Effect of cooling rate on the phase transformation and stability of the mushy zone during the solidification of Waspaloy

Scanning electron microscopy（SEM） and energy dispersive X-ray analysis（EDAX） were used to study the microstructure,microsegregation, and fluid flow tendency of the superalloy Waspaloy in the mushy zone,which had been solidified at different cooling rates. The investigation was accompanied with the calculation of Rayleigh numbers.It is found that Ti is the main segregating element and the content of Ti is the highest in the final liquid at the cooling rates of 3-6℃/min.The eta phase（η） precipitate presented in the residual liquid at the cooling rates higher than 6℃/min is responsible for the fluctuations in the curves of Ti content.The dendrite arm spacing is found to markedly decrease with the increase of cooling rate.The maximum relative Rayleigh number occurs at 10-20℃below the liquidus temperature at a cooling rate of 1℃/min,where the mushy zone is most unstable and fluid flow is most prone to occur.

Radial integral boundary element method for simulating phase change problem with mushy zone

A radial integral boundary element method(BEM)is used to simulate the phase change problem with a mushy zone in this paper.Three phases,including the solid phase,the liquid phase,and the mushy zone,are considered in the phase change problem.First,according to the continuity conditions of temperature and its gradient on the liquid-mushy interface,the mushy zone and the liquid phase in the simulation can be considered as a whole part,namely,the non-solid phase,and the change of latent heat is approximated by heat source which is dependent on temperature.Then,the precise integration BEM is used to obtain the differential equations in the solid phase zone and the non-solid phase zone,respectively.Moreover,an iterative predictor-corrector precise integration method(PIM)is needed to solve the differential equations and obtain the temperature field and the heat flux on the boundary.According to an energy balance equation and the velocity of the interface between the solid phase and the mushy zone,the front-tracking method is used to track the move of the interface.The interface between the liquid phase and the mushy zone is obtained by interpolation of the temperature field.Finally,four numerical examples are provided to assess the performance of the proposed numerical method.

Microstructure evolution and phase transformation of the mushy zone in a quenchedβ-solidifying TiAl alloy

This study investigates the phase constitutions and transformations that occur in the mushy zone and in the adjacent phase fields of a directionally solidified Ti-44Al-8Nb-1Cr alloy via quenching technique.The results indicate that the mushy zone consists of unmeltedβdendrites and interdendritic liquid,whose formation can be attributed to the difference in melting point aroused by local heterogeneity in solutecontent.Theβdendrite is composed of numerous subgrains with various orientations.During quenching,theβdendrite transforms into Widmanstättenαvia a precipitation reaction,owing to the decreasing cooling rate caused by heat transfer from the surrounding liquid.Additionally,after quenching,the interdendritic liquid is transformed intoγplates.Within the singleβphase field and the lower part of the mushy zone,a massive transformation ofβtoγoccurs.Conversely,in theβ+αphase field,bothβandαphases are retained to ambient temperature.During the heating process,the transformation ofα→βgives rise to the formation ofβvariants,which affects the orientation ofβdendrites in the mushy zone.The growth kinematics of theα→βtransformation was elucidated,revealing the preferential growth directions of111and112forβvariants.Furthermore,this study presents an illustration of the formation process of the mushy zone and the microstructural evolution during the heating and quenching process.

Effect of phosphorus on the microstructure and mechanical properties of strip cast carbon steel

Carbon steel strips with different phosphorus and carbon contents were produced by using the twin roll strip casting process. Fine grains and dendrite structure were observed in high-P steels. Negative phosphorus segregation was found in strip cast high-P and high-C steels. For the steels with different carbon contents, phosphorus distribution in the thickness direction of the strip is obviously different. This is because solutes are redistributed in the melting pool and the phosphorus segregation rate is associated with the length of the mushy zone and the phosphorus solubility in different phases. Phosphorus as a solute in ferrite can lead to the higher hardness and strength with lower plastic propertyies.

A MATHEMATICAL HEAT TRANSFER MODEL INSTATIC AND CONTINUOUS CASTING

The application of a heat flow model to describe the thermal characteristics of freezing alloys with narrow mushy zones from a refrigerated mould wall was outlined. The extension of the model was to treat the continuous casting of metals with low thermal conductivity, such as steels, which will be outlined. The model was based on the mathematical expedient for replacing thermal resistance of the metal/mould interface by virtual adjuncts of metal/mould material. It provided a good description of the pool profile and the technique exhibits advantages in terms of both computation and versatility of application.

Solidification in Soft-Contact Continuous Casting Mold with Alternating Electromagnetic Field

The mechanical behavior of the mushy zone was studied in detail. Based on the analysis of conventional high temperature mechanical model, considering the physical phenomenon in casting process, a mechanical model suitable for describing the mechanical behavior of the casting material in twin roll casting process was built. Comparing experimental results to the numerical simulated ones, the mechanical model was proved to be correct.

