Showing papers in "Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science in 1996"

Effects of forced electromagnetic vibrations during the solidification of aluminum alloys: Part I. solidification in the presence of crossed alternating electric fields and stationary magnetic fields

Abstract: A new magnetohydrodynamic method of transmitting forced vibrations to solidifying aluminum alloy melts has been developed. Contrary to the case of the conventional mechanoacoustic systems, this device lends itself very well to a fundamental investigation. The relatively accurate knowledge of both the electromagnetic pressure and the local velocity peaks has enabled us to reveal the specific effects of the oscillatory flow and of the cavitation phenomena on grain refinement. It has been shown that the cavitation threshold depends both on the surface state of the crucible internal walls and on the electromagnetic pressure peak. In the presence of well-developed cavitation situations, a very fine and homogeneous microstructure has been observed throughout the ingot. A laboratory prototype of a new magnetohydrodynamic cavity resonator, allowing for significant energy saving and likely to be used for industrial applications, including the elaboration of metal matrix composites by means of a preform infiltration process, was also the subject of experimentation.

Prediction of dendrite arm spacing for low alloy steel casting processes

Abstract: Simple mathematical expressions to predict the primary dendrite arm spacing (PDAS) and secondary dendrite arm spacing (SDAS) suitable for steel casting processes are presented. The equations of the PDAS and SDAS were based on previously published experimental data for low alloy steels. Good agreement was obtained between previous measurements of dendrite arm spacing (DAS) and the model predications in the range of cooling rate occurring in steel casting processes. The results indicated that the cooling rate and carbon content basically govern the calculation of PDAS, especially for low carbon steel. However, the carbon content governs the selection of mathematical expression to predict SDAS for low alloy steels. Dendritic growth is the most common crystallization mechanism in industrial steels. Many empirical expressions have been employed to correlate the PDAS and SDAS with growth rate, temperature gradient, cooling rate, and local solidification time. However, the comparative advantages of the various expressions with respect to the accuracy with different types of steels remain unclear. Thus, the aim of the present study is to develop simple expressions to predict DAS as a function of carbon content and thermal conditions of low alloy steels.

Flow and thermal behavior of the top surface flux/powder layers in continuous casting molds

Abstract: Steady-state finite-element models have been formulated to investigate the coupled fluid flow and thermal behavior of the top-surface flux layers in continuous casting of steel slabs. The three-dimensional (3-D) FIDAP model includes the shear stresses imposed on the flux/steel interface by flow velocities calculated in the molten steel pool. It also includes different temperature-dependent powder properties for solidification and melting. Good agreement between the 3-D model and experimental measurements was obtained. The shear forces, imposed by the steel surface motion toward the submerged entry nozzle (SEN), create a large recirculation zone in the liquid flux pool. Its depth increases with increasing casting speed, increasing liquid flux conductivity, and decreasing flux viscosity. For typical conditions, this zone contains almost 4 kg of flux, which contributes to an average residence time of about 2 minutes. Additionally, because the shear forces produced by the narrowface consumption and the steel flow oppose each other, the flow in the liquid flux layer separates at a location centered 200 mm from the narrowface wall. This flow separation depletes the liquid flux pool at this location and may contribute to generically poor feeding of the mold-strand gap there. As a further consequence, a relatively cold spot develops at the wideface mold wall near the separation point. This nonuniformity in the temperature distribution may result in nonuniform heat removal, and possibly nonuniform initial shell growth in the meniscus region along the wideface off-corner region. In this way, potential steel quality problems may be linked to flow in the liquid flux pool.

Thermodynamics of phosphorus in molten silicon

Abstract: Removal of phosphorus is one of the major problems on the purification of molten silicon for solar cell. The Gibbs energy change of phosphorus dissolution into molten silicon was determined in the temperature range from 1723 to 1848 K by equilibrating a molten silicon-phosphorus alloy in a controlled phosphorus partial pressure and is expressed by the following equations. $$\begin{gathered} \frac{1}{2}P_2 (g) = \underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle-}$}}{P} (mass pct, in Si) \hfill \\ \Delta G^\circ = - 139,000{\text{ }}( \pm 2000){\text{ }} + {\text{ }}43.4{\text{ }}( \pm 10.1T){\text{ }}(J/mol) \hfill \\ \end{gathered} $$ The possibility of removing phosphorus from silicon melts by vacuum treatment and the accompanying yield of silicon during the refining process are discussed.

