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...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.展开更多
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 b...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.展开更多
The bonding of steel and mushy Al 28Pb alloy was studied. The relationship model about preheat temperature of steel plate, solid fraction of mushy Al 28Pb alloy, rolling speed and interfacial shear strength of the bon...The bonding of steel and mushy Al 28Pb alloy was studied. The relationship model about preheat temperature of steel plate, solid fraction of mushy Al 28Pb alloy, rolling speed and interfacial shear strength of the bonding plate was established by artificial neural network perfectly. This model can be optimized by a genetic algorithm, and the optimum bonding parameters for the largest interfacial shear strength are: 546 ℃ for preheat temperature of steel plate, 43.5% for solid fraction of mushy Al 28Pb alloy and 8.6 mm/s for rolling speed, and the corresponding largest interfacial shear strength of bonding plate is 70.3 MPa. [展开更多
Liquid permeability of the mushy zone is important for porosity formation during the solidification process. In order to investigate the permeability of the mushy zone, an integrated model was developed by combining t...Liquid permeability of the mushy zone is important for porosity formation during the solidification process. In order to investigate the permeability of the mushy zone, an integrated model was developed by combining the phase field model and computational fluid dynamics (CFD) model. The three-dimensional multigrain dendrite morphology was obtained by using the phase field model. Subsequently, the computer-aided design (CAD) geometry and mesh were generated based on calculated dendrite morphologies. Finally, the permeability of the dendritic mushy zone was obtained by solving the Navier-Stokes and continuity equations in ANSYS Fluent software. As an example, the dendritic mushy zone permeability of Al-4.5wt%Cu alloy and its relationship with the solid fractions were studied in detail. The predicted permeability data can be input to the solidification model on a greater length scale for macro segregation and porosity simulations.展开更多
The density of Ni-Cr alloy in the mushy state has been measured using the modified sessile drop method. The density of Ni-Cr alloy in the mushy state was found to decrease with increasing temperature and Cr concentrat...The density of Ni-Cr alloy in the mushy state has been measured using the modified sessile drop method. The density of Ni-Cr alloy in the mushy state was found to decrease with increasing temperature and Cr concentration in alloy. The molar volume of Ni-Cr alloy in the mushy state therefore increases with increasing the Cr concentration in alloy. The ratio of the difference of density divided by the temperature difference between liquidus and solidus temperatures decreases with increasing Cr concentration. The density of the alloy increased with the precipitation of a solid phase in alloy during the solidification process. The temperature dependence of the density of alloy in the mushy state was not linear but biquadratic.展开更多
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 t...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.展开更多
The influence of solid fraction of Al 28Pb alloy mushy on distribution of Pb in Al Pb ingot was studied. The special electromagnetic stirring apparatus was used to prepare Al 28Pb alloy mushy and the relationship betw...The influence of solid fraction of Al 28Pb alloy mushy on distribution of Pb in Al Pb ingot was studied. The special electromagnetic stirring apparatus was used to prepare Al 28Pb alloy mushy and the relationship between solid fraction and stirring temperature was gotten. The results show that when solid fraction of Al 28Pb alloy mushy is less than 45%, Pb precipitation usually happens in the ingot. However, Pb precipitation in casting reduces gradually with increasing solid fraction of mushy, and when solid fraction is larger than 45%, Pb precipitation in casting can be removed. [展开更多
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 pr...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.展开更多
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 segr...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.展开更多
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 continuo...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.展开更多
A comprehensive 3D turbulent CFD study has been carried out to simulate a Low-Head(LH)vertical Direct Chill(DC)rolling ingot caster for the common magnesium alloy AZ31.The model used in this study takes into account t...A comprehensive 3D turbulent CFD study has been carried out to simulate a Low-Head(LH)vertical Direct Chill(DC)rolling ingot caster for the common magnesium alloy AZ31.The model used in this study takes into account the coupled laminar/turbulent melt flow and solidification aspects of the process and is based on the control-volume finite-difference approach.Following the aluminum/magnesium DC casting industrial practices,the LH mold is taken as 30 mm with a hot top of 60 mm.The previously verified in-house code has been modified to model the present casting process.Important quantitative results are obtained for four casting speeds,for three inlet melt pouring temperatures(superheats)and for three metal-mold contact heat transfer coefficients for the steady state operational phase of the caster.The variable cooling water temperatures reported by the industry are considered for the primary and secondary cooling zones during the simulations.Specifically,the temperature and velocity fields,sump depth and sump profiles,mushy region thickness,solid shell thickness at the exit of the mold and axial temperature profiles at the center and at three strategic locations at the surface of the slab are presented and discussed.展开更多
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 suit...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.展开更多
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 betwe...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.展开更多
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 l...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.展开更多
