The influence of buoyant forces and volume fraction of particles on the particle pushing/entrapment transition during directional solidification of Al/SiC and Al/graphite composites

Directional solidification experiments in a Bridgman-type furnace were used to study particle behavior at the liquid/solid interface in aluminum metal matrix composites. Graphite or siliconcarbide particles were first dispersed in aluminum-base alloysvia a mechanically stirred vortex. Then, 100-mm-diameter and 120-mm-long samples were cast in steel dies and used for directional solidification. The processing variables controlled were the direction and velocity of solidification and the temperature gradient at the interface. The material variables monitored were the interface energy, the liquid/particle density difference, the particle/liquid thermal conductivity ratio, and the volume fraction of particles. These properties were changed by selecting combinations of particles (graphite or silicon carbide) and alloys (Al-Cu, Al-Mg, Al-Ni). A model which considers process thermodynamics, process kinetics (including the role of buoyant forces), and thermophysical properties was developed. Based on solidification direction and velocity, and on materials properties, four types of behavior were predicted. Sessile drop experiments were also used to determine some of the interface energies required in calculation with the proposed model. Experimental results compared favorably with model predictions. BRU K. DHINDAW Visiting Scholar with the Solidification Laboratory at the time this work was performed.     

Experimental Investigations and Computer Simulation of the Liquid Fraction Evolution during the Solidification of Alloys Suitable for Semi‐Solid Processing

One important parameter for the processing of materials by semi‐solid forming is the actual distribution of the solid and liquid phases in the semi‐solid range. This parameter defines the process stability for the forming step. Therefore it is necessary to obtain information about the materials behaviour in the semi‐solid state for different materials grades. This kind of information can be obtained by experimental studies in the interesting temperature range or by calculations with simulation programs using thermodynamic data validated by experiments. This work shows the results of experimental studies and thermodynamic calculations of the solidification and heat treatment behaviour of the aluminium alloy A319 and the steel X210CrW12. The experimental studies of solidification and heat treatment of these alloys were carried out using a differential thermal analysis system (DTA). The theoretical fraction of liquid content was calculated from the DTA signal by using a software module called Corrdsc. The experimental data obtained were used to validate the thermodynamic simulations of the solidification of semi‐solid alloys. The simulations of the solidification process were carried out for equilibrium conditions, with the Scheil‐Gulliver model as well as with diffusion calculations. The equilibrium and Scheil‐Gulliver calculations were performed by the program Thermo‐Calc, and the diffusion by the program DICTRA. The required thermodynamic and mobility data for multicomponent systems were taken from the data bases COST 507 light alloys, TCFE2000 Steel/Alloys and MOB2 mobility and from newly added data. The comparison of calculated phase transformations and fractions of liquid content with experimental data revealed a good agreement.     

Solidification Microstructure of Aluminum AA 6016-T4 Resistance Spot Welds Using a Short-Pulse Technique

The influence of the duration of the current pulse on the solidification microstructure of resistance spot welded (RSW) samples of aluminum alloy 6016-T4 with the short-pulse technique was investigated through experiments and numerical modeling. Microstructure was analyzed in terms of morphology and size, and primary and secondary dendrite arm spacing were measured on experimental samples. A reduction in pulse width resulted in a fine, columnar-dendritic microstructure in the outer regions of the fusion zone as well as a larger equiaxed-dendritic zone in the fusion zone center. A two-dimensional, axisymmetric finite element model of the spot welding process with new methods for calculation of the solidification parameters G (thermal gradient in the solid behind the solid–liquid interface) and R (velocity of the solid–liquid interface) was used for investigation of influence of pulse time on solidification microstructure and comparison to experimental results. Morphological trends in the solidification structure showed good agreement between experiments and simulations, and the influence of the pulse duration on the solidification parameters evolved because of changing heat transfer conditions. Simulated solidification data suggest that the solidification of aluminum during RSW falls in the regime of rapid solidification.

