Zhao Jiuzhou

EFFECT OF FORCED FLOW ON THREE DIMENSIONAL DENDRITIC GROWTH OF Al-Cu ALLOYS

The dendrite morphology is determined by the interaction between the capillarity effect and the transports of heat and solute,and is significantly altered by the presence of fluid flow during solidification.A lot of numerical models have been developed to investigate the effect of fluid flow on the dendritic growth of pure materials.But up to date,only a few researches were carried out on the effect of fluid flow on the dendritic growth of alloys.The effect of fluid flow on three dimensional (3D) dendrite tip selection parameter of alloys remains an unsolved scientific problem.A 3D cellular automaton(CA) model for dendritic growth of alloys was developed in this paper.3D CA is solved in coupling with a momentum transport model in order to predict the evolution of dendritic morphology during solidification of alloys in the presence of flow.The dendrite growth with a forced flow in an undercooled melt of an Al-4%Cu(mass fraction) alloy was simulated.The effect of forced flow on dendritic growth was investigated.The results show that a forced flow affect the three dimensional dendritic growth of an alloy significantly.The growth of the primary and secondary arm in the upstream direction is much greater than that in the downstream direction.The growth direction of the primary arm perpendicular to the flow direction tilted into the upstream direction.The dendrite tip of the primary arm perpendicular to the flow direction shows an asymmetric morphology.The degree of the tilt and the asymmetry of the tip become stronger with the increase of the forced flow velocity. With the increase of the flow velocity the growth velocity of the upstream dendrite tip increases,the radius and the selection parameter of the upstream dendrite tip decrease.For a given undercooling, the effect of forced flow on the selection parameter of the upstream dendrite tip becomes stronger with the increase of the anisotropy of the interfacial energy.For a given alloy,the effect of forced flow on the selection parameter of the upstream dendrite tip also becomes stronger with the increase of undercooling.

DEVELOPMENT OF CELLULAR AUTOMATON MODELS AND SIMULATION METHODS FOR SOLIDIFICATION OF ALLOYS

Dendritic structure is the most frequently observed solidification microstructure of alloys. It has a dominant effect on the mechanical properties of alloys. The formation of the dendritic microstructure has attracted extensive attentions. It has been demonstrated that numerical simulation is a powerful tool for studying the microstructure formation during the solidification of alloys. Various models, such as the front-tracking(FT) model, the phase-field(PF) model and the cellular automaton(CA) model have been proposed to simulate the formation process of dendrite. Compared with other methods, CA is an effective numerical simulation method with high calculation efficiency and clear physical meaning. It is more suitable to be applied to simulate the formation kinetics of the dendritic microstructure of alloys. It has been widely applied in the investigation of the solidification of alloys.This paper makes a detailed introduction to the common process of CA modeling and simulation, the constructing method of CA model and the calculation method for some key parameters such as nucleation rate of nuclei, growth velocity of dendrite, etc. A review of the development of the CA models for the solidification of alloys is carried out. The features and applications of the existing CA models are critically assessed. The applications of the CA models in the investigations of the practical solidification process are summarized. The problems to be solved and the future development of CA models are also pointed out.

SOLIDIFICATION OF MONOTECTIC ALLOY UNDER LASER SURFACE TREATMENT CONDITIONS

Monotectic alloys have great potentials to be used in industry due to their special physical and mechanical properties.But these alloys have an essential drawback that just the miscibility gap in the liquid state poses problem during solidification.When a homogeneous,single-phase liquid is cooled into the miscibility gap,the components are no longer miscible and two liquid phases develop.Generally,the liquid-liquid decomposition causes the formation of the microstructure with serious phase segregation.Recent researches demonstrate that the only effective method to prevent the formation of the phase segregated microstructure in the immiscible alloys is using the rapid or sub-rapid solidification processing techniques.Laser surface treatment is a well known rapid solidification technique. It is widely applied in the industry to improve the surface properties of different type of alloys. But up to date,the solidification behaviors of the monotectic alloys under the laser surface treatment conditions have not been investigated.In this work,laser surface treatment experiments with Al-Pb alloys were carried out.A numerical model was developed to describe the microstructure evolution in the surface layer of Al-Pb alloys under the conditions of laser surface treatment.The model was applied to calculate the microstructure formation in the surface layer.The numerical results have a good agreement with the experimental ones.Both of the numerical and experimental results indicate that the microstructure of the laser treated surface layer is determined by the re-melting,composition homogenization and solidification of the alloy.Laser surface treatment can lead to the formation of an Al-Pb surface layer with well dispersed microstructure.