This paper describes a comprehensive model for predicting the evolution of the velocity and temperature fields in electromagnetically-driven flows during solidification.The electromagnetic field was formulated using t...This paper describes a comprehensive model for predicting the evolution of the velocity and temperature fields in electromagnetically-driven flows during solidification.The electromagnetic field was formulated using the mutual inductance method,which accounts for the metal,chill blocks,and magnetic shields.The model solves the heat transfer equation throughout the system,as well as the fluid flow equations in the liquid and mushy regions.A two-zone model for the mushy region that accounts for dampening of momentum by the turbulent field has been developed.The turbulence in the molten region was determined using the k-s model.Calculations were performed for unidirectional solidification in a bottom chill mold placed in a stationary magnetic field.Computed results show that the flow field at the beginning of solidification shows typical four recirculating loops,and later evolves to two recirculating loops as solidification progresses.The magnitude of the velocity in the bulk liquid was found to decrease as solidification progresses.展开更多
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 tran...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.展开更多
Phase change energy storage technology has great potential for enhancing the efficient conversion and storage of energy.While triply periodic minimal surface(TPMS)structures have shown promise in improving heat transf...Phase change energy storage technology has great potential for enhancing the efficient conversion and storage of energy.While triply periodic minimal surface(TPMS)structures have shown promise in improving heat transfer,research on their application in phase change heat transfer remains limited.This paper presents numerical simulations of composite phase change materials(PCMs)featuring TPMS skeletons,specifically gyroid,diamond,primitive,and I-graph and wrapped package-graph(I-WP)utilizing the lattice Boltzmann method(LBM).A comparative analysis of the effects of four TPMS skeletons on enhancing the phase change process reveals that the PCM containing the gyroid skeleton melts the fastest,with a complete melting time of 24.1%shorter than that of the PCM containing the I-WP skeleton.The PCM containing the gyroid skeleton is further simulated to explore the effects of the Rayleigh(Ra)number,Prandtl(Pr)number,and Stefan(Ste)number on the melting characteristics.Notably,the complete melting time is reduced by 60.44%when Ra is increased to 10^(6)compared to the case with Ra at 10^(4).Increasing the Pr number accelerates the migration of the mushy zone,resulting in fast melting.Conversely,the convective heat transfer effect from the heating surface decreases as the Ste number increases.The temperature differences caused by the local thermal non-equilibrium(LTNE)effect over time are significant and complex,with peaks becoming more pronounced nearer the heating surface.This study intends to provide theoretical support for the further development of TPMS skeletons in enhancing the phase change process.展开更多
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 SI...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.展开更多
基金supported by the National Natural Science Foundation of China(No.51831001)the Funds for Creative Research Groups of China(No.51921001)+1 种基金the Beijing Natural Sci-ence Foundation(No.2222092)the National Science and Tech-nology Major Project(No.J2019-Ⅵ-0003-0116).
文摘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.
基金supported by the school fund of Nanjing University of Information Science and Technol ogy
文摘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.
文摘The bonding of steel and mushy Al 28Pb alloy was studied. The relationship model about preheat temperature of steel plate, solid fraction of mushy Al 28Pb alloy, rolling speed and interfacial shear strength of the bonding plate was established by artificial neural network perfectly. This model can be optimized by a genetic algorithm, and the optimum bonding parameters for the largest interfacial shear strength are: 546 ℃ for preheat temperature of steel plate, 43.5% for solid fraction of mushy Al 28Pb alloy and 8.6 mm/s for rolling speed, and the corresponding largest interfacial shear strength of bonding plate is 70.3 MPa. [
基金financially supported by the National Key Research and Development Program of China(No.2016YFB0700503)National Natural Science Foundation of China(No.51701013)Beijing Laboratory of Metallic Materials and Processing for Modern Transportation
文摘Liquid permeability of the mushy zone is important for porosity formation during the solidification process. In order to investigate the permeability of the mushy zone, an integrated model was developed by combining the phase field model and computational fluid dynamics (CFD) model. The three-dimensional multigrain dendrite morphology was obtained by using the phase field model. Subsequently, the computer-aided design (CAD) geometry and mesh were generated based on calculated dendrite morphologies. Finally, the permeability of the dendritic mushy zone was obtained by solving the Navier-Stokes and continuity equations in ANSYS Fluent software. As an example, the dendritic mushy zone permeability of Al-4.5wt%Cu alloy and its relationship with the solid fractions were studied in detail. The predicted permeability data can be input to the solidification model on a greater length scale for macro segregation and porosity simulations.
文摘The density of Ni-Cr alloy in the mushy state has been measured using the modified sessile drop method. The density of Ni-Cr alloy in the mushy state was found to decrease with increasing temperature and Cr concentration in alloy. The molar volume of Ni-Cr alloy in the mushy state therefore increases with increasing the Cr concentration in alloy. The ratio of the difference of density divided by the temperature difference between liquidus and solidus temperatures decreases with increasing Cr concentration. The density of the alloy increased with the precipitation of a solid phase in alloy during the solidification process. The temperature dependence of the density of alloy in the mushy state was not linear but biquadratic.
