Showing papers in "Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science in 1998"

Factors affecting the immobilization of metals in geopolymerized flyash

Abstract: Geopolymerization, a fairly new technology based on a very old principle, has emerged during the last few years as a possible solution to some waste stabilization and solidification problems. Some commercial successes have been achieved, although the technique remains fairly unknown as well as seemingly unpopular. It has been shown that most waste materials containing sources of silica and alumina should be capable of taking part in a geopolymerization reaction. In this article, flyash was used as a reactant in creating a geopolymeric matrix for the immobilization of process water containing 25,000 ppm of Cu or Pb cations. By means of X-ray diffraction, scanning electron microscopy (SEM), infrared spectroscopy, Brunauer-Emmett-Teller (BET), compressive strength, as well as kinetic leaching analyses, the main factors influencing matrix stability, immobilization efficiency, and therefore leaching behavior were investigated and discussed qualitatively. It was found that relatively high strengths could be obtained using low Ca flyash. The environment and coordination number of source aluminum and silica seemed to play a major role in the eventual matrix stability. Other factors influencing matrix stability include the alkali metal cation used as well as the type of metal being immobilized. The kinetics of leaching of immobilized metals from the geopolymerized flyash were qualitatively found to proceed along a combination of pore diffusion and boundary diffusion control mechanisms. It is finally concluded that immobilization of metals in geopolymerized flyash proceeds by a combination of physical encapsulation and chemical bonding, with adsorption also thought to play a role.

A computational model for the prediction of steel hardenability

Abstract: A computational model is presented in this article for the prediction of microstructural development during heat treating of steels and resultant room-temperature hardness. This model was applied in this study to predict the hardness distribution in end-quench bars (Jominy hardness) of heat treatable steels. It consists of a thermodynamics model for the computation of equilibria in multicomponent Fe-C-M systems, a finite element model to simulate the heat transfer induced by end quenching of Jominy bars, and a reaction kinetics model for austenite decomposition. The overall methodology used in this study was similar to the one in the original work of Kirkaldy. Significant efforts were made to reconstitute the reaction kinetics model for austenite decomposition in order to better correlate the phase transformation theory with empiricism and to allow correct phase transformation predictions under continuous cooling conditions. The present model also expanded the applicable chemical composition range. The predictions given by the present model were found to be in good agreement with experimental measurements and showed considerable improvement over the original model developed by Kirkaldy et al.

Review and modeling of viscosity of silicate melts: Part I. Viscosity of binary and ternary silicates containing CaO, MgO, and MnO

Abstract: A structurally related model for the calculation of the viscosity of silicate melts is proposed based on the general behavior of the viscosity of binary silicate melts. It relates viscosity to the degree of polymerization, as represented by the three types of oxygen in the melts. The model parameters for binary systems were optimized to give best fit to the experimental values. For ternary systems, it was assumed as a first approximation that the model parameters were linear functions of the parameters of the two binary silicate systems. The model has been applied to the CaO-SiO2, MgO-SiO2, and MnO-SiO2 binary systems and the CaO-MgO-SiO2 and CaO-MnO-SiO2 ternary systems. Good agreement was obtained between calculated values and experimental data over the composition and temperature ranges in which experimental data exist. Comparison was made between the present model and the Urbain model. The present model has the capability of representing changes in viscosity due to substitution of cation species in silicate melts.

Activities of SiO2 and Al2O3 and activity coefficients of FetO and MnO in CaO-SiO2-Al2O3-MgO slags

Abstract: The activities of SiO2 and Al2O3 in CaO-SiO2-Al2O3-MgO slags were determined at 1873 K along the liquidus lines saturated with 2CaO · SiO2, 2(Mg,Ca)O · SiO2, MgO, and MgO · Al2O3 phases using a slag-metal equilibration technique. Based on these and previous results obtained in ternary and quaternary slags, the isoactivity lines of SiO2 and Al2O3 over the liquid region on the 0, 10, 20, 30, and 40 mass pct Al2O3 planes and those on the 10 and 20 mass pct MgO planes were determined. The activity coefficients of Fe t O and MnO, the phase boundary, and the solubility of MgO were also determined.

