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Numerical Modeling And Experiment Investigation Of Liquid Structure And Solidification Behavior Of Al-Cu-Zn Alloys

Posted on:2005-05-21Degree:DoctorType:Dissertation
Country:ChinaCandidate:F Y ChenFull Text:PDF
GTID:1101360155477389Subject:Materials Processing Engineering
Abstract/Summary:PDF Full Text Request
Solidification is one of the most important routes for modern metallic material processing, a great variety of microstructures, and thus of material properties, can be obtain by varying alloy composition and process condition. Over the last decade, important advances have been made in our fundamental understanding solidification nature; however, the solidification process is highly complex, especially for engineering alloys. Several interacting physical phenomena not completely understood yet are responsible for the solidification behaviours of a casting. The present research is a contribution to the knowledge on solidification feature of multi-component Aluminium alloy in the following three main parts: liquid structure, partitioning behavire and Microsegregation.Part â… . Various models including Lennard-Jones model, Meadima model, associated solution model and Vogel-Fulcher-Tammann model were used to study the liquid structure and dynamic nucleation feature of Al and Al-Cu melt.The structural and thermodynamical properties of pure Al melts were modeled by Lennard-Jones melts and investigated by the molecular dynamical simulation during the heating and cooling process. The results show that the structure transition was found at reduced temperature 0.51 during cooling process. The atom configuration of the system was randomly arranged and the first peak of the pair distribution function varied between 4 and 5 in heating process. The system gradually evolved from a disordered structure into a structure with the long range order in the cooling process as indicated by the data from the atom configuration and pair distribution function of the system.Proper analysis of the solidification charactertics of multi-component alloys was mainly dependent on the thermodynamic description of binary alloys. A new model was developed to calculate the excessive free energy and the influence of the chemical short range order in melt structure of Al-Cu alloys. The new model was applied to calculate the thermodynamic properties in Al-Cu alloys containingdifferent chemical short range order. Compared with other theoretical models, such as Meadema model and sub-regular solution model, the new model is more reasonable, and agrees well with the experimental results.A new associated solution model was proposed to incorporate the AICU3 associate in Al-Cu melt into classic substitution model as kind of short range order. The model was described by Redlich-Kister polynomial and was able to deal with the phase calculation by Thermo-Calc software. The model was applied to improve the thermodynamic description of the liquid phase in the Al-Cu binary alloy. The enthalpy of mixing, solidus and equilibrium partition coefficient were calculated in several binary alloys. It was found that the present model offers more exact description than that of Saunder's model and was suitable to other binary alloys, for example, Mg-Sn and In-Sb alloys.One of the problems in the classical nucleation theory was the method to determine the size and amount of critical clusters in metallic melts. The Vogel-Fulcher-Tammann model was improved to describe the formation of clusters and their feature during homogeneous nucleation in the paper. The viscosity and mass transport property measurement was used to identify the a-p1 bifurcation temperature and VFT parameters. The model can be used to calculate the nucleation temperature, size of critical clusters in melts and capacity jump between the liquid and crystalline and also can be used to explain the effects of melt rapid heat treatment on homogeneous nucleation.Part II. The equilibrium solute partition coefficients in Al-Cu-Zn alloy were measured experimentally. Their thermodynamics model was proposed and used to calculate the solute trapping phenomena during rapid solidification.The non-equilibrium phase tends to precipitate during solidification of multicomponent alloy and leads to detrimental effect on the soundness of the castings. The LMC directional solidification and quenching technology was used to investigate the crystal-melt interface morphology and segregation of the Al-1.5Cu-3Zn alloy at various temperature gradient G and growth rate R. Thecellular growth interface and dendritic growth interface were observed and characterized. The distribution of solutes Cu and Zn was measured along the crystal-melt interface. From the results the equilibrium partition coefficient ko for Cu and Zn was obtained to be 0.47 and 0.6 respectively, koof Cu was found to become higher than that in the binary Al-Cu alloy and Zn remain unchanged. It was indicated that the solute interaction in the Al-1.5Cu-3Zn alloy