” in the cast-filling process.The VOF-PLIC algorithm was used to track the interface front,the CSF model was used to calculate thesurface tension,and the equivalent specific heat and temperature correction methods worked together to deal with the latent heat.A cast-filling process solver was developed based on the CFD open source code OpenFOAM.Two filling benchmark experiments(an S-shaped channel water filling experiment and an aluminum alloy benchmark test)were calculated to verify the accuracy of the basic algorithm of flow field model(gas-liquid two-phase flow calculation).Computation and comparison of the calculation results of a bottom-pouring casting process under different mushy zone flow models verified the rationality of the calculation model of mushy region flow behavior in this paper.Secondly,the formation of cold shut is due to that a liquid front has been partially solidified,and atoms are less diffusive and less athletic,unable to form strong bonds with other grains at the atomic scale.The main factors affecting the cold shut defect include the interface fronting solidification state,the degree of collision and the degree of convergence.Considering that the implicit interface tracking algorithm can only get the diffusion interface front,an algorithm for judging the interface front was proposed for the cold shut defect prediction.Considering that the cold shut defect was produced by the intersection of liquid metal fronts,an algorithm to determine the frontal collision of the interface was proposed,with reference to the idea of Lagrangian particle tracking.Combining with the cold shut defect evolution mechanism,a formation model of cold shut,which is related to solid-fraction,velocity and volume-fraction of metal phase,was developed here.A predictive solver for cold shut defects during the cast-filling process was developed,on the basis of the obtained cast-filling process solver.Thirdly,the formation of oxide inclusion defects is a complex process of physical and chemical changes,and in the gas-liquid-solid multiphase coupling flow state.The main influencing factors of oxide inclusion defects during the cast-filling process are the liquid metal-air contact degree,the liquid metal temperature,and the generated oxide inclusion.Combining with the existing chemical reaction models(a chemical combustion rate calculation model and an core gas formation rate calculation model),a formation rate model that is relevant to the volume fraction of liquid metal,the temperature,and the current oxide inclusion density,was proposed here.A boundary condition was developed to handle the adhesion and accumulation on the wall.A predictive solver for oxideinclusion defects during the cast-filling process was developed,on the basis of the obtained cast-filling process solver.Finally,a set of low pressure die casting crafts with different wall thickness were designed,and the filling processes and casting final filling heights under different wall thickness were analyzed and compared,which verified the accuracy of the proposed calculation model of mushy region flow behavior in predicting the misrun defect.Two zinc alloy HPDC(high pressure die casting)processes with different runners were designed,the filling process under different runners and the distribution of final air entrapment defects in the castings were analyzed.The predicted results of the air entrapment defects corresponded well with the actual casting pore distributions,which verified the practicality of the adopted gas-liquid-solid multiphase flow model in predicting the air entrapment defect.Aimed at two copper alloy LPDC(low pressure die casting)processes with different runners,the filling and cold shut defect evolution processes were calculated and compared.The predicted results of cold shut defects corresponded well with the actual cold shut distribution of castings,which validated the practicability of the predictive model of cold shut defect.Aimed at a representative aluminum alloy casting technology and a practical copper alloy casting technology,the evolution processes of oxide inclusion defects were compared and analyzed.The prediction results of oxide inclusion defects corresponded well with the actual distribution of oxide inclusion defects on the casting surface,which validated the practicability of the predictive model of oxide inclusion defect.The above-mentioned benchmark tests and casting experiments proved the feasibility and practicability of the numerical simulation technology of forming defects in the cast-filling process,which could provide the scientific guidance for the actual casting production.