Modeling freckle formation in three dimensions during solidification of multicomponent alloys

Abstract: The formation of macrosegregation defects known as “freckles” was simulated using a three-dimensional finite element model that calculates the thermosolutal convection and macrosegregation during the dendritic solidification of multicomponent alloys. A recently introduced algorithm was used to calculate the complicated solidification path of alloys of many components, which can accommodate liquidus temperatures that are general functions of liquid concentrations. The calculations are started from an all-liquid state, and the growth of the mushy zone is followed in time. Simulations of a Ni-Al-Ta-W alloy were performed on a rectangular cylinder until complete solidification. The results reveal details of the formation of freckles not previously observed in two-dimensional simulations. Liquid plumes in the form of chimney convection emanate from channels within the mushy zone, with similar qualitative features previously observed in transparent systems. Associated with the formation of channels, there is a complex three-dimensional flow produced by the interaction of the different solutal buoyancies of the alloy solutes. Regions of enhanced solid growth develop around the channel mouths, which are visualized as volcanoes on top of the mushy zone. The prediction of volcanoes differs from our previous calculations with multicomponent alloys in two dimensions, in which the volcanoes were not nearly as apparent. These and other features of freckle formation phenomena are illustrated.

The kinetics and mechanism of the pyrite-to-pyrrhotite transformation

Abstract: The kinetics of the transformation of pyrite to pyrrhotite have been investigated. The study was performed using thermogravimetric analysis over the temperature range of 620 to 973 K in atmospheres of H2, He, Ar, and in vacuo over a wide range of pressures: 0.20 Pa to 4.24 MPa. Based on the kinetic results, a mechanistic picture of the various steps exerting control over the transformation is proposed. The thermal decomposition proceeds via a two-step, consecutive process. The rate-controlling step is the desorption of sulfur vapor from the surface. The presence of H2 introduces different rate-controlling steps into the sequence, providing the H2 exists at a pressure sufficiently high to suppress the rate of thermal decomposition. Rates at which the H2 reduction occurs with pyrite samples from different sources depends upon the samples’ impurity level and the extent to which various crystallographic faces are exposed.

Mathematical Simulation on Coupled Flow, Heat, and Solute Transport in Slab Continuous Casting Process

Abstract: A three-dimensional comprehensively coupled model has been developed to describe the transport phenomena, including fluid flow, heat transfer, solidification, and solute redistribution in the continuous casting process. The continuous casting process is considered as a solidification process in a multicomponent solid-liquid phase system. The porous media theory is used to model the blockage of fluid flow by columnar dendrites in the mushy zone. The relation between flow pattern and the shape of the solid shell is demonstrated. Double diffusive convection caused by thermal and concentration gradients is considered. The change in the liquidus temperature with liquid concentration is also considered. The formation mechanism of macrosegregation is investigated. Calculated solid shell thickness and temperature distribution in liquid core are compared with the measured quantities for validating the model.

Effects of CaO, Al2O3, and MgO additions on the copper solubility, ferric/ferrous ratio, and minor-element behavior of iron-silicate slags