     

Limit of absolute stability for crystal growth into undercooled alloy melts

The limit of absolute stability for crystal growth of binary alloys into an undercooled melt is discussed in respect of a positive temperature gradient in the solid and different thermal properties of liquid and solid. It is shown that the positive temperature gradient and the better heat conductivity and diffusivity of the solid phase reduce the velocity corresponding to the limit of absolute stability. Thus although the solid grows into an undercooled melt a planar interface may be stable with a quite lower growth rate than the absolute thermal velocity.     

方坯连铸凝固传热的复合数值模拟

根据连铸机特点和铸流场特性，在拉坯方向上将整个铸流长度划分为上部计算域和下部计算域。对于铸坯温度场的数值模拟，上部计算域充分耦合钢液对流对传热的影响，采用了三维稳态流动传热耦合模型；下部计算域则将铸流场对传热的影响考虑为有效导热系数，并忽略拉坯方向上的传热效果，采用了二维非稳态有效导热系数模型。计算过程和结果表明，采用此复合模拟方法保证了数值模拟的准确性并降低了仿真程序的计算成本。     

Mathematical modeling of macrosegregation of iron carbon binary alloy: Role of double diffusive convection

During alloy solidification, macrosegregation results from long range transport of solute under the influence of convective flow and leads to nonuniform quality of a solidified material. The present study is an attempt to understand the role of double diffusive convection resulting from the solutal rejection in the evolution of macrosegregation in an iron carbon system. The solifification process of an alloy is governed by conservation of heat, mass, momentum, and species and is accompanied by the evolution of latent heat and the rejection or incorporation of solute at the solid liquid interface. Using a continuum formulation, the goverming equations were solved using the finite volume method. The numerical model was validated by simulating experiments on an ammonium chloride water system reported in the literature. The model was further used to study the role of double diffusive convection in the evolution of macrosegregation during solidification of Fe 1 wt pct C alloy in a rectangular cavity. Simulation of this transient process was carried out until complete solidification, and the results, depicting the influence of flow field on thermal and solutal field andvice versa, are shown at various stages of solidification. Under the given set of parameters, it was found that the thermal buoyancy affects the macrosegregation field globally, whereas the solutal buoyancy has a localized effect.     

Computational modelling of heat⧹mass transfer near the LIQUID–SOLID interface during rapid solidification of binary metal alloys under laser treatment

This paper presents the results of an investigation into the problem of planar solid–liquid interface stability during rapid solidification of binary metal alloys under laser treatment. A new quantitative model is proposed. This model describes the self-organized development of stable spatially-periodic vortices in the melt near the solid–liquid interface due to concentration- (or thermal) capillary effectsfn2 together with effects due to the influence of normal concentration or temperature gradients directed from the interface towards the melt. These vortices give rise to a cellular structure at the solid–liquid interface of rapidly frozen melts.A computer code was developed to solve the set of second-order linear differential equations which describe heat and mass transfer at the liquid–solid interface. This model allows calculation of the liquid phase velocity field, the second component concentration field in the melt, as well as the temperature field in the liquid and solid phases near the solid–liquid interface at a given solidification rate.     

铌硅基高温合金定向凝固铸造温度场模拟计算

以铌硅基高温合金定向凝固铸造过程为对象,通过实验测试和反求法确定了铌硅基高温合金和型壳的热物性参数,以及凝固过程各界面换热系数等边界条件;利用ProCAST软件对不同抽拉速率下铌硅基高温合金凝固过程温度场进行了模拟。结果表明,随着抽拉速率由5 mm·min?1增加到10 mm·min?1,固/液界面离液态金属锡表面的距离由12.1 mm下降到8.2 mm;平均糊状区宽度逐渐变窄,由11.5 mm减小到10.4 mm。针对抽拉速率为5 mm·min?1的实验结果表明,数值模拟结果与实际定向凝固实验获得的一次枝晶间距差异在6%以内,从一个方面验证了温度场模拟结果的正确性,相关结果可为铌硅基高温合金叶片定向凝固铸造参数的确定提供参考。      

Assessment of Heat Transfer During Solidification of Al–22% Si Alloy by Inverse Analysis and Surface Roughness Based Predictive Model