基金the National Natural Science Foundation of China(No.11672064)。
文摘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.
文摘The influence of solid fraction of Al 28Pb alloy mushy on distribution of Pb in Al Pb ingot was studied. The special electromagnetic stirring apparatus was used to prepare Al 28Pb alloy mushy and the relationship between solid fraction and stirring temperature was gotten. The results show that when solid fraction of Al 28Pb alloy mushy is less than 45%, Pb precipitation usually happens in the ingot. However, Pb precipitation in casting reduces gradually with increasing solid fraction of mushy, and when solid fraction is larger than 45%, Pb precipitation in casting can be removed. [
基金supported by the National Key Research and Development Program of China[2017YFA0403804]National Natural Science Foundation of China[51425402,51671073]。
文摘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.
基金This work is partially supported from the National Sciences and Engineering Research Council(NSERC)of Canada Discovery Grant RGPIN48158 awarded to M.Hasan of McGill University,Montreal,for which the authors are grateful.
基金supported by the Major State Basic Research Development Program of China(No.2004CB619108) the Science Foundation of the Ministry of Education of China(No.NECT-04-0278)
文摘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.
文摘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.
文摘A comprehensive 3D turbulent CFD study has been carried out to simulate a Low-Head(LH)vertical Direct Chill(DC)rolling ingot caster for the common magnesium alloy AZ31.The model used in this study takes into account the coupled laminar/turbulent melt flow and solidification aspects of the process and is based on the control-volume finite-difference approach.Following the aluminum/magnesium DC casting industrial practices,the LH mold is taken as 30 mm with a hot top of 60 mm.The previously verified in-house code has been modified to model the present casting process.Important quantitative results are obtained for four casting speeds,for three inlet melt pouring temperatures(superheats)and for three metal-mold contact heat transfer coefficients for the steady state operational phase of the caster.The variable cooling water temperatures reported by the industry are considered for the primary and secondary cooling zones during the simulations.Specifically,the temperature and velocity fields,sump depth and sump profiles,mushy region thickness,solid shell thickness at the exit of the mold and axial temperature profiles at the center and at three strategic locations at the surface of the slab are presented and discussed.
基金Item Sponsored by National Natural Science Foundation of China(59734080)National Fundamental Research and Development Plan of China(G1998061510)
文摘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.
文摘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.
文摘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.
基金Item Sponsored by National Science Foundation under grant CMMI-0856320
文摘This paper describes a comprehensive model for predicting the evolution of the velocity and temperature fields in electromagnetically-driven flows during solidification.The electromagnetic field was formulated using the mutual inductance method,which accounts for the metal,chill blocks,and magnetic shields.The model solves the heat transfer equation throughout the system,as well as the fluid flow equations in the liquid and mushy regions.A two-zone model for the mushy region that accounts for dampening of momentum by the turbulent field has been developed.The turbulence in the molten region was determined using the k-s model.Calculations were performed for unidirectional solidification in a bottom chill mold placed in a stationary magnetic field.Computed results show that the flow field at the beginning of solidification shows typical four recirculating loops,and later evolves to two recirculating loops as solidification progresses.The magnitude of the velocity in the bulk liquid was found to decrease as solidification progresses.
文摘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.
基金supported by the National Natural Science Foundation of China(Grant No.51976111).
文摘Phase change energy storage technology has great potential for enhancing the efficient conversion and storage of energy.While triply periodic minimal surface(TPMS)structures have shown promise in improving heat transfer,research on their application in phase change heat transfer remains limited.This paper presents numerical simulations of composite phase change materials(PCMs)featuring TPMS skeletons,specifically gyroid,diamond,primitive,and I-graph and wrapped package-graph(I-WP)utilizing the lattice Boltzmann method(LBM).A comparative analysis of the effects of four TPMS skeletons on enhancing the phase change process reveals that the PCM containing the gyroid skeleton melts the fastest,with a complete melting time of 24.1%shorter than that of the PCM containing the I-WP skeleton.The PCM containing the gyroid skeleton is further simulated to explore the effects of the Rayleigh(Ra)number,Prandtl(Pr)number,and Stefan(Ste)number on the melting characteristics.Notably,the complete melting time is reduced by 60.44%when Ra is increased to 10^(6)compared to the case with Ra at 10^(4).Increasing the Pr number accelerates the migration of the mushy zone,resulting in fast melting.Conversely,the convective heat transfer effect from the heating surface decreases as the Ste number increases.The temperature differences caused by the local thermal non-equilibrium(LTNE)effect over time are significant and complex,with peaks becoming more pronounced nearer the heating surface.This study intends to provide theoretical support for the further development of TPMS skeletons in enhancing the phase change process.
基金Supported by the National Natural Science Foundationof China-Bao Steel Conjunct Foundation ( No.5 0 1 74 0 31 )
文摘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.