Abstract: The effects of CaO, Al2O3, and MgO additions, singly or in combination, on the copper solubility, the Fe3+/Fe2+ ratio in slag, and on the minor-element behavior of silica-saturated iron silicate slags were examined at 1250 °C and a p O2 of 10−12 to 10−6 atm. The results indicated that copper solubility in slag was lowered with the addition of CaO, MgO, and Al2O3, in decreasing order. The Fe3+/Fe2+ ratio in the slag decreased with the additions, but this effect was smaller at lower oxygen potentials. The presence of small amounts (about 4 pct) of CaO, Al2O3, and MgO in the slag resulted in increased absorption of Bi and Sb into molten copper, but had a smaller effect at large additions (about 8 to 11 pct). The distribution behavior of Pb was a function of oxygen partial pressure, which indicates the oxidic dissolution of Pb in the slag as PbO, while the behavior of Bi, Sb, and As was found to be independent of oxygen potential, supporting the atomic (neutral) dissolution hypothesis of these elements in the slag. The distribution behavior of Pb and As was not significantly affected by the additions. The activity coefficients of Bi and Sb in the slags were determined to be as follows: (1) for no addition, γ Bi=40 and γ Sb=0.4; (2) for small additions (about 4.4 pct), γ Bi=70 to 85 and γ Sb=0.8; and (3) for large additions (about 8 to 11 pct), γ Bi=60 to 75 and γ Sb=0.5 to 0.7.

Simulation of microporosity formation in modified and unmodified A356 alloy castings

Abstract: In order to comprehensively model both the performance and inspectability of early design stage safety critical aluminum castings, the size, shape, and location of defects such as pores should be determined by simulation. In this article, a two-dimensional (2-D) model to predict grain size, pore size, pore morphology, and location is presented. The proposed model couples hydrogen gas evolution and microshrinkage pore formation mechanisms with a grain growth simulation model. The nucleation and growth of grains are modeled with a probabilistic method that uses the information from a macroscale heat transfer simulation to determine the rules of transition for grain evolution. Microshrinkage pores and the combination of microshrinkage and gas pores are addressed. The proposed model and postprocessing can provide direct simulated views of the microstructure of the solidifying casting. In the present work, the effect of Sr modifier and hydrogen content on pore size and morphology for equiaxed aluminum alloy A356 is modeled. The simulation results correlate well with the experimental observation of cast structures and other published data. In addition, Sievert’s law and the conditions for spontaneous growth of a gas pore are derived from first principles in the Appendix.

Solidification behavior and microstructural evolution during laser beam—material interaction

Abstract: A laser-assisted visualization technique has been used to monitor the solidification behavior at the tail of a molten pool created by scanning high energy density laser beam. A high speed digital camera with spatial resolution of 64×64 pixels and temporal resolution of 40,500 frames/s has been employed along with a novel concept of illuminating the interaction zone by a secondary visible probe laser. This technique enabled in situ monitoring of the solid/liquid interface due to the characteristic difference in the reflectivity between solid and liquid surfaces. It is observed that the solidification behavior is unstable and is highly influenced by the instabilities in the flow, which develop from the complex laser-material interaction process. Quite often the growth front remelted back due to the fluctuating thermal field driven by flow instability. The fluctuations in the growth front and the fluctuations in the laser-material interaction process have been monitored simultaneously, however, no correlation is apparent. The influence of flow instability on the resulting microstructure has been analyzed.

Review and modeling of viscosity of silicate melts : Part II. Viscosity of melts containing iron oxide in the CaO-MgO-MnO-FeO-Fe2O3-SiO2 system

Abstract: The viscosity of iron-containing silicate slags in the CaO-MgO-MnO-FeO-Fe2O3-SiO2 system has been reviewed by analyzing the available experimental data and the results of modeling studies. The model proposed by the present authors[1] for the calculation of viscosity of silicate melts has been found to provide a reasonably good description of the behavior of the viscosity of these melts with respect to both the change in silica content and the effect of different cations on viscosity. The results of the present model are compared with those of the Urbain model[2] and that proposed by Utigard and Warczok (the UW model).[3] The present model and the UW model gave a similar order of accuracy, while the results of the Urbain model tended to be higher than the experimental data for most systems examined. Although none of these models is capable of representing some peculiar behaviors, such as the peak observed around the fayalite composition in the FeO-SiO2 binary system and the isoviscosity contours found in the MnO-FeO-SiO2 system, they are considered to be very useful for representing the general behavior of viscosity in the systems studied.