A water model study of the flow asymmetry inside a continuous slab casting mold

Abstract: The work studies the extent of asymmetric flow in water models of continuous casting molds of two different configurations. In the molds where fluid is discharged through multiple holes at the bottom, the flow pattern in the lower portion depends on the size of the lower two recirculating domains. If they reach the mold bottom, the flow pattern in the lower portion is symmetrical about the central plane; otherwise, it is asymmetrical. On the other hand, in the molds where the fluid is discharged through the entire mold cross section, the flow pattern is always asymmetrical if the aspect ratio is 1:6.25 or more. The fluid jet swirls while emerging through the nozzle. The interaction of the swirling jets with the wide sidewalls of the mold gives rise to asymmetrical flow inside the mold. In the molds with lower aspect ratios, where the jets do not touch the wide side walls, the flow pattern is symmetrical about the central plane.

A study of the thermal decomposition of BaCO3

Abstract: In the present work, the decomposition reaction, BaCO3 (solid) = BaO (solid) + CO2 (gas), was investigated by thermogravimetric analysis (TGA) and differential thermal analysis (DTA) methods. Both shallow powder beds and densely compacted spheres of the carbonate were employed. In the case of the shallow powder beds, TGA and DTA were carried out simultaneously. The DTA curves showed that BaCO3 exhibited two phase transformations, the transformation of orthorhombic to hexagonal occurring at 1079 K and that of hexagonal to cubic at 1237 K. The activation energy and the forward reaction rate constant of the decomposition of BaCO3 were evaluated from the thermogravimetric results of the powder beds. The activation energy of the decomposition was found to be 305(± 14) kJ • mole−1. The experimental results obtained with the compacted spheres were compared with those corresponding to the powder beds. After the initial stages, the formation of liquid due to the eutectic reaction between BaCO3 and BaO appears to play an important role in the reaction kinetics.

Effects of forced electromagnetic vibrations during the solidification of aluminum alloys : Part II. Solidification in the presence of colinear variable and stationary magnetic fields

Abstract: The influence on grain refinement of electromagnetic vibrations imposed during solidification of various aluminum alloys has been examined. The vibrations were produced, without any material contact with the solidifying alloys, by the simultaneous application of a stationary magnetic fieldB 0 and a periodic magnetic fieldb(t) of 50 Hz frequency. Extensive grain refinement has been observed in both continuous casting and batch-type mold casting. This investigation shows that the mean grain size obtained by this electromagnetic vibrational method is smaller than that produced by the variable magnetic field acting alone (electromagnetic stirring), particularly when the alloys are characterized by a narrow freezing range.

Reduction of FeO in smelting slags by solid carbon: Experimental results

Abstract: The reduction of CaO-SiO2-Al2O3-FeO slags containing less than 10 wt pct FeO by solid carbonaceous materials such as graphite, coke, and coal char was investigated at reaction temperatures of 1400 °C to 1450 °C. The carbon monoxide evolution rate from the system was measured using stationary and rotating carbon rods, stationary horizontal carbon surfaces, and pinned stationary spheres as the reductants. The measured reaction rate ranged from 3.25 × 10−7 mol cm−2 s−1 at 2.1 pct FeO under static conditions to 3.6 × 10−6 mol cm−2 s−1 at 9.5 pct FeO for a rotating rod experiment. Visualization of the experiment using X-ray fluoroscopy showed that gas evolution from the reduction reaction caused the slag to foam during the experiment and that a gas film formed between the carbon surface and the slag at all times during experimentation. The reaction rate increased with increased slag FeO contents under all experimental conditions; however, this variation was not linear with FeO content. The reaction rate also increased with the rotation speed of the carbon rod at a given FeO content. A small increase in the reaction rate, at a given FeO content, was found when horizontal coke surfaces and coke spheres were used as the reductant as compared to graphite and coal char. The results of these experiments do not fit the traditional mass transfer correlations due to the evolution of gas during the experiment. The experimental results are consistent, however, with the hypothesis that liquid phase mass transfer of iron oxide is a major factor in the rate of reduction of iron oxide from slags by carbonaceous materials. In a second article, the individual rates of the possible limiting steps will be compared and a mixed control model will be used to explain the measured reaction rates.