Heat flux transients were estimated during unidirectional downward solidification of Al?C22% Si alloy against copper, die steel and stainless steel chills. The chill instrumented with thermocouples was brought into contact with the liquid metal so as to avoid the effect of convection associated with the pouring of liquid metal. Heat flux transients were estimated by solving the inverse heat conduction problem. Higher thermal conductivity of chill material resulted in increased peak heat flux at the metal/chill interface. Peak heat flux decreased when 100???m thick alumina coating was applied on the chill surface. The lower thermal conductivity of alumina based coating and the presence of additional thermal resistance decreases the interfacial heat transfer. For uncoated chills, the ratio of the surface roughness (R_a) of the casting to chill decreased from 6.5 to 0.5 with decrease in the thermal conductivity of the chill material. However when coating was applied on the chill, the surface roughness ratio was nearly constant at about 0.2 for all chill materials. The measured roughness data was used in a sum surface roughness model to estimate the heat transfer coefficient. The results of the model are in reasonable agreement with experimentally determined heat-transfer coefficients for coated chills.     

Rapid melting and solidification of a surface due to a moving heat flux

Rapid melting and solidification of a semi-infinite substrate subjected to a high intensity heat flux over a circular region on its bounding surface moving with a constant velocity is considered. General expressions are developed for the coefficients in the finite difference equation governing the heat transfer in moving orthogonal curvilinear coordinate systems. These expressions are reduced to their specific forms in terms of dimensionless nodal temperature and enthalpy for a moving oblate spheroidal coordinate system. Quasisteady state conditions are assumed and the thermal properties of the substrate in the liquid and solid phase are considered constant and equal. It is also assumed that the substrate melts and solidifies at a single temperature. Temperature distributions in the molten region and the adjacent heat affected zone are computed along with the liquid-solid interface shape, its velocity and other important solidification variables. Both uniform and Gaussian heat flux distributions within the circular region are considered. The results are presented in their most general form—in terms of dimensionless numbers when possible. Specific criteria for the melting of the substrate are established. It is shown that the three variables, absorbed heat fluxq, the radius of the circular regiona and the velocity of the moving fluxU, could be combined into two independent variables. That is, the dimensionless temperature distribution in the metal pool and the solid substrate remain the same as long as the productsqa andUa orU/q are kept constant. The effect of these variables on cooling rate in the liquid and the ratio of temperature gradient to growth rate at the solid-liquid interface are discussed using an aluminum substrate as an example.     

Two-dimensional model for twin-roll continuous casting

A numerical algorithm for the two-dimensional solidification problem in the twin-roll continuous casting system is presented in this paper. Attention is focused on the elucidation of heat transfer and flow characteristics in both the liquid and the solid phases. The present mathematical model can be applied to general full Navier-Stokes and energy equations, thereby covering the wide range of twin-roll casting conditions. The boundary fixing method (BFM) is adopted to handle the moving boundary, and the resultant transformed governing equations for the solid and liquid regions are solved separately by using a usual explicit-type finite difference method. In this paper, a general numerical methodology is presented, and the quantitative relationships between the important control parameters in continuous casting of twin-roll type (such as the roll speed, the roll gap, the initial temperature of molten materials, the material properties, the solidification profile, and the endpoint of solidification) are clarified in detail. The present numerical results have been compared with experimental results obtained separately to check the validity of the proposed method.     

Thermal Analysis and Microstructure of ZA8 Alloy Solidifying Against Chills

Thermal analysis during solidification of ZA8 alloy against copper, hot die steel and stainless steel chills instrumented with thermocouples was carried out in the present work. The investigation showed that the chill material and coating had a significant effect on the cooling curve of the casting. When casting was solidified against chills, the liquidus and eutectic start temperature of the casting remained nearly the same whereas eutectoid transformation occurred at a higher temperature. Cooling rate curve of the casting solidified against coated chill indicated that formation of solid shell and subsequent re-melting in the case of high thermal conductivity coated chill whereas in lower thermal conductivity coated chill, the re-melting of solid shell was absent. It was found that chilling during solidification causes the morphology of dendrites transform to nearly rounded shape with refinement of lamellar eutectic.     