NUMERICAL SIMULATION OF CHANNEL SEGREGATION IN Al-4.5wt-%Cu ALLOY

Equations for numerical simulation of channel segregation based on a model of continuum ap- proach to porous medium were advanced.In order to solve the non-linear computation prob- lem caused by the interactions between unknown variables in the equations,the trial-and-er- ror method is used.The computation results showed that the radial back-flow due to natural convection in mushy zone indeed exists in A1-4.5wt-%Cu alloy ingot under the condition of low cooling rate,and that the numerical simulation of channel segregation is applicable due to the coincidence of calculated concentration with the experimental values.

MODELING STUDY OF CONVECTION AND CONSTITUTION VARIATION IN INGOT DURING SOLIDIFICATION

Convection and constitution variation of liquid metal during solidification has been studied us- ing NH_4Cl aqueous solution.The liquid in mushy zone attached to the side wall of the mold will flow upward and form a low concentration region,in which no significant convection oc- curs,on the top of liquid zone.This region develops downward gradually during solidification. Addition of adequate component may suppress the formation of this region.

Numerical Simulation of Multicomponent Alloy Solidification

A non-equilibrium model of multicomponent melt solidification has been developed in which a Stefan problem with two boundaries is solved numerically, the boundaries being between the solid phase and the two-phase transition zone and between the two-phase transition zone and the liquid phase. The two-phase zone is represented as a porous medium with variable porosity. The additional force resisting the melt flow due to porosity and introduced by analogy with Darcy＇s law is taken into account. Computer simulation has been performed of the experiment on Sn-20 wt.%Pb binary alloy solidification by the method of downward-directed crystallization along the gravity vector. The paper shows the results of a quasi two-dimensional benchmark experiment on horizontal （i.e., at the right angle to the gravity vector） directional solidification of a binary Sn-3 wt.%Pb alloy. The calculations were done using two crystallization models： the equilibrium model and the non-equilibrium one. It is shown that the non-equilibrium model gives a better description of the thermal field evolution and solute distribution caused by natural convection.

Mathematical Model of Fluid Flow and Solidification in Mold Region of Continuous Slab Casting

To simulate the phenomena in the mold region of continuous casting by coupling fluid flow and solidification, a three-dimensional mathematical model has been developed based on the K-ε turbulence equations and the SIMPLER algorithm. A pseudo source term was introduced into the energy equation to account for the latent heat and kinetic energy. The fluid flow in the mushy zone was calculated by defining the fluid viscosity as a function of the solid fraction in the mushy zone. Fine meshes in the solid region improve convergence and reduce iteration time. Comparison of the fluid flow and temperature distribution with and without solidification shows that although the solid shell in the mold is thin, it still greatly affects the flow pattern. The numerical results obtained provide details of the fluid flow and solidification phenomena which can be used to optimize the nozzle structure and other process parameters in continuous casting.

Optimizing the microstructures and mechanical properties of Al-Cu-based alloys with large solidification intervals by coupling travelling magnetic fields with sequential solidification

Alloys with large solidification intervals are prone to issues from the disordered growth and defect formation;accordingly, finding ways to effectively optimize the microstructure, further to improve the mechanical properties is of great importance. To this end, we couple travelling magnetic fields with sequential solidification to continuously regulate the mushy zones of Al-Cu-based alloys with large solidification intervals. Moreover, we combine experiments with simulations to comprehensively analyze the mechanisms on the optimization of microstructure and properties. Our results indicate that only downward travelling magnetic fields coupled with sequential solidification can obtain the refined and uniform microstructure, and promote the growth of matrix phase -Al along the direction of temperature gradient.Additionally, the secondary dendrites and precipitates are reduced, while the solute partition coefficient and solute solid-solubility are raised. Ultimately, downward travelling magnetic fields can increase the ultimate tensile strength, yield strength, elongation and hardness from 196.2 MPa, 101.2 MPa, 14.5 % and85.1 kg mm-2 without travelling magnetic fields to 224.1 MPa, 114.5 MPa, 17.1 % and 102.1 kg mm-2,and improve the ductility of alloys. However, upward travelling magnetic fields have the adverse effects on microstructural evolution, and lead to a reduction in the performance and ductility. Our findings demonstrate that long-range directional circular flows generated by travelling magnetic fields directionally alter the transformation and redistribution of solutes and temperature, which finally influences the solidification behavior and performance. Overall, our research present not only an innovative method to optimize the microstructures and mechanical properties for alloys with large solidification intervals,but also a detailed mechanism of travelling magnetic fields on this optimization during the sequential solidification.

Modeling and Measurements of Heat Transfer Phenomena in Two-Phase PbSn Alloy Solidification in an External Magnetic Field

The main aim of this work is to study numerically the influence of an external magnetic field on the solidification processes of two-component materials. Based on the continuum model of two-phase flow a mathematical model for the directional solidification of a binary alloy in a magnetic field is presented. The model includes mass, momentum, energy and species mass conservation equations written in compressible form and additional relationships describing the temperature-solute coupling. The geometry under study is a cylindrical mold with adiabatic walls and cooled bottom. The macroscale transport in the solidification of alloys is governed by the progress of the two-phase mushy zone, which is treated by means of a porous medium approach. The volume fraction of liquid and solid phases, respectively, is calculated from a 2D approximation of the phase diagram. The results of calculation are compared with experimental data.