Modeling Of The Synergistic Interaction Between Microstructural Evolution And Gas Pore Formation During Alloy Solidification

Solidification microstructures and gas porosity defects are important factors impacting on the mechanical properties of materials.The evolution and formation of porosity defects are strongly associated with the evolution of dendritic/eutectic growth during solidification of alloys.In this thesis,the mechanisms of solidification microstructure evolution and porosity formation as well as their interaction are intensively investigated utilizing the numerical simulations and in situ observation experiments of directional solidification in transparent alloys.It has important academic and practical significance for improving the understanding of the underlying physics of solidification,guiding alloy design,optimizing process parameters,controlling microstructures and predicting porosity defects.A cellular automaton-finite difference(CA-FD)model for simulating the dendrite solidification,combined with in situ observation experiments on the directional solidification of transparent alloys,is employed to investigate the mechanisms of competitive growth among columnar dendrites during directional solidification of a SCN-2 wt.%ACE alloy.The mechanisms of dendrite competitive growth and unusual overgrow phenomenon at the converging bicrystal grain boundaries are explained by analyzing the solute diffusion behavior at front of dendrite tips.The solute diffusion is the dominant factor determining the dendrite competitive evolution at slower temperature gradients and cooling rates.The solute diffusion channel at front of dendrites determines the degree of solute enrichment at the dendrite tip,and the dendrites with heavier solute enrichment at the tips are eliminated.On the other hand,higher temperature gradients and cooling rates result in faster dendrite growth,with the tertiary branches at the diverging bicrystal boundaries being able to grow more quickly into a primary dendrite truck.The simulation results are in good agreement with the experimental observations.A cellular automaton-finite difference-lattice Boltzmann(CA-FD-LB)coupled model for simulating dendrite solidification and gas pore formation is applied,in conjunction with in situ observation experiments on the directional solidification of transparent alloys,to study the evolution of the interaction between columnar dendrites and the gas bubble during the directional solidification of the SCN-2 wt.%ACE alloy.The multiphase LB models are tested by comparing Laplace’ law and contact angle theory,which verifies the correctness of the LB simulation code for gas pore formation and the reasonableness of the simulation parameters.Simulation results show that the presence of the bubble disturbs the solute field near dendrite tips,which in turn affects the morphological evolution of the subsequent dendrite growth.After the dendrite touches the bubble,a solid phase envelope is rapidly formed on the bubble surface,followed by the development of new primary dendrite trunks on the envelope.The number of the new primary dendrites formed on the bubble surface increases with the wrapped bubble size.The new primary dendrites grow in the same direction as the dendrites of the wrapped bubble.The simulation results are in good accordance with the relevant experimental observations.The simulation analysis reveals that the solute diffusion layer in front of dendrites becomes wider due to the growth of new small branches on the wrapped bubble.A multiphase CA-FD-LB model coupled dendrite/eutectic growth and gas pore formation is developed.The CA-FD-LB model enables a reasonable description of the microstructural evolution,hydrogen porosity formation(including natural nucleation,growth,movement and coalescence),the evolution of concentration fields of solute and hydrogen,and the gas/liquid/solid phase interaction in the process of primary dendritic and irregular eutectic solidification.Simulation results of the synergistic growth evolution of equiaxed dendrite,irregular eutectic and hydrogen gas pore during solidification of Al-Si alloys utilizing the CAFD-LB model show that the initial Si composition has a significant influence on the size and distribution of the microporosity in the final solidified microstructures.The final gas porosities formed in the solidified alloys with a higher initial Si composition are smaller in size,more numerous and dispersed.The initial H concentration in the alloy melt and cooling rate have a significant effect on the gas pore nucleation behavior and the final percentage of porosity.Higher initial H concentration and lower cooling rate result in the nucleation of gas pores at lower solid phase fraction and higher final porosity percentage.The simulated final solidification microstructures,pore morphology and gas porosity percentages agree well with the experimental data reported in literature.A multicomponent and multiphase CA-FD-LB-CAPHAD coupled model is developed to simulate the dendritic/eutectic growth and gas porosity formation in ternary alloys.The model has the capability of reproducing the morphological evolution of columnar/equiaxed dendrites,binary/ternary eutectic growth,the natural nucleation and growth behaviour of hydrogen porosity,the concentration field evolution of solutes(Si,Mg)and hydrogen,and the gas/liquid/solid interaction during the solidification of Al-Si-Mg alloys.The simulation results show that the initial H concentration and cooling rate have significant effects on the nucleation and the final porosity percentage.Lower initial H concentration and higher cooling rate slow down the gas pore nucleation relatively,resulting in a lower final percentage of porosity.The simulated solidification microstructures,pore morphology,and gas porosity percentages are in good agreement with the experimental results reported in literature.