Activities in MnO-SiO2-AI2O3 slags and deoxidation equilibria of Mn and Si

Abstract: The activities of MnO and SiO2 along the liquidus line in the MnO-SiO2-Al2O3-FetO (1.2 to 6.7 mass pct) system were determined at 1823 and 1873 K by using a slag-metal equilibration technique. On the basis of the re-evaluated MnO iso-activity curves, the SiO2 and Al2O3 iso-activity curves were determined by using the ternary Gibbs-Duhem relation. The control of inclusions composition in Si-Mn killed steels is discussed based on the equilibria between inclusion and steel with respect to Si, Mn, Al, and O.

Liquidus temperatures for primary crystallization of cryolite in molten salt systems of interest for aluminum electrolysis

Abstract: Temperatures for primary crystallization of Na3AlF6 in multicomponent electrolyte systems of interest for the aluminum electrolysis process were determined by thermal analysis. The results are presented as binary and quasibinary diagrams and discussed in view of the literature data. An empirical equation describing liquidus temperatures for primary crystallization of Na3AlF6 was derived: $$\begin{gathered} t/(^\circ C) = 1011 + 0.50[AlF_3 ] - 0.13[AIF_3 ] - \frac{{3.45[CaF_2 ]}}{{1 + 0.0173[CaF_2 ]}} \hfill \\ + 0.124[CaF_2 ] \cdot [AlF_3 ] - 0.00542([CaF_2 ] \cdot [AlF_3 ])^{1.5} \hfill \\ - \frac{{7.93[Al_2 O_3 ]}}{{1 + 0.0936[Al_2 O_3 ] - 0.0017[Al_2 O_3 ]^2 - 0.0023[AlF_3 ] \cdot [Al_2 O_3 ]}} \hfill \\ - \frac{{8.90[LiF]}}{{1 + 0.0047[LiF] + 0.0010[AlF3]^2 }} - 3.95[MgF_2 ] - 3.95 \hfill \\ \end{gathered} $$ wheret is the temperature in degree Celsius and the square brackets denote the weight percent of components in the system Na3AlF6-AlF3-CaF2-Al2O3-LiF-MgF2-KF. The composition limitations are [AlF3] ≈ [CaF2] ≈ [LiF] < 20 wt pct, [MgF2] ≈ [KF] < 5 wt pct, and [A12O3] up to saturation.

Representation of mixed reactive gases on free energy (Ellingharn-Richardson) diagrams

Abstract: Free energyvs temperature (Ellingham-Richardson) diagrams provide a convenient and useful means of portraying equilibria for a variety of systems, such as oxides, sulfides, and carbides. The oxide diagrams typically include nomographic scales that facilitate determination of the oxygen partial pressure, the H2/H2O ratio, and the CO/CO2 ratio in equilibrium with the oxides at any temperature. In addition, monatomic hydrogen provides a potentially useful means for the reduction of oxides in metal processing schemes but, at present, can only be produced in H2 at fractions less than unity. In the present work, a modification of conventional free energy diagrams that includes a means of portraying the equilibria between H, H2, O2, and H2O is developed. The modification is based on the oxidation reaction of H/H2 mixtures at any fraction H, and the equilibria are portrayed on the free energy diagram by means of a two-dimensional nomographic scale. The diagram illustrates the potential utility of H and H/H2 mixtures for oxide reduction and can be used in essentially the same manner as conventional free energy diagrams. The modification can conceivably be extended to other gaseous systems or diagram types, such as those for sulfides.

Dilution in single pass arc welds

Abstract: A study was conducted on dilution of single pass arc welds of type 308 stainless steel filler metal deposited onto A36 carbon steel by the plasma arc welding (PAW), gas tungsten arc welding (GTAW), gas metal arc welding (GMAW), and submerged are welding (SAW) processes. Knowledge of the arc and melting efficiency was used in a simple energy balance to develop an expression for dilution as a function of welding variables and thermophysical properties of the filler metal and substrate. Comparison of calculated and experimentally determined dilution values shows the approach provides reasonable predictions of dilution when the melting efficiency can be accurately predicted. The conditions under which such accuracy is obtained are discussed. A diagram is developed from the dilution equation which readily reveals the effect of processing parameters on dilution to aid in parameter optimization.