Evaluation of Formation and Evolution of Microporosity in Anodic Copper Solidification Processes: Simulation and Experimental Validation

The current study analyzes the formation and evolution of microporosity during the solidification of anodic cooper. The aim of this study is to develop a thermofluid-formulation including microstructural evolution and to perform experiments to validate some measured variables with the respective numerical predictions. To this end, a set of experiments is carried out in copper testing primary and eutectic phase formation together with porosity evolution. To evaluate the formation of different microstructural phases and porosity, anodic copper (99.80 pct purity, approximately) is poured into different types of molds. The effect of heat extraction on the thermofluid-microstructural response is evaluated using graphite and steel molds to promote different cooling rates. The microporosity depends on the microstructural formation; hence the microstructure needs to be firstly described. The proposed microstructural model takes into account nucleation and grain growth laws based on thermal undercooling together with microstructural evolution. The primary phase evolution model is based on both solute diffusion at the grain scale and the dendrite tip growth kinetics, while the eutectic evolution is assumed proportional to the copper initial composition and eutectic undercooling. The microporosity model accounts for the partial pressures of gases and the solute distribution in the liquid and solid phases. The corresponding numerical formulation is solved in the framework of the finite element method. Finally, the computed temperature, solid, and liquid volumetric fractions, and pressure histories together with the final values for the radius, density, and pore volumetric fraction, are all compared and validated with the experimental measurements.     

Design of the Precipitation Process for Ni-Al Alloys with Optimal Mechanical Properties: A Phase-Field Study

An attempt to design the heat treatment schedule for binary Ni-Al alloys with optimal mechanical properties was made in the present work. A series of quantitative three-dimensional (3-D) phase-field simulations of microstructure evolution in Ni-Al alloys during the precipitation process were first performed using MICRESS (MICRostructure Evolution Simulation Software) package developed in the formalism of the multi-phase field model. The coupling to CALPHAD (CALculation of PHAse Diagram) thermodynamic and atomic mobility databases was realized via TQ interface. Moreover, the temperature-dependent lattice misfits and elastic constants were utilized for simulation. The effect of the alloy composition and aging temperature on microstructure evolution was extensively studied with the aid of statistical analysis. After that, an evaluation function was proposed for evaluating the optimal heat treatment schedule by choosing the phase fraction, grain size, and shape factor of γ′ precipitate as the evaluation indicators. Based on 50 groups of phase-field-simulated and experimental microstructure information, as well as the proposed evaluation function, the optimal alloy composition, aging temperature, and aging time for binary Ni-Al alloy with optimal mechanical properties were finally chosen. The successful application in the present Ni-Al alloys indicates that it is possible to design the optimal alloy composition and heat treatment for other binary and even multicomponent alloys with optimal mechanical properties based on the evaluation function and the sufficient microstructure information. Additionally, the combination of the present method and the key experiments can definitely accelerate the material design and improve the efficiency and accuracy.     

Heat-flux measurements of industrial on-site continuous copper casting and their use as boundary conditions for numerical simulations

An embedded sensor, designed for rapid and accurate response times and using wireless data transmission, has been developed for the on-site measurement of temperatures in industrial continuous casting moulds. The sensor has been used to measure the temperature at several points in the mould during production in a Southwire copper casting process. The measured data has been used to calculate the temperature gradient in the mould to estimate the heat flux through it; this is then used as a boundary condition for numerical simulations of solidification. For these, we employ a method that tracks the solidification front explicitly; this has an advantage over fixed-grid methods in simulations for materials having a short solidification interval, since the release of latent heat at the solidification front can be resolved without resorting to a very fine mesh. The special considerations required for setting the initial condition for the numerical scheme and the time taken for the superheated melt to form a solid shell are also discussed.     

Characterisation of 316L powder injection moulding feedstock for purpose of numerical simulation of PIM process

Abstract

Viscosity, specific heat and thermal conductivity of the standard feedstock of 316L stainless steel have been measured under the typical conditions of a real powder injection moulding (PIM) process. The viscosity was measured in a wide range of shear rates at four different temperatures. The experimental viscosity data were fitted into the Carreau-Yasuda model. Both specific heat and thermal conductivity were measured in the temperature range that overlaps the recommended processing range for the studied feedstock. It has been shown that at high cooling rates the transition temperature of the binder material is shifted towards lower temperatures. Tabulated values of thermal conductivity and specific heat for the studied feedstock are presented. The obtained data can be used for numerical simulation of the powder injection moulding process.     