led to increase k0 of Cu and lessen its micro-segregation.Solute partitioning is one of the complicated but important phenomena in the solidification process, which controlling segregations and morphologic evolution during the solidification of multicomponent alloy. Two thermodynamic models, i.e., activity model and concentration model, were developed to calculate equilibrium solute partition ratios of multicomponent alloy. Based on these models, the equilibrium solute partition ratios of Cu and Zn in Al-Cu-Zn alloy were calculated. It was found that the results obtained by the two models were close each other, k, was slightly dependent on the temperature. The increase of Zn concentration led to the decrease of the equilibrium partition ratio of Cu. The effect of impurities, such as Fe, Si, Cr, on the equilibrium partition ratio of Cu in the ternary Al-Cu-X alloy was analyzed and compared with the experimental results.The formula to calculate the Gibbs energy and interface temperature during solute trapping in the ternary alloy was present based on the continuous growth solute trapping model of Aziz and Kaplan, and was used to model the solute trapping behavior during the primary phase solidification of Al-Cu-Zn alloys. The kinetic solidus and liquidus was found to approach to To curve with increasing growth velocity on two different isopleths of Al-Cu-Zn alloys, the solute trapping coefficient increased sharply with increasing growth speed, and the applicability of equilibrium partitioning coefficient ko was calculated, and it was found that ko was used reasonably for the solidification with cooling rate below 7.1 K/s. In assumption of fixed solid composition, the interface temperature rised with increasing growth velocity and reached its peak near the solute diffusive speed vd, however, with fixedliquid concentration, the interface temperature decreased with increasing growth speed.Part III. The thermodynamics-calculated phase diagram and experimental phase diagram were used to predict the solidification path, these calculated results were compared with experiment results, the numerical model of solute redistribution during dendritic solidification was proposed, and the effect of solid diffusion on the solute redistribution was analysised during solidification of Al-Cu-Zn alloy.The solidification path and microsegregation during primary phase solidification of Al-Cu-Zn alloy were evaluated by CALPHAD method, simple Scheil equation and experimental measurements. CALPHAD results showed that the end point of primary solidification changed synchronistically with the start point, solute content solidified in dendritic arm and partition coefficient increased with increasing of solid fraction. Simple Scheil equation was shown to be able to calculate the eutectic mount on the base of partition coefficient calculated by CALPHAD. Its prediction form the different solute was consistent and agreeable with the CALPHAD method. The eutectic amount and secondary dendritic arm spacing of the Al-Cu-Zn alloy were measured in experimental sample and compared to prediction value. It was shown that experimental data and model prediction result are in agreement.The liquidus surface and solute partition coefficient in Al-rich corner of the Al-Cu-Zn alloy were determined by Differential Thermal Analysis and quenching during LMC unidirectional solidification. A numerical model for the calculation of solidification path was developed based on the plate-like dendrite geometry. The equilibrium solute concentration and liquidus temperature of the Al-1.5Cu-3Zn alloy were calculated. The solidification path ended in the (Al) phase domain in the case of equilibrium solidification while it ended in the liquidus valley in the case of the scheil non-equilibrium solidification, and the amount of the primary phase was 97.4% and that of eutectic was 2.6%. The effect of incomplete mixing of the rejected solute in the liquid on the solidification path was also discussed.Solute redistribution controlled by solute partition and solute diffusion during dendritic solidification of the solid solution alloy was described by solute diffusion equation and species conserve equation. The Crank Nicholson finite difference scheme and the trapezoid law were used to solve these equations. A new iteration equation was derived to calculate the solute concentration at the solid and liquid interface. The solute concentration field during solidification was calculated on TDMA-based rapid algorithm. The solute redistribution during solidification of Al-4.5Cu and Al-1.5Cu-3Zn alloys was analyzed by the algorithm and compared with the results of analytical formula, and it was proved that the numerical model was efficient and consistent.
Keywords/Search Tags:Solidification, Alloy, metallic solution model, associated melt, Al-Cu-Zn alloy, thermodynamic model, solute partition ratio, solute trapping, interface kinetics, microsegregation, CALPHAD, solidification path
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