Solid-state contributions to densification during liquid-phase sintering

Abstract: Densification via liquid-phase sintering generally requires transport of substantial amounts of dissolved solid through the liquid However, in composite systems, such as W-Cu, solid solubility in the liquid is almost negligible, and densification is hindered by the low amount of total mass transport In this case, solid-state sintering of the skeletal solid structure in the presence of the liquid is a significant densification mechanism In this article, the relative contributions to densification of both liquid and solid mass transport mechanisms are considered A computer simulation is constructed to predict the densification behavior and concurrent microstructural development of liquidphase sintered composites for realistic heating cycles Governing differential equations for densification are derived from idealized models of the microstructure, considering grain size, diffusion distance from vacancy source to sink, pore size, and pore morphology Temperature-dependent terms, including the diffusivity, solubility, and surface energy, govern densification and microstructural parameters, such as the grain size, dihedral angle, and contiguity Predictions for the sintered density, grain size, and contiguity are compared to experimental results for the W-Cu and W-Cu-Ni systems with approximately 20 vol pct liquid For W-Cu, which has almost no intersolubility, solid-state sintering of W in the presence of liquid Cu is the dominant densification mechanism Nickel additions increase solid solubility in the liquid and improve typical liquid-phase sintering contributions to densification Alternatively, high sintered densities can be achieved in the absence of solubility with a sufficiently small particle size due to the solid-state contribution

Thermomechanics of the cooling stage in casting processes : three-dimensional finite element analysis and experimental validation

Abstract: A thermomechanical three-dimensional (3-D) finite element analysis of solidification is presented. The heat transfer model is based on a multidomain analysis accounting for noncoincident meshes for the cast part and the different mold components. In each subdomain, a preconditioned conjugate gradient solver is used. The mechanical analysis assumes the mold is rigid. A thermoelastic-viscoplastic rheological model is used to compute the constrained shrinkage of the part, resulting in an effective local air gap width computation. At each time increment, a weak coupling of the heat transfer and mechanical analyses is performed. Comparisons of experimental measurements and model predictions are given in the case of a hollow cylindrical aluminum alloy part, showing a good quantitative agreement. An application to an industrial aluminum casting is presented, illustrating the practical interest of thermomechanical computations in solidification analysis.

Transient thermal analysis of solidification in a centrifugal casting for composite materials containing particle segregation

Abstract: One-dimensional heat-transfer analysis during centrifugal casting of aluminum alloy and copper base metal matrix composites containing Al2O3, SiCp, and graphite particles has been studied. The model of the particle segregation is calculated by varying the volume fraction during centrifugal casting, and a finite difference technique has been adopted. The results indicate that the thickness of the region in which dispersed particles are segregated due to the centrifugal force is strongly influenced by the speed of rotation of the mold, the solidification time, and the density difference between the base alloy and the reinforcement. In the case where the base alloy density is larger than that of the particles, the thickness of the particle-rich region near the inner periphery decreases with an increase in speed, thereby increasing the volume fraction of dispersion. The solidification time of the casting is also dependent upon the speed of rotation of the mold, and it decreases with an increase in speed. This study also indicates that the presence of particles increases the solidification time of the casting.

Scaling of intragranuiar dendritic microstructure in ingot solidification

Abstract: Analytic scaling formulas of complete constitutional generality for forced velocity cells and dendrites were in earlier research perfected forin situ steady-state solidification conditions involving binary organic alloys. As a further test, these were used, given the velocity and gradient control parameters, to predict the primary and secondary dendrite arm spacings of unidirectionally cooled Al-Cu alloys for which a large data set is available. Numerical methods were employed to determine the control parameters that exist under unsteady-state ingot solidification conditions according to the Scheil formulation. Primary and secondary arm spacings, corrected empirically for ripening, that by and large agree with the Al-Cu experimental data were obtained, demonstrating that the formulas are adequate for the prediction of dendrite scales in steady and unsteady-state conditions. The predictions have been incorporated into a computer program that displays the time-dependent columnar microstructure and mushy zone in an ingot cross section of an oriented single crystal together with the thermal and liquid-solid distributions.

Preoxidation and hydrogen reduction of ilmenite in a fiuidized bed reactor

Abstract: Studies on preoxidation and hydrogen reduction of Quilon-grade ilmenite have been carried out in a fluidized bed reactor. During preoxidation, the effect of various parameters such as temperature, gas flow rate, and period of reaction has been studied. Hydrogen reduction studies have been carried out both for raw and preoxidized ilmenite. Results obtained on the conversion rate of iron oxide to metallic iron for the preoxidation as well as the reduction period showed three distinct stages: (1) initial slow induction stage; (2) intermediate acceleratory stage; and (3) final slowing down process. Kinetic data plot on -In (1 -x) vs time for metallization of iron oxide showed a linear trend for preoxidation and hydrogen reduction.