Material property database for heat treatment simulation of tool materials

A materials data base has been set up with mechanical, thermophysical and transformation properties of tool and creep resistant steels. In particular powder metallurgical materials have been covered. The intended use of the data is for numerical simulation of heat treatment with focus on computation of residual stresses and distortion in Compound materials (macro composites). The main body of the data was generated in new experiments. In this paper data for the powder metallurgical high speed steel ASP 2060 will be presented. The material properties determined include thermophysical data such as thermal conductivity and specific heat, mechanical data such as yield strength and work hardening rate, and thermal expansion data. Most data were determined at different temperatures and after heat treatments to different microstructures. Additional data added to the database include literature data for elastic properties and transformation induced plasticity. Quenching of a Compound ring consisting of an inner high speed steel and an outer tool steel was simulated numerically using two codes, DistSIMR and Trast7. With the help of a database interface input data adjusted to the formats of the two codes were generated. The results of the simulations showed good agreement between the two models both for the computed residual stresses and distortions, which confirms the reliability of the results.     

Thermophysical Properties of Platinum-Copper Alloys

Platinum and copper along with their alloys have been used in a broad range of applications including jewelry, coinage, electrical and electronic devices, and many others. Their thermophysical properties play an important role in casting processes and are required as input data for casting simulation. The focus of this work was to investigate these properties by different methods. Platinum, copper, and four platinum-copper alloys, namely, Pt96Cu04, Pt68Cu32, Pt50Cu50, and Pt25Cu75, were investigated within this work. The melting range and thermal expansion were measured at fem by differential scanning calorimetry and dilatometry, respectively. At TU Graz, wire-shaped samples were investigated by an ohmic pulse heating technique. This technique delivers thermophysical properties of electrically conducting materials far into the liquid phase. These measurements allow the calculation of specific heat capacity and the temperature dependencies of electrical resistivity, enthalpy, and density of these alloys in the solid and liquid phases. Thermal conductivity and thermal diffusivity as a function of temperature are estimated from resistivity data using the Wiedemann?CFranz law at the end of the solid phase and at the beginning of the liquid phase. The results are compared with the available literature values.     

Heat flow in atomized metal droplets

The solidification of spherical droplets with a discrete melting temperature is analyzed using an enthalpy model. Equations describing the cooling of the initially superheated liquid droplet and a numerical heat flow model for its subsequent solidification are presented. Important parameters like times for initiation and completion of solidification, cooling rates and interface velocities in aluminum, iron, and nickel are related to the process variables governing the rate of heat extraction from the droplets. The analysis is performed for the range of Biot numbers of practical interest where Newtonian cooling models are not considered applicable, 0.01 ≤ Bi ≤ 1.o, and the results are presented in the form of normalized or dimensionless quantities. It is shown that the average cooling rate in the liquid prior to solidification can be computed with the Newtonian cooling expressions. However, significant temperature gradients are noted at the droplet surface even for Biot numbers as low as 0.01. Reducing the droplet diameter reduces the time necessary for the initiation and completion of solidification, increases the interface velocities at equivalent fractions solidified and decreases theG _L /R ratio. Although smaller droplet diameters promote higher cooling rates in the liquid at the beginning and in the solid at the end of solidification, the effect at the intermediate stages is more complex and depends on the initial superheat, the Biot number and the thermophysical properties of the material. Formerly Professor in the same Department.     

An analysis of the temperature distribution and the location of the solidus,mushy, and liquidus zones for binary alloys in remelting processes

The solidification of a binary alloy in an electroslag-remelting process is considered by means of mathematical analysis. A heat transfer analysis is presented from which the temperature distribution in a cylindrical ingot is determined. Since alloy solidification is considered, a third region, the “mushy zone”, is introduced in addition to the solid and liquid regions. From the temperature distribution the shape of the respective zones are determined when the process reaches a quasi-steady state condition. Results are presented for the binary alloy of Al-4.5 pct Cu.