The effect of the interaction between the minority phase droplets on the nucleation behavior during the liquid-liquid phase transformation

The microstructure evolution during the liquid-liquid phase transformation of Al-Pb alloy was calculated. The numerical results indicate that the interaction between the minority phase droplets has effect on the nucleation process of the droplets, and the effect increases with the cooling rate and the content of Pb.

EFFECT OF Sn ON THE SOLIDIFICATION PROCESS AND MICROSTRUCTURE OF Al-Pb MONOTECTIC ALLOYS

When a homogeneous, single-phase liquid of monotectic alloy is cooled into the miscibility gap, the components are no longer miscible and two liquid phases develop. Generally, the liquid-liquid decomposition causes the formation of the microstructure with serious phase segregation. Many efforts have been made to use the demixing phenomenon for the production of well dispersed composite materials. It has been demonstrated that the rapid directional solidification technique is an effective method to prevent the formation of the phase segregated microstructure in immiscible alloys. Directional solidification experiments were carried out to study the influence of the addition of Sn on the solidification process of Al-Pb alloys. The experimental results show that the addition of a small amount of Sn causes a decrease in the interface energy between the matrix and the minority phase liquids and, thus, an increase in the nucleation rate of the minority phase droplets during the liquid-liquid phase transformation or a decrease in the average size of the minority phase particles. With the increase of the Sn content, both the volume fraction of the minority phase droplets and the temperature range of the liquid-liquid phase zone and the liquid-liquid-solid tertiary phase zone of the phase diagram increase. These are favorable for the coarsening of the minority phase droplets. The addition of Sn leads to the formation of a dendrite solid/liquid interface. This may promote the formation of a well dispersed microstructure and shows great effect on the distribution of the minority phase particles.

DIRECTIONAL SOLIDIFICATION OF MONOTECTIC COMPOSITION Al-Bi ALLOY

Monotectic systems are a kind of extensive alloys.Many of them have great potentials for practical industry application.A lot of work has been carried out to study the solidification of monotectic alloys.But most of them focused on the microstructure formation during cooling a hyper- monotectic alloy through the miscibility gap in the liquid.Little work was done on the solidification behaviors of an alloy of monotectic composition.Directional solidification experiments were carried out with Al- Bi alloy of the monotectic composition(Al-3.4%Bi,mass fraction).The influences of the solidification velocity on the microstructure were investigated.The microstructure evolution during solidification was analyzed.The results indicate that a solute-rich layer forms in front of the solidification interface and the liquid-liquid decomposition occurs there.When the alloy is solidified at such a high velocity so that the minority phase droplets of all sizes in front of the solidification interface are migrating to the solidification interface,the size distribution of the minority phase particles shows only one peak.When the alloy is solidified at a relatively low velocity,the minority phase droplets within a certain size range may move away from the solidification interface under the concurrent actions of the Marangoni migration,Stokes motion of the droplets as well as the movement of the sample.Al-3.4%Bi alloy solidified under such conditions shows a size distribution of the minority phase particles with two peaks.The average radius of the minority phase particles R depends on the solidification velocity V_0 exponentially according toR∝V_0~(-1/2).With the decrease of the solidification velocity,the dependence of the average radius of the minority phase particles varies towards to R∝ V_0~(-1/3).

STUDY OF THE SOLIDIFICATION OF Ni-Ag MONOTECTIC ALLOY

The Ni-Ag alloy has good mechanical properties,high corrosion resistance and electrical conductivity.It is an excellent candidate to be used in many high-tech fields of aerospace,energy resource and chemical engineering etc.This alloy,however,is a typical monotectic system.Generally, the liquid-liquid phase transformation leads to the formation of a solidification microstructure with serious phase segregation.The manufacturing of this alloy is thus extremely difficult.Injection casting has already been carried out with the Ni-Ag monotectic alloy.The sample with composite microstructure, in which Ag-rich particles dispersed homogeneously in Ni matrix has been obtained.A model describing the microstructure evolution during injection casting of the Ni-Ag monotectic alloy has been proposed.The process of microstructure formation has been simulated and discussed in details. The results indicate that the Ostwald coarsening of Ag-rich droplets is very weak during cooling in miscibility gap under injection casting cooling conditions.The dispersivity of the primary Ag-rich phase is controlled by the nucleation of Ag-rich droplets during the liquid-liquid transformation.The number density(TV)and average radius((R))of primary Ag-rich particles depend exponentially on the cooling rate of the alloy during the nucleation of Ag-rich droplets(T_(nuc))according to N∝T_(Nuc)~(1.8) and(R)∝T_(Nuc)~(-0.6).