Reasearch On The Prediction Of Solidification Mushy Zone Mechanical Properties And Hot Tearing Defects Of Cast Aluminum Alloy

The development trend of increasingly large,complex and integrated castings has higher requirements for casting technology and design capability.At present,casting simulation technology is widely used in industry to assist the optimization of casting structure and casting process.The material physical parameter is crucial in the casting simulation,and its accuracy greatly effect the casting simulation results.The solid/liquid mushy zone is a special region in the solidification process of castings.Latent heat release,solute segregation and solidification shrinkage occur in the mushy zone during solidification,and most casting defects also occur in the mushy zone(such as porosity,cold shut,hot tearing,etc.).Accurate acquisition of material parameters of the solid/liquid mushy zone is crucial to describe the heat,stress and other transfer and accumulation process in the mushy zone,also for the accurately prediction of casting defects.Especially for the mechanical parameters of the solid/liquid mushy zone,due to the complexity of measurement methods and the lack of unified standards,how to accurately obtain the mechanical parameters of the solid/liquid mushy zone has always been a difficulty in the solidification field.For the above problems in the industries,this research obtained the microstructures morphologies of the solid/liquid mushy zone in different stages using phase field simulation.The "representative volume element" of the mushy zone was constructed through image processing and model reconstruction to carry out the micromechanical calculation,and obtain the corresponding mechanical parameters of mushy zone in different stages.The micromechanical calculation results were used to provide accurate mechanical parameters for macro-casting simulation and optimize the solidification model.On this basis,the model and research results obtained in this study were verified by experiments using Al-4.5wt%Cu alloy.The focus of this research is proposed an integrated method to accomplish the multi-scale analysis and application of aluminum alloy casting from microstructure simulation to macro casting defect prediction.The conclusions are as follows.A phase field model was constructed based on the phase field principle and free energy functional theory,and calculated the evolution of microstructure morphology of mushy zone.The phase field control equation and the solute diffusion equation in mushy zone were derived,and the process of solving the phase field model of Al-Cu alloy by difference method is established.The detailed parameters in the simulation are determined based on the influencing factors of microstructure evolution.For Al-4.5wt%Cu,the evolution process of its mushy zone morphology was obtained,and the phase field simulation results were processed by image processing and geometric model reconstruction,providing accurate model preparation for establishing micromechanical calculation to study the solidification mechanical properties of the mushy zone.An integrated model was developed in this work by combining the phase field model and CFD model to study the liquid permeability in mushy zone.The method introduces the precise microstructure of mushy zone,and greatly reduce the computation time for solving the flow equations.The cloud diagrams of liquid phase pressure distribution in mushy zone were obtained according to the CFD calculation results with different solid fraction,and then obtained the mushy zone permeability.The values of permeability decrease from 10-9.8 to 10-12.5 m2 as the solid fraction change from 0.3 to 0.9.The relationship between the mushy zone permeability K and the specific surface area of the solid-liquid interface Sv with the solid fraction was established.Moreover,the value of the Carman-Kozeny coefficient Kc was confirmed to equal approximately 4.5 at low solid(fs<0.7),however,the value of Kc increases to 5.5 or even higher with increasing of solidfraction(fs>0.8),which provides more accurate parameters support for the using of Carman-Kozeny equation.An integrated method was developed to study the constitutive behavior and rheological properties of the solidification mushy zone by combining phase field simulations and micromechanical calculation.The numerical model was constructed based on the microstructure morphologies of mushy zone obtained via phase field simulation.Coupled Eulerian-Lagrangian(CEL)method was applied to solve the solid dendrite deformation and interdendritic liquid flow.The method introduced the precise microstructure morphology of mushy zone,and can assigned the materials parameters of the solid and liquid phases,respectively,which is more accurate than the method that ignores the liquid phase effect or gives it complete plastic characteristics.The prediction and experimental results were in agreement at the corresponding solid fractions for Al-4.5 wt%Cu alloy,which indicates that the proposed method is reliable for predicting the constitutive behavior of A1 alloy mushy zone.The mushy zone permeability and constitutive behavior data obtained in this study were used in casting simulation to optimize the solidification model,and predict the hot tearing defects through casting thermal stress analysis.The rheological property term was introduced into the elastic-plastic(E-P)constitutive model,and establied an elastic-viscoplastic(E-VP)model.The analysis results show that E-VP model is consistent with the deformation mechanism of mushy zone,and obtained more accurate hot tearing prediction results than E-P model.A comparison of the modified permeability with the default constant permeability model shows higher accuracy on the hot tearing prediction;The results of micromechanics calculation in the mushy zone were applied to the prediction of casting defects,which bridge the connection between the microscopic phase field calculation and the macro-casting simulation,providing a method and reference for the multi-scale analysis and control of alloy casting process research.A mushy zone deformation and hot tearing propagation mechanism was established based on the constrained rod casting(CRC)experimental results.The mechanism shows that hot tearing propagation includes three stages.Rheological deformation occurs during the initial stage.Thereafter,solid bridging occurred at the hot tearing interface as solidification progressed.Finally,hot tearing propagation terminated with elastic-plastic deformation.The growing dendrites underwent rheological deformation under thermal stress at the initial stage of hot tearing propagation.With increasing of the casting temperature,the length and area of the rheological structure increased,which is one of the possible causes for the decreased severity of hot tearing with increasing casting temperature.CRC experimental results show that E-VP model is reliable for hot tearing prediction.At the same time,the microstructure of the hot tearing fracture proved that the E-VP model constructed in this study is consistent with the mushy zone deformation and hot tearing propagation mechanism.