METALLURGICAL AND MATERIALS TRANSACTIONS B
ABSTRACTS
Volume 27B, No. 1, February 1996

This Month Featuring: Pyrometallurgy; Electrometallurgy; Transport Phenomena; Process Control; Physical Chemistry; Solidification; and Solid State Reactions. View February 1996 Table of Contents.

PYROMETALLURGY

Reaction Equilibria in the Production of Manganese Ferroalloys
WEIZHONG DING and SVERRE E. OLSEN
A laboratory investigation has been carried out to determine slag/metal and slag/metal/gas equilibria relevant to production of manganese ferroalloys. The metal phase was normally composed of Mn-Si-Csat alloys, but in some experiments, the alloys contained up to 15 wt pct Fe. Different slag systems were used: MnO-SiO2, MnO-SiO2-CaO, MnO-SiO2-Al2O3, and quaternary MnO-SiO2-CaO-Al2O3 with fixed CaO/Al2O3 weight ratios of 1.5 and 3. The experiments were normally made in CO gas atmosphere at temperatures ranging from 1450°C to 1600°C. The results give comprehensive information about equilibrium relations. Partial and complete equilibria are illustrated in equilibrium diagrams. Partial equilibrium is a situation in which equilibrium is established with respect to certain variables but not to others, in this case, between slag and metal but not with the gas phase. The effect of temperature was found to be of minor importance for the partial slag/metal equilibrium, whereas the complete slag/metal/gas equilibrium is considerably influenced by both temperature and CO pressure. As expected, increasing temperature and decreasing CO pressure will reduce the equilibrium MnO content of slags. The influence of alumina addition to the slag phase and of iron to the metal phase is also discussed.

ELECTROMETALLURGY

Physical Modeling Studies of Electrolyte Flow Due to Gas Evolution and Some Aspects of Bubble Behavior in Advanced Hall Cells: Part III. Predicting the Performance of Advanced Hall Cells
R. SHEKHAR and J.W. EVANS
For the effective removal of bubbles from the anode-to-cathode gap (ACG), an earlier study had suggested that advanced Hall-Heroult cells be operated with a "grooved" anode in the near-horizontal electrode configuration. However, it was observed that a large number of bubbles did not enter the grooves readily; the avoidance of this phenomenon could further lower the interpolar resistance. To circumvent this problem, an "improved" anode design which would promote the entry of gas bubbles into the grooves was conceived. A further objective was to analyze the performance of advanced Hall cells equipped with this improved anode. For this purpose, measurements of (simulated) electrolyte velocities, interpolar resistance, and mass transfer coefficient at the cathode were carried out in a "water" model. An important suggestion emerges from this work: energy savings of 2 to 2.5 kWh/kg aluminum may result when a conventional Hall cell is replaced by an advanced Hall cell equipped with this improved anode.

TRANSPORT PHENOMENA

Prediction of Liquid Metal Viscosities Using an Adjustable Hard Sphere Radial Distribution Curve
ANTHONY L. HINES and TSAIR-WANG CHUNG
Predictions of liquid metal viscosities have been made using the Born-Green equation. The pair potential was calculated by the Percus-Yevick and hypernetted chain equations with the predicted structure factor and was used with the Mie-Gruneisen potential to make the calculations. Instead of using an experimentally determined radial distribution curve, however, an adjustable hard sphere radial distribution curve is employed to obtain the pair potential. The first peak of the hard sphere radial distribution curve used here was adjusted to represent the radial distribution curve at the melting point for any liquid metal. This was done by using the number of nearest neighbors and the distance to the first maximum in the radial distribution curve. This is possible because of the similarities in the radial distribution curves of liquid metals. A temperature correction is employed so that the viscosities can be calculated over a wide temperature range. The results are compared with literature values.

Water Model Experiment on the Liquid Flow Behavior in a Bottom Blown Bath with Top Layer
MANABU IGUCHI, OLUSEGUN J. ILEGBUSI, HIROSHI UEDA, TOMOAKI KURANAGA, and ZEN-ICHIRO MORITA
Water model experiments were performed to study the effect of top slag on the mean flow and turbulence characteristics in a steel bath agitated by bottom gas injection. The slag was modeled by silicone oil with a density of 0.968 g/cm3 and a kinematic viscosity 100 times larger than that of water at 25°C. Velocity measurements were made using a two-dimensional (2-D) laser Doppler velocimeter (LDV) in the absence of swirl motions. The output signals of the LDV system were processed on a personal computer to obtain the axial and radial mean velocity components, the root-mean-square (rms) values of the axial and radial turbulence fluctuations, the Reynolds shear stress, and the turbulence production for two cases with and without top slag. The bubbling jet (or the bubble dispersion) region was localized near the centerline of the bath by the presence of the top oil layer. The mean flow and turbulence motions in the recirculation region located outside the bubbling jet region were also suppressed significantly by the top layer. This result could be attributed to the entrainment of top slag into steel in a real system.

Probing the Initial Stage of Synthesis of Al2O3/Al Composites by Directed Oxidation of Al-Mg Alloys
H. VENUGOPALAN, K. TANKALA and T. DEBROY
In the directed oxidation of Al-Mg alloys, the amount of MgO that forms in the initial stage prior to the incubation period affects the rate of oxidation of Al to Al2O3 in the composite growth stage. The mechanism of formation of MgO and the duration of the initial stage were investigated experimentally and theoretically. The variables studied were total pressure in the reaction chamber, partial pressure of oxygen, and the nature of the diluent gas which affects the diffusion coefficients of magnesium vapor and oxygen in the gas phase. The oxidation rate in the initial stage was proportional to both the oxygen partial pressure and the diffusivity of oxygen. The duration of the initial stage decreased with the increase in oxygen pressure. To understand the role of magnesium evaporation in the oxidation behavior of the alloy, the velocity, temperature, and concentration fields in the gas phase were simulated numerically. The calculated concentration profiles of magnesium vapor and oxygen as a function of time were consistent with the experimentally measured oxidation rates and confirm reaction-enhanced gaseous diffusion-limited vaporization of magnesium in the initial stage of oxidation of Al-Mg alloys. The region where the magnesium vapor is oxidized in the gas phase moved progressively closer to the alloy surface during the initial stage of oxidation. The end of the initial stage and the start of the incubation period corresponded to the arrival of the oxygen front close to the surface when the spinel formation occurred.

PROCESS CONTROL

Variation of Contact Angles with Temperature and Time in the Al-Al2O3 System
WOOHYUN JUNG, HUESUP SONG, SANG WHAN PARK, and DOH-YEON KIM
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.

PHYSICAL CHEMISTRY

Solubility of Carbon in CaO-Al2O3 Melts
MASAKI KUWATