A comparative study on Sn macrosegregation behavior of ternary Al-Sn-Cu alloys prepared by gravity casting and squeeze casting

A comprehensive study on Sn macrosegregation behavior in ternary Al-Sn-Cu alloys was carried out by comparative analysis between gravity casting and squeeze casting samples.The microstructure and Sn distribution of the castings were characterized by metallography,scanning electron microscopy(SEM),energy-dispersive X-ray(EDX)spectroscopy,and a direct reading spectrometer.Results show that there are obvious differences in Sn morphology between gravity casting and squeeze casting alloys.Under squeeze casting condition,the grain size of the casting is smaller and the distribution ofβ(Sn)is uniform.This effectively reduces the segregation of triangular grain boundary as well as the segregation of Sn.The segregation types of Sn in gravity casting and squeeze casting samples are obviously different.The upper surfaces of gravity casting samples show severe negative segregation,while all the lower surfaces have positive segregation.Compared with gravity casting,squeeze casting solidifies under isostatic pressure.Due to the direct contact between the upper surface of the casting and the mold,the casting solidifies faster under higher undercooling degree and pressure.Consequently,the uniform distribution of Sn reduces the segregation phenomenon on the surface of the casting.

Effects of macrosegregation on mechanical and tribological properties of squeeze casting immiscible bearing alloys

The macrosegregation behaviors of Al-Sn-Cu ternary immiscible alloy castings and their effects on mechanical and tribological properties were investigated.The results demonstrate that Sn and Cu segregate in the casting simultaneously,and the mass fraction of the two elements has a"U"shaped distribution.Significantly,positive and negative segregation occur in the casting,with positive segregation appearing on the top and lower surfaces and negative segregation on the remaining surfaces,with the 1/2 surface(hot node location)having the highest degree of negative segregation.Furthermore,the results of Vickers hardness,tensile strength,and elongation show that Sn and Cu cooperatively affect the mechanical properties of castings.The higher the mass fraction of Sn and Cu elements,the higher the hardness,the greater the tensile strength,and the better the elongation.The findings of the step-by-step loading tests demonstrate that the segregation of Sn and Cu significantly impacts the tribological characteristics of the castings.The higher the mass fraction of Sn and Cu on the sample surface,the better the tribological characteristics.

MICROSTRUCTURE AND MACROSEGREGATION OF Al-7%Si ALLOY SOLIDIFIED UNDER COMPLEX EFFECTS OF ELECTROMAGNETIC AND CENTRIFUGAL FORCES

Macrosegregations and microstructures of Al-7%Si alloy solidified under complex of fects of magnetic field and centrifugal forces are studied by means of a set of selfdesigned electromagnetic centrifugal casting (EMCC) device. It is shown that electromagnetic field (EMF) has an important effect on the macrosegregation of centrifugal casting specimen of Al-7%Si alloy in two respects: one is that there exists always a kind of convection in the liquid in front of the S/L interface caused by effect ofthe electromagnetic force; the other is that different atomic clusters of solidparticles with different physical characteristics are subjected to quite different electromagnetic (Lorentz) force. Therefore, their movements get changed. In addition, the formation process of a complex band structure consisting of primary α-Al dendrites and (α-Al+β-Si) eutectics in hypoeutectic Al-Si alloys during EMCC and the effect of EMF are discussed.

Structures and macrosegregation of a 2024 aluminum alloy fabricated by direct chill casting with double cooling field

Central region coarse grains and centerline segregation are common defects in aluminum ingots fabricated by direct chill(DC)casting.A double cooling field was introduced into the DC casting process to reduce these defects,whereby the external cooling was supplied by the mold and water jets,and intercooling was achieved by inserting a rod of the same alloy into the molten pool along the central axis of the ingot.Rather than forming a good metallurgical interface during solid-liquid compound casting,in the present work,the purpose of inserting the rod is to enforce internal cooling and consequently decrease the sump depth.Moreover,the insertion provides more nucleation sites with the unmoltenα-Al particles.The structure and the macrosegregation of 2024 Al alloy ingots prepared by DC casting with and without the inserts were investigated.Results show that when the inserting position is 50 mm above the upper edge of the graphite ring,significant grain refinement in the central region of the ingot and a reduced centerline segregation are achieved.

Grain refinement and macrosegregation behavior of direct chill cast Al-Zn-Mg-Cu alloy under combined electromagnetic fields

The combined electromagnetic fields were achieved by the application of an alternating magnetic field and a stationary magnetic field and were used during direct chill(DC) casting process to control the microstructure and macrosegregation of an Al-Zn-Mg-Cu alloy. Ingot microstructures were analyzed under an optical microscope(Leica DMR). The composition at different locations in the ingots was measured with inductively coupled plasma mass spectrometry(ICP) method. The results showed that the grain structure is transformed from dendrite to equiaxed structure and significantly refined with the application of combined electromagnetic fields. The uniformity of microstructure is also greatly improved. The combined electromagnetic fields show a significant effect on the distribution of elements. The negative macrosegregation in the centre area of the ingot is obviously reduced.

Simulation of macrosegregation in a 36-t steel ingot using a multiphase model

Macrosegregation is the major defect in large steel ingots caused by solute partitioning and melt convection during casting.In this study,a three-phase(liquid,columnar dendrites,and equiaxed grains)model is proposed to simulate macrosegregation in a 36-t steel ingot.A supplementary set of conservation equations are employed in the model such that two types of equiaxed grains,either settling or adhering to the solid shell,are well simulated.The predicted concentration agrees quantitatively with the experimental value.A negative segregation cone was located at the bottom owing to the grain settlement and solute-enriched melt leaving from the mushy zone.The interdendritic liquid flow was carefully analyzed,and the formation of A-type segregations in the mid-height of the ingot is discussed.Negative segregation was observed near the riser neck due to the specific relationship between flow direction and temperature gradient.Additionally,the as-cast macrostructure of the ingot is presented,including the grain size distribution and columnar–equiaxed transition.

Macrosegregation Quality Criteria and Mechanical Soft Reduction for Central Quality Problems in Continuous Casting of Steel

In order to study the central quality of continuously cast tool steel slabs, the simple model has been developed to simulate the macrosegregation quality criteria. The model calculates different quality criteria such as average macro-segregation level criterion “ASL”, its fluctuation level “FSL” and its segregation quality number “SQN”. These criteria are calculated based on the previous measurements of carbon and sulfur concentrations distributions in final region of spray zones and centerline area of lower and upper slab sides. The effect of mechanical soft reduction Technique “MSR” on the slab centerline quality is examined and analyzed. The model results show that MSR affects the quality of centerline areas significantly by different ways based on the casting speed. The experimental and theoretical results clarify that the qualities of different slab sides are different for all collected samples. The model results show also that the accuracy of the macro-segregation quality criteria increases quantitatively with increasing the number of analyzed segregated elements. Therefore, the macrosegregation quality criteria and their distributions can be considered as the most simple and vital tool to evaluate the various slab qualities. Finally, the mechanism of centerline segregation formation with mechanical soft reduction is discussed in this study.

NUMERICAL SIMULATION OF HEAT,MASS AND MOMENTUM TRANSFERS IN METALLIC INGOTS AND PREDICTION OF MACROSEGREGATION

A continuum model and numerical methods were established for description of heat,mass andmomentum transfers as well as macrosegregation formations in metallic ingots.Numericalsimulation of temperature,composition and liquid flow fields during the solidification of anAl-4.5% Cu ingot were performed on an IBM personal computer.The model and numericalmethods were verified through two experiments.

Finite element modeling of macrosegregation coupled with shrinkage cavity in steel ingots using arbitrary Lagrangian-Eulerian model

Shrinkage cavity has significant influence on macrosegregation in steel ingots. An arbitrary Lagrangian-Eulerian (ALE) model based on volume averaging method is developed to predict the coupled formation progress of macrosegregation and shrinkage cavity during solidification of steel ingots. The combined effect of thermal-solutal convection and solidification shrinkage on macrosegregation is considered in the model. A specially designed mesh update algorithm is proposed to consider the formation of shrinkage cavity. The streamline-upwind/Petrov–Galerkin (SUPG) stabilized finite element algorithm is adopted to solve the conservation equations. Two solution methods for the energy conservation equation are proposed, i.e. the temperature-based solver and enthalpy-based solver. A Pb-48wt.%Sn solidification benchmark is used for validation. Then, the ALE model is applied to a Fe-3.6wt.%C industrial steel ingot. The formation progress of macrosegregation coupled with shrinkage cavity is predicted. By comparison with the predictions of the finite element model and finite volume model, the effect of shrinkage cavity formation on macrosegregation is investigated. Results show that the formation of shrinkage cavity can significantly change the segregation region and segregation degree at the hot top. It is demonstrated that the ALE model can predict the coupled formation of macrosegregation and shrinkage cavity in steel ingots.

INFLUENCE OF MOULD ROTATION ON MACROSEGREGATION IN INGOT

The macrosegregation formation in an ingot and the influence of rotating mould upon it wereanalysed on the basis of interdendritic flow.When the ingot mould rotates unidirectionally,thecentrifugal volume force acts on all inner parts,and when the ingot mould rotates alternately,the tangential alternate force is also formed,and both have various influences on the differentmodes of interdendritic flow.It is shown that as the ingot mould rotates unidirectionally oralternately with a moderate angular speed,the interdendritic unstable flow may be controlledeffectively and the formation of segregation may be restrained.Thus,it seems to be significantfor reducing the segregation such as A-type etc.,in the steel ingot.