Intermixing Model of Continuous Casting during a Grade Transition

Abstract: To investigate the composition distribution that develops in continuously cast steel during a grade change, an efficient, accurate, and user-friendly computational model has been developed. The model is fully transient and consists of three submodels, which account for mixing in the tundish, mixing in the liquid core of the strand, and solidification. The first submodel of mixing in the tundish consists of two plug flow zones, two back-mixing boxes, and two dead volumes. The second submodel solves a one-dimensional (1-D) diffusion equation in series with two back-mixing boxes to calculate concentration histories in the strand, and the third submodel transforms these histories into slab compositions. The model was calibrated using both concentration histories measured on tundish water models and calculations from a three-dimensional (3-D) model. It was then verified with several sets of composition measurements along the surface and centerlines of slabs. The model is capable of tracking mixing phenomena for arbitrary tundish filling and casting speed histories. It has been used to compare the effects of different grade change procedures on the amount of intermixed steel, including standard sequence casting, flying tundish change, and insertion of grade separators. Mixing in the strand was found to be very important. Without a grade separator, a flying tundish change had very little benefit on reduced intermixing, for the typical conditions considered.

A thermodynamic evaluation of the Ti-Mo-C system

Abstract: A thermodynamic assessment of the Ti-Mo-C system has been made, employing a two-sublattice regular solution model for the solid solution and carbide phases and an ordinary subregular solution model for the liquid phase. A set of thermodynamic parameters describing the Gibbs energy of each individual phase in the Ti-Mo-C as well as the Ti-Mo systems was evaluated from thermochemical and phase equilibria information available in the literature through a computer-aided optimization procedure called the CALPHAD method. The comparison between the calculated and experimental results was made and practically important phase diagrams are also presented.

Studies of interface deformations in single- and multi-layered liquid baths due to an impinging gas jet

Abstract: An impinging gas jet on a molten bath having a slag layer on top is encountered in various metal processing operations. The impinging region was studied using a physical model consisting of an air jet and water bath. Kerosene and corn oil were used as the second layer to investigate the role of the slag layer properties on interface shape and bath circulation. The interface shapes were measured both photographically and by using a surface-tracking resistance probe. The limiting condition at which the jet breaks through the kerosene or corn oil layer and reaches the water layer was determined experimentally. A phenomenological model for the prediction of penetration depth is developed for both short and long jet heights for liquid baths with and without a second liquid layer on top.

Studies on the corrosion and the behavior of inert anodes in aluminum electrolysis

Abstract: The corrosion rates of inert anodes based on tin oxide and nickel ferrite cermet materials were studied as a function of some operating parameters. To reach a better understanding of the corrosion mechanism, the behavior of the anodes was observed under some specific conditions, such as in pure cryolite, at high current densities, at different potentials, and at varying cathode surface areas. It was confirmed that low alumina concentrations led to catastrophic corrosion of the anodes and that high current densities and high as well as low NaF/AlF3 molar ratios were also detrimental. The corrosion rate of tin oxide based anodes showed a minimum (so-called “normal corrosion”) at anodic potentials of 2.2 to 2.4 V with respect to aluminum. The normal corrosion is due to chemical dissolution of the anode material and reduction of the corrosion products into the cathode metal. The corrosion rate increased with increasing cathode surface area. At potentials higher than ∼2.5 V, the anodes showed catastrophic corrosion. Catastrophic corrosion can be ascribed to decomposition of the anode material by depletion of alumina at the anode surface provoked by low bulk concentration of alumina and/or high current density.

Electrical conductivity of molten cryolite-based mixtures obtained with a tube-type cell made of pyrolytic boron nitride