Chemical reactions between aluminum and fly ash during synthesis and reheating of Al-fly ash composite

Abstract: Thermodynamic analysis indicates that there is the possibility of chemical reactions between aluminum melt and cenosphere fly ash particles. These particles contain alumina, silica, and iron oxide, which, during solidification processing of aluminum-fly ash composites or during holding of such composites at temperatures above the melting temperature of aluminum, are likely to undergo chemical reduction. These chemical reactions between the fly ash and molten aluminum have been studied by metallographic examination, differential thermal analysis (DTA), scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX) and X-ray analysis after holding the aluminum-fly ash composites for different periods above the liquidus temperature. The experiments indicate that there is progressive reduction of silica and mullite in the fly ash, and formation of alumina with holding time of composites at a temperature of 850 °C. The walls of the cenosphere fly ash particles progressively disintegrate into discrete particles as the reaction progresses. The rate of chemical reaction was high at the start of holding the composite at a temperature of 850 °C, and then the rate significantly decreased with time. The reaction was almost complete after 10 hours.

Modeling the microstructural changes during hot tandem rolling of AA5XXX aluminum alloys: Part I. Microstructural evolution

Abstract: A comprehensive mathematical model of the hot tandem rolling process for aluminum alloys has been developed. Reflecting the complex thermomechanical and microstructural changes effected in the alloys during rolling, the model incorporated heat flow, plastic deformation, kinetics of static recrystallization, final recrystallized grain size, and texture evolution. The results of this microstructural engineering study, combining computer modeling, laboratory tests, and industrial measurements, are presented in three parts. In this Part I, laboratory measurements of static recrystallization kinetics and final recrystallized grain size are described for AA5182 and AA5052 aluminum alloys and expressed quantitatively by semiempirical equations. In Part II, laboratory measurements of the texture evolution during static recrystallization are described for each of the alloys and expressed mathematically using a modified form of the Avrami equation. Finally, Part III of this article describes the development of an overall mathematical model for an industrial aluminum hot tandem rolling process which incorporates the microstructure and texture equations developed and the model validation using industrial data. The laboratory measurements for the microstructural evolution were carried out using industrially rolled material and a state-of-the-art plane strain compression tester at Alcan International. Each sample was given a single deformation and heat treated in a salt bath at 400 °C for various lengths of time to effect different levels of recrystallization in the samples. The range of hot-working conditions used for the laboratory study was chosen to represent conditions typically seen in industrial aluminum hot tandem rolling processes, i.e., deformation temperatures of 350 °C to 500 °C, strain rates of 0.5 to 100 seconds and total strains of 0.5 to 2.0. The semiempirical equations developed indicated that both the recrystallization kinetics and the final recrystallized grain size were dependent on the deformation history of the material i.e., total strain and Zener-Hollomon parameter (Z), where \(Z = \dot \varepsilon exp \left( {\frac{{Q_{def} }}{{RT_{def} }}} \right)\) and time at the recrystallization temperature.

Investigation of inclusion re-entrainment from the steel-slag interface

Abstract: The mechanism of inclusion elimination from continuous steel casting is investigated at the steelslag interface. Inclusion impact at the interface is considered under the concept of energy balance, with buoyancy forces, fluid dynamic forces, interfacial adhesion, and rebound forces determining whether the particle will pass through the interface or be retained by it. The effects of the inclusion, slag, and steel properties, as well as the effect of inclusion impact velocity, are considered at the interface. The interfacial tension between the slag and the inclusion should be smaller than that between the steel and the inclusion (negative wettability), so that the inclusions can pass into the slag layer and avoid re-entrainment. The inclusion particles that reach an equilibrium state at the steel-slag interface are subject to re-entrainment back into the steel, due to lift forces applied to them by the turbulent boundary layer at the interface. A removal criterion dependent upon the shear stress is introduced, and then the removal rates are calculated from the turbulent burst theory. It is found that the smaller diameter inclusions are trapped at the interface. Of the particles that remain at the interface, it is the larger ones that are more easily removed by the lift forces due to the turbulent shear stress. High slag viscosity is desirable, since it makes inclusion re-entrainment into the casting product more difficult.