Effects of vertical electromagnetic stirring on grain refinement and macrosegregation control of bearing steel billet in continuous casting

The grain refinement and macrosegregation control of GCr15 bearing steel were investigated under a type of rarelyused electromagnetic stirring,vertical electromagnetic stirring( V-EMS),in continuous casting. V-EMS can create an upward electromagnetic force and generate longitudinal loop convection,which enables the better mixing of the upper part with the lower part of the liquid steel. The results showed that applying V-EMS can enlarge the region of the equiaxed grain,decrease the secondary dendrite arm spacing( SDAS) and reduce the segregation of both carbon and sulfur. After applying V-EMS,liquid steel with a high solute concentration is brought to the dendrite tips,making the dendrite arms partially melt. The length of the dendrite fragment is approximately 1. 8 mm,10 to 12 times the SDAS. Upon increasing the amount of cooling water from 2. 0 to 3. 5 m^3/h,the dendrite fragments exhibit an obvious aggregation following V-EMS. Finally,a criterion for dendrite fragmentation under VEMS was derived based on the dendrite fragmentation theory of Campanella et al.

nfluences of superheat and cooling intensity on macrostructure and macrosegregation of duplex stainless steel studied by thermal simulation

The influences of superheat and cooling intensity on macrostructure and macrosegregation of one new kind duplex stainless steel(DSS)were studied.Thermal simulation equipment was applied to prepare samples,which could reproduce the industrial processes of DSS manufactured by a vertical continuous slab caster.Macrostructure and macrosegregation were analyzed using the digital single lens reflex and laser-induced breakdown spectroscope(LIBSOPA-200),respectively.The percentage of both chill zone and center equiaxed zone increases with the superheat decreasing,while that of the columnar zone decreases.There is only equiaxed grain existing as the superheat is 10 and 20℃.The lower the superheat is,the coarser the gain size is.High cooling intensity in mold could remarkably decrease the chill zone length and refine the grains in chill zone and center equiaxed zone.The influences of cooling intensity on macrosegregation are greater than those of superheat.The macrosegregation of Si,Mn and Cr is slightly dependent on superheat,while that of Cu,Mo and Ni changes greatly with superheat increasing.

Numerical simulation of macrosegregation in continuously cast gear steel 20CrMnTi with final electromagnetic stirring

A 3D/2D hybrid multi-physical-field mathematical model,which takes into consideration the thermosolutal buoyance,was developed to predict the macrosegregation of gear steel 20CrMnTi continuously cast by a curved billet caster with size of 160 mm×160 mm,and investigated the effect of final electromagnetic stirring(F-EMS)on the fluid flow,heat transfer and solute distribution in the liquid core of continuously cast steel.The results show that the application of F-EMS eliminates the effect of thermosolutal buoyancy on the asymmetric distribution of carbon concentration in the cross section of billet and accelerates the final solidification of resident molten steel in the liquid core of strand,but promotes the negative carbon segregation near the billet center.When the gear steel 20CrMnTi is cast at the temperature of 1803 K and speed of 1.7 m/min,the solidification end advances forward from 9.84 to 9.72 m,and center carbon segregation ratio of billet decreases from 1.24 to 1.17 with the increase in current density of F-EMS from 0 to 350 A.

Numerical simulation of macrosegregation in billet continuous casting influenced by electromagnetic stirring

A three-dimensional numerical model coupling the macrosegregation and magnetohydrodynamic simulations was developed to investigate the effects of electromagnetic stirring(EMS)on the macrosegregation.The results show that a significant swirling flow was induced by the in-mold EMS,which further changed the shape of the solidification shell and homogenized the solute elements in mold.However,the effects were only confined to the initial billet shell.The improvement in centerline segregation was observed with the usage of the final EMS(F-EMS),which led to the forced convection at the final solidification stage.The solute elements in the mushy zone were significantly even,with the maximum segregation degree of solute C reducing from 1.311 to 1.237.In addition,the effects of the stirrer positions and currents of F-EMS on the macrosegregation alleviation were numerically studied.Different values of centerline segregation were predicted with various stirrer positions and currents,and there is an optimum stirrer parameter to obtain the best macrosegregation alleviation.In the experimental conditions,the optimum position was about 7 m away from meniscus,and the optimum current was 300 A.

A review on prediction of casting defects in steel ingots:from macrosegregation to multi-defect model

Due to the nature of the solute redistribution,the reduction in the solidification rate with time in a square root relationship,and the multiphase melt flow during the solidification,casting defects such as macrosegregation,shrinkage cavity,and porosity will inevitably occur in the steel ingot and intensify with the increase in ingot size.These defects directly affect the performance of the final product and severely restrict the choice of subsequent thermal processing methods and process windows.Therefore,the solidification defects including macrosegregation,shrinkage/porosity,and inclusions encountered in the preparation of large steel ingots and their formation mechanisms were reviewed.The development progress and the latest development of the macrosegregation model for steel ingots were introduced in detail,especially the latest progress in the coupling prediction of macrosegregation and shrinkage as well as macrosegregation and inclusions.Some methods to reduce macrosegregation of ingots were discussed as well.Finally,a new casting method called layered casting was introduced in detail.This method can effectively improve the uniformity of the macrostructure and reduce the macrosegregation of the large ingots and therefore is a promising method for preparing large ingots with high homogeneity.