Abstract: A pyrolytic boron nitride tube-type cell was used to measure the electrical conductivity for molten cryolite, for binary mixtures of cryolite with Al2O3, AlF3, CaF2, KF, Li3AlF6, and MgF2, and for ternary mixtures Na3AlF6-Al2O3-CaF2 (MgF2) and Na3AlF6-AlF3-KF (Li3AlF6). The cell constant was about 40 cm−t. The temperature and concentration dependence of the conductivity in the investigated concentration range was described by the equation $$\begin{gathered} \kappa /S cm^{ - 1} = 7.22 exp\left( { - 1204.3/T} \right) - 2.53\left[ {Al_2 O_3 } \right] - 1.66\left[ {AlF_3 } \right] \hfill \\ - 0.76\left[ {CaF_2 } \right] - 0.206\left[ {KF} \right] + 0.97\left[ {Li_3 AlF_6 } \right] - 1.07\left[ {MgF_2 } \right] \hfill \\ - 1.80\left[ {Al_2 O_3 } \right]\left[ {CaF_2 } \right] - 2.59\left[ {Al_2 O_3 } \right]\left[ {MgF_2 } \right] \hfill \\ - 0.942\left[ {AlF_3 } \right]\left[ {Li_3 AlF_6 } \right] \hfill \\ \end{gathered} $$ whereT represents the temperature in Kelvin and the brackets represent the mole fractions of the additions. The standard deviation was found to be 0.026 S cm−1 (∼1 pct). For practical reasons, it is often desired to express composition in weight percent. In that case, it holds that $$\begin{gathered} \ln \kappa = 1.977 - 0.0200\left[ {Al_2 O_3 } \right] - 0.0131\left[ {AlF_3 } \right] - 0.0060\left[ {CaF_2 } \right] \hfill \\ - 0.0106\left[ {MgF_2 } \right] - 0.0019\left[ {KF} \right] + 0.0121\left[ {LiF} \right] - 1204.3/T \hfill \\ \end{gathered} $$ whereT represents the temperature in Kelvin and the brackets denote the concentration of the additives in weight percent. However, in this case, the maximum relative error of the conductivity equation can reach up to 2.5 pct.

High-Speed imaging and analysis of the solidification of undercooled nickel melts

Abstract: Rapid solidification of undercooled pure nickel has been imaged at sufficiently high spatial resolution (64 ×X 64 pixels) and temporal resolution (40,500 frames/s) to observe interface shape and motion at solidification velocities exceeding 45 m/s. Imaging was of 8 g, quartz-enclosed melts at undercoolings of 70 to 300 K. Dendrite velocities within the melt were calculated from the surface velocities observed employing a simple geometric model of growth. Solidification was found to proceed invariably from a single nucleation point; growth velocity then followed an approximate power-law relationship with respect to undercooling up to some critical value ΔT*, where 150 K < ΔT* < 180 K. At higher undercoolings, velocity increased less rapidly than predicted by the power-law relationship and the interface morphology changed in appearance from angular to macroscopically smooth.

Electrochemical behavior of the dissolution of gold-silver alloys in cyanide solutions

Abstract: The dissolution behavior of gold and silver from Au/Ag alloys in aerated cyanide solutions has been investigated using rotating disc electrodes. The variables studied included concentration of cyanide, oxygen partial pressure, and rotating speed of the disc. The dissolution potential and the rate of dissolution were obtained in view of the anodic and cathodic current-potential relationships. The results were discussed in terms of the mixed potential theory. The results showed that the dissolution rate of gold and silver from the alloys was partially controlled by chemical reaction but largely controlled by transport of either oxygen or cyanide, depending on their relative concentrations under the experimental conditions employed in this study. The diffusion coefficient of free cyanide, Dcn−, was found to be (1.25 ± 0.05) X 10−5 cm2/s. The diffusion coefficient of oxygen,\(D_{O_2 } \), was calculated to be (1.29 ± 0.02) X 10−5 cm2/s.

Critical evaluation and optimization of the thermodynamic properties of liquid tin solutions

Abstract: Thermodynamic and phase equilibrium data for the following 18 elements in molten Sn were collected and critically evaluated: Al, Ca, Ce, Co, Cr, Cu, Fe, H, Mg, Mo, Na, Ni, O, P, S, Se, Si, and Ti. Binary and ternary data were optimized to give polynomial expressions for the excess Gibbs energies as functions of temperature and composition. For some solutes, the optimized expressions are valid over the entire composition range 0 ≤ XSn ≤ 1. In other cases, the expressions apply to Snrich solutions. Solute-solute interaction terms were estimated where data were not available. The optimized Gibbs energy expressions are also presented in the form of interaction parameters, and the equivalence between the polynomial and interaction parameter formalisms is discussed. Through the Kohler equation, or the modified interaction parameter formalism, the thermodynamic properties of the multicomponent solution of 18 elements in Sn can be calculated. The database is suitable for computer storage and manipulation.