Bioleaching model of a copper-sulfide ore bed in heap and dump configurations

Abstract: A two-dimensional (2-D) model for a heap or dump bioleaching of a copper ore containing mainly chalcocite and pyrite has been developed. The rate of the mineral sulfide dissolution was related to the rate of oxidation by bacteria attached onto the ore surface. The latter was calculated using the model of Michaelis-Menten, where both temperature and dissolved oxygen in the leach solution were taken into account by the kinetic equation. Oxygen transport through the ore bed was associated with natural air convection originating from the decrease in gas density inside the ore bed, which was attributable not only to heating, but also to humidification and decrease in the oxygen concentration. The model was used to estimate air-velocity fields and profiles of temperature and oxygen concentrations as well as mineral conversions during the bioleaching operation for ore beds with different pyrite contents, bacterial populations, widths, heights, and permeabilities. The model provides a useful tool for the design, improvement, and optimization of industrial operating conditions.

Numerical investigation of the free surface in a continuous steel casting mold model

Abstract: This article presents a numerical investigation of the turbulent flow in a water model, simulating a continuous steel casting mold. Special attention is given to the free-surface oscillations. The governing differential equations are discretized in a curvilinear coordinate system moving with the free surface by the Finite Volume Methodology (FVM). The results indicate that the free-surface wave has a predominant length and frequency and the wave amplitude scales with the flow dynamic head. Wave instability, which may be associated with emulsification, is predicted at a critical casting speed.

Assessment of the Fe-Ti-C system, calculation of the Fe-TiN system, and prediction of the solubility limit of Ti(C,N) in liquid Fe

Abstract: The literature on the Fe-Ti-C, Fe-Ti-N, and Fe-Ti-C-N systems is reviewed and the experimental information is reproduced by thermodynamic calculations. In the Fe-Ti-C system, interaction parameters are evaluated for the liquid and fcc phases. In the Fe-Ti-N and Fe-Ti-C-N systems, on the other hand, no interactions are evaluated because of the very low solubility of TiN and Ti(C,N) in liquid Fe, bcc(Fe), and fcc(Fe). The Fe-Ti-C and Fe-Ti-N phase diagrams are presented through a series of sections and projections, and the solubility limit of Fe(Ti,N) in liquid Fe is calculated

Assessment of the Fe-Ti System

Abstract: The literature on the Fe-Ti system is reviewed and the thermodynamic description of the system is reassessed checking the ternary extrapolations in the Fe-Ti-C and Fe-Ti-N systems. The reproduction of the thermochemical and phase diagram information is presented through a series of figures and tables.

The Kinetics of Dephosphorization of Carbon-Saturated Iron Using an Oxidizing Slag

Abstract: The kinetics of dephosphorization of carbon-saturated iron by oxidizing slags were studied at 1330 °C. Nine slag compositions were investigated in the systems CaO-Fe2O3-SiO2-CaF2 and CaO-Fe2O3-SiO2-CaCl2. Increasing Fe2O3 up to 50 pct was found to increase the rate and extent of dephosphorization, whereas further increases were found to decrease the rate and extent of dephosphorization. This was explained in terms of two competing effects on the driving force, where increased levels of iron oxide increase the oxygen potential for dephosphorization, hence the driving force, but simultaneously dilute the basic components in the slag, lowering the driving force for dephosphorization. CaF2 and CaCl2 were found to decrease the rate and extent of dephosphorization at levels higher than 12 pct. The rate of dephosphorization was found to be first order with respect to phosphorous in the metal and was controlled by mass transport in the slag. The oxygen potential at the slag/metal interface was controlled by the FeO activity in the slag. When the kinetic results were analyzed to take account of different driving forces, Fe2O3, CaF2 and CaCl2 were all found to increase the mass transfer coefficient of phosphorous in the slag, and a quantitative relationship has been demonstrated between these mass transfer coefficients and the slag viscosity for each system studied.

Formation of hexavalent chromium by reaction between slag and magnesite-chrome refractory

Abstract: The goal of this work was to understand how Cr6+ formation is affected by the interaction between chromite phases present in magnesite-chrome refractory and different slag compositions. In addition, the formation of Cr6+ as a function of chromite particle size and cooling rate due to the chromite phase/slag interaction was investigated. The following slag compositions were studied: calcium aluminate, calcium aluminum silicate, and calcium silicate. It was found that the presence of uncombined CaO in the calcium aluminate slags is responsible for a higher yield of Cr6+. However, the replacement of Al2O3 by SiO2 in calcium aluminate slags decreases the formation of Cr6+. SiO2 reacts with CaO to form stable 2CaO·Al2O3·SiO2 and CaO·SiO2 phases, consequently decreasing the amount of uncombined CaO available to react with the chromite phase to form Cr6+. Moreover, the content of Cr6+ is decreased by increasing chromite particle size and increasing the cooling rate of magnesite-chrome refractory.

Formation of TiN/TiC-Fe composites from ilmenite (FeTiO3) concentrate

Abstract: The production of a ceramic hard material-metal composite directly from a mineral concentrate has great potential application. An homogenizing pretreatment of a mixture of ilmenite (FeTiO3) and graphite, followed by annealing under an argon ambient, showed the formation of titanium carbide and elemental iron. Annealing of the same powder in nitrogen resulted in the formation of a composite of elemental iron and titanium nitride. The nitride was formed at a lower temperature than the carbide with almost complete conversion after 1 hour at 1000 °C. The rate of carbide formation was controlled by carbon diffusion, whereas the nitridation reaction was controlled by either oxygen or nitrogen diffusion. The TiC was found to form via TiC0.5, which slowly increased its carbon content until near stoichiometric TiC was formed; stoichiometric TiN formed directly with no intermediate phases. Titanium carbide showed the presence of a second phase with a slightly smaller unit cell size; this was due to interdiffusion between the iron and TiC. The titanium carbide composite was found to be composed of 3 to 4 µm anhedral iron grains dispersed in the titanium-rich matrix. There was no segregation in the iron/titanium nitride composite with apparently submicron distribution.

Microstructural effects on distortion and solid-liquid segregation during liquid phase sintering under microgravity conditions

Abstract: Tungsten heavy alloys with compositions ranging from 78 to 98 wt pct tungsten were liquid phase sintered at 1507 °C under microgravity conditions for 120 minutes. The sintered microstructures were quantitatively measured for solid volume fraction, grain size, connectivity, and contiguity. Links between these microstructural parameters were analyzed and compared to previously derived empirical equations. The macrostructures of the samples were also quantified and correlated to the underlying microstructures. Critical values of solid volume fraction, contiguity, and connectivity required for free-standing structural rigidity were defined for various degrees of bond rigidity as represented by the dihedral angle. The results are used to predict the degree of solid-liquid segregation due to density differences between the solid and the liquid.

Transport phenomena in electric smelting of nickel matte : Part II. Mathematical modeling

Abstract: A three-dimensional mathematical model was developed to simulate the distributions of electrical potential, heat release, temperature, and velocity in the slag and matte in a six-in-line 36 MVA capacity furnace for smelting nickel calcine. From Part I of this series, it was found that there was a substantial electrical potential drop at the electrode surface, likely due to arcing through evolved carbon monoxide. The incorporation of this phenomenon into the model permitted accurate calculation of the current, power, and temperature distributions in the slag and matte. The slag was found to be thermally homogenized due to the evolved gas, and to a lesser extent by natural convection. In contrast, the matte was thermally stratified; this finding was attributed to poor momentum transfer across the slag/matte interface. Ninety percent of the electrical energy was used in smelting reactions in the calcine; to simulate the heat transfer from the slag to the calcine, a heat transfer coefficient was deduced from plant data. The implications of these findings for stable furnace operation are discussed.