Numerical Simulation of Macrosegregation During Steel Ingot Solidification Using Continuum Model

A continuum model is adopted to study the macrosegregation phenomena during solidification of large steel ingots.Evolution of temperature,melt velocity,and compositional concentration field during a 22 t steel ingot solidification are illustrated by using the finite volume method.Numerical results of temperature distribution are validated by experiments.The influence of local permeability relates to the friction that the melt experienced in mushy region is investigated.It is shown that the continuum model is able to predict the temperature field,and the variation of permeability obviously affects the melt flowing behavior and the final compositional distribution.

Numerical Simulation of Macrosegregation During the Solidification of Alloys under Magnetic Fields

In this paper,a mathematical model considering the electric potential is established for describing the transfer behavior of multicomponent alloy solidification process under steady magnetic fields.By using finite volume method,the corresponding simulation program is self-developed.A validation numerical example is calculated to verify the correctness of the model and solution method,and the model also is applied to analyze steel alloys solidification transportation mechanism and the influence of magnetic parameter.The numerical results indicate that the suitable magnetic field intensity is beneficial to reduce the type and degree of macrosegregation.

Influence of Electromagnetic Stirring on the Macrosegregation in Arc Remelted Ti and Zr Alloy Ingots

A numerical model of the VAR process has been developed in close collaboration with titanium and zirconium producers.The model is based on the solution of the coupled heat,mass and momentum transport equations in the whole ingot(liquid pool,mushy zone and solidified part)using a finite volume method.It accounts for electromagnetic stirring,buoyancy flows and turbulence effects.Solidification mechanisms implemented in the model include a full coupling between energy and solute transport in the mushy zone.The numerical model has been applied to simulate the remelting of Ti6-4.The influence of the applied magnetic field on the fluid flow and segregation behaviour,for a number of different stirring practices,is presented and discussed.Also,the macrosegregation in Zy4 ingots is investigated.The comparison between the predicted segregation and the experimental results shows the importance of accounting for both the stirring and thermosolutal convection to forecast properly the segregation in remelted ingots.

Numerical Simulation of Macrosegregation Caused by Thermal–Solutal Convection and Solidification Shrinkage Using ALE Model

Solidifi cation shrinkage has been recognized as an important factor for macrosegregation formation. An arbitrary Lagrangian–Eulerian(ALE) model is constructed to predict the macrosegregation caused by thermal–solutal convection and solidi-fi cation shrinkage. A novel mesh update algorithm is developed to account for the domain change induced by solidifi cation shrinkage. The velocity–pressure coupling between the non-homogenous mass conservation equation and momentum equation is addressed by a modifi ed pressure correction method. The governing equations are solved by the streamline-upwind/Petrov–Galerkin-stabilized fi nite element algorithm. The application of the model to the Pb-19.2 wt%Sn alloy solidifi cation problem is considered. The inverse segregation is successfully predicted, and reasonable agreement with the literature results is obtained. Thus, the ALE model is established to be a highly effective tool for predicting the macrosegregation caused by solidifi cation shrinkage and thermal–solutal convection. Finally, the effect of solidifi cation shrinkage is analyzed. The results demonstrate that solidifi cation shrinkage delays the advance of the solidifi cation front and intensifi es the segregation.

Application of heat absorption method to reduce macrosegregation during solidification of bearing steel ingot

The control of macrosegregation is still a difficult problem for the production of large steel ingots.In order to develop new techniques for producing low-macrosegregation and high-quality steel ingots,the effect of the heat absorption method(HAM)used by the inorganic material rods to cool the liquid steel on the formation of macrosegregation during solidification of a 5-t GCr15SiMn bearing steel ingot was studied using experiment and mathematical simulation.The inorganic material was a mixture of CaF2 and CaO.The levels of macrosegregation in the longitudinal sections of two ingots with and without HAM were compared.Experimental results showed that the application of HAM reduced the positive segregation in the upper part of the ingot and the negative segregation in the lower part.The levels of carbon segregation along the longitudinal centerline and horizontal direction at different heights were all alleviated and the fluctuation of carbon segregation was significantly reduced.The simulation results confirmed that the melting and floating of the inorganic material could carry the sensible heat to the top of the liquid steel quickly.This leads to the acceleration of the cooling rate of the liquid steel,thereby alleviating the level of macrosegregation.