Thermodynamics of calcium and oxygen in molten titanium and titanium-aluminum alloy

Abstract: The deoxidation equilibrium of molten titanium and titanium-aluminum alloys saturated with solid CaO has been measured in the temperature range from 1823 to 2023 K. The equilibrium constant of reaction CaO (s)=Ca (mass pct in Ti,Ti-Al)+O (mass pct in Ti,Ti-Al) and the interaction parameter between calcium and oxygen were determined for Ti, TiAl, and TiAl3. The standard Gibbs energy of reaction for TiAl was obtained as follows: $$\Delta G^\circ = 279,000 - 103TJ/mol$$ The possibilities for the deoxidation of titanium and titanium-aluminum alloys by using calcium-based fluxes are discussed.

Solidification of particle-reinforced metal-matrix composites

Abstract: The solidification behavior of ceramic particle-reinforced metal-matrix composites (MMCs) is different from that of the bare matrix, not only because of the presence of the ceramic particles, but also due to their redistribution in the melt that results in nonhomogeneous thermophysical properties. The MMCs comprised of 10-to 15-μm SiC particles of varying volume fractions, dispersed uniformly in a modified aluminum A356 alloy by the melt stirring technique, were solidified unidirectionally in a thermocouple-instrumented cylindrical steel mold. The cooling rates were continually monitored by measuring temperatures at different depths in the melt, and the solidified MMCs were sectioned into disks and chemically analyzed for SiC volume fraction. The results point out that the cooling rate increased with increasing volume fraction of SiC particles. A small increase in the bulk SiC volume fraction of the cast MMC was observed due to particle settling during solidification. A one-dimensional enthalpy model of MMC solidification was formulated, wherein particle settling occurring in the solidifying matrix was coupled to the enthalpy equation by means of the Richardson-Zaki hindered settling correlation. A comparative study of simulations with experiments suggested that the thermal response of SiC particles used in this study was similar to that of single crystals, and their presence increased the effective thermal conductivity of the composite.

Transient Thermal Model of the Continuous Single-Wheel Thin-Strip Casting Process*

Abstract: A transient heat-transfer model (STRIP1D) has been developed to simulate the single-roll continuous strip-casting process. The model predicts temperature in the solidifying strip coupled with heat transfer in the rotating wheel, using an explicit finite difference procedure. The model has been calibrated using strip thickness data from a test caster at ARMCO Inc. (Middletown, OH) and verified with a range of other available measurements. The strip/wheel interface contact resistance and heat transfer were investigated in particular, and an empirical formula to calculate this heat-transfer coefficient as a function of contact time was obtained. Wheel temperature and final strip thickness are investigated as a function of casting speed, liquid steel pool depth, superheat, coatings on the wheel hot surface, strip detachment point, wheel wall thickness, and wheel material.

Variation of contact angles with temperature and time in the Al-Al2O3 system

Abstract: The contact angles of liquid Al on polycrystalline Al2O3 determined by the conventional sessile drop method were obtuse (∼120 deg) up to 900 °C but decreased rapidly at 1000 °C. When the molten Al was squeezed through a narrow orifice and dropped onto the substrate, the contact angle at 900 °C was 77 deg and decreased linearly with temperature. At 1000 °C and 1100 °C, the contact angles decreased slowly with holding times up to 50 and 6 hours, respectively. At 1200 °C, the contact angle also decreased with holding time up to 40 minutes, after which it oscillated, resulting in a ring pattern on the substrate. The structural change of the Al2O3 substrate surface is suggested to be an important variable that determines the wetting behavior of the Al-Al2O3 system.

The measurement of hydrogen activities in molten copper using an oxide protonic conductor

Abstract: We have developed a hydrogen sensor for in situ measurements of hydrogen activities in molten copper. The sensor consists of a concentration cell utilizing a proton conductor, CaZr0.9In0.1O3-δ, as the solid electrolyte. The electromotive force (emf) of the cell was generated by both hydrogen and oxygen activity gradients across the cell in a high-temperature region simulating the fire refining processes of copper. However, accurate hydrogen activity in molten copper could be evaluated from the emf if oxygen activity in molten copper was determined simultaneously by another concentration cell and if the hydrogen and the oxygen activities at the reference electrode were known. The performance of the sensor was studied under various conditions. The observed good response and reliability of the sensor show that it should be a powerful tool for improvement of the fire refining process of the molten copper. Theoretical treatment of the calculation of the emf of the concentration cells using a mixed ionic conductor, i.e., protonic and oxide ionic conductor, as solid electrolytes is also discussed briefly.