Thermodynamic Properties of Aluminum, Magnesium, and Calcium in Molten Silicon

Abstract: The thermodynamic properties of aluminum, magnesium, and calcium in molten silicon were investigated using a chemical equilibration technique at 1723 to 1848 K, 1698 to 1798 K, and 1723 to 1823 K, respectively. The activity coefficient of aluminum in molten silicon was determined by equilibrating molten silicon-aluminum alloys with solid Al2O3 and Al6Si2O13, that of magnesium was determined by equilibrating molten silicon-magnesium alloys and MgO-SiO2-Al2O3 melts doubly saturated with MgSiO3 and SiO2, and that of calcium was determined by equilibrating molten silicon-calcium alloys with SiO2-saturated CaO-SiO2 melts. The activity coefficients at infinite dilution relative to the pure liquid state were determined as follows: $$\begin{array}{*{20}c} {log \gamma _{Al(l) in Si}^ \circ = - \frac{{1570}}{T} + 0.236 (1723 to 1848 K)} \\ {log \gamma _{Mg(l) in Si}^ \circ = - \frac{{4900}}{T} + 1.96 (1698 to 1798 K)} \\ {log \gamma _{Ca(l) in Si}^ \circ = - \frac{{7670}}{T} + 1.53 (1723 to 1823 K)} \\ \end{array} $$

Influence of melt carbon and sulfur on the wetting of solid graphite by Fe-C-S melts

Abstract: The interfacial phenomena between carbonaceous materials such as graphite, coke, coal, and char and Fe-C-S melts are important due to the extensive use of these materials in iron processing furnaces However, the understanding of the interfacial phenomena between these kinds of carbonaceous materials and molten iron alloys is far from complete In this study, graphite was selected as the solid carbonaceous material because its atomic structure has been well established The sessile drop method was adopted in this investigation to measure the contact angle between solid graphite and molten iron and to study the interfacial phenomena The influence of carbon and sulfur content in Fe-C-S melts on the wettability of solid graphite has been investigated at 1600 °C The melt carbon content was in the range of 013 to 224 wt pct, and the melt sulfur content was in the range of 005 to 037 wt pct X-ray energy-dispersive spectrometer (EDS) analysis was conducted on an HITACHI S-4500 scanning electron microscope to detect composition distribution at the interfacial region It was found that contact of solid graphite with Fe-C-S melts will result in a nonequilibrium reactive wetting It involved carbon transfer from the solid to the liquid and iron transfer from the liquid to the solid The Fe-C-S melts exhibited relatively poor wetting when the reactions were absent The mass transfer between solid graphite and Fe-C-S melts was observed to strongly enhance the wetting phenomena It is proposed that the decrease of system free energy corresponding to the mass transfer reactions strongly influences the formation of the interface region and results in the progressive spreading of the wetting line The composition and thickness of the graphite/iron interfacial layer was dependent on the intensity of mass transfer across the interface The resulting change in the interfacial energy γls is a strong function of mass transfer, and it varies in accordance with time of contact The influence of carbon content on the wetting phenomena could only be seen at in the initial stages, whereas the influence of sulfur on the wettability was found when the system approached equilibrium Therefore, the interfacial tension in its equilibrium condition at the graphite/Fe-C-S melt interface was determined only by the extent of sulfur adsorption at this interface

Thermodynamic modeling of liquid Fe-Ni-Cu-Co-S mattes

Abstract: The modified quasichemical model for short-range ordering is described for liquid metal-sulfur solutions. Available thermodynamic data for molten Fe-S, Ni-S, Cu-S, and Co-S solutions are collected, critically evaluated, and optimized by means of the model. Very good descriptions of the thermodynamic properties are obtained with few parameters. Using only these binary parameters, the model predicts the thermodynamic properties of Fe-Ni-S, Fe-Cu-S, Ni-Cu-S, and Fe-Ni-Cu-S mattes over a wide range of composition and temperature within experimental error limits.

Modeling of Porosity during Spray Forming: Part I. Effects of Processing Parameters

Abstract: In this article, a porosity model is developed based on particle packing theory, fluid mechanics, and particle thermal and dynamic behavior during spray forming. According to this model, the amount of porosity in as-deposited materials can be estimated on the basis of the average fraction of solid in the incident spray and the solid particle packing density. A porosity coefficient Φ is introduced. By using this model, the effects of deposition distance, atomization gas pressure, melt flow rate, and melt superheat on porosity are investigated. The amount of porosity demonstrates distinct V-shaped variations with the processing parameters. Finally, the optimal processing parameters for achieving low porosity are discussed on the basis of the calculated results.

A three-dimensional model of the spray forming method

Abstract: A three-dimensional model has been formulated to calculate the shape of the general preform, using vector calculus. The shape of a rod, tube, plate, or irregular preform can be calculated at given spray forming conditions. The shape of a spray-formed rod was analyzed at various spray forming conditions using the three-dimensional model. The effects of spray forming parameters, such as spray distribution parameters, angular velocity of rotation, withdrawal velocity, spray angle, and eccentric distance on rod shape, were analyzed. The most important parameters affecting the shape of rods are the spray distribution parameters and the withdrawal velocity. The dynamic evolution of rod shape with a stepwise variation of the withdrawal velocity during spray forming was investigated. The effect of a stepwise change of the withdrawal velocity was the same as that of the scanning atomizer. The calculated surface profiles were compared with those of spray-formed 7075 aluminum alloy rods prepared on a pilot scale. The calculated results for the surface profiles were in agreement with those of the spray-formed rods.

Mixing time and fluid flow phenomena in liquids of varying kinematic viscosities agitated by bottom gas injection

Abstract: A model experiment was carried out to investigate the mixing condition and related fluid flow phenomena in a slag layer of metal-refining processes agitated by bottom gas injection. Silicone oil was used as a model for the molten slag. Mixing time in a silicone oil bath was measured with a newly developed laser optical sensor. Measured mixing time values increased with an increase in the kinematic viscosity of the silicone oil. In order to explain the relation between mixing time and the kinematic viscosity of silicone oil, the rising velocity of bubbles and the vertical and horizontal velocities of silicone oil flow were measured with an electroresistivity probe and a laser Doppler velocimeter, respectively. The increase in the mixing time with the kinematic viscosity of silicone oil was caused mainly by the suppression of upward motion of bubbles and silicone oil in the bubbling jet region. An empirical correlation for the mixing time was derived as a function of the kinematic viscosity of silicone oil, in addition to conventionally used parameters such as the gas flow rate, bath diameter, and bath depth.

A correlation to describe interfacial heat transfer during solidification simulation and its use in the optimal feeding design of castings

Abstract: It is known from experimental data that for pure aluminum castings manufactured via the gravity die casting process, the interfacial heat-transfer coefficient can vary in the range 500 to 16,000 W/m2K. These coefficients are of significant importance for the numerical simulation of the solidification process. The experimentally determined variation of interfacial heat-transfer coefficients with respect to time has been recalculated to highlight the variation with respect to casting temperature at the interface. This variation was observed to be of an exponential nature. Also, the pattern of variation was found to be similar in all the experimental results. It has been found that all these patterns of interfacial heat-transfer coefficient variation can be matched by a unique equation that has been proposed as a correlation to model the metal-mold interfacial heat transfer. The benefit of this correlation is in its ability to approximate the combined effects of geometry variation, insulation, chills, die coatings, air gap formation, etc. during the numerical simulation and its use in the optimal design of heat transfer at the metal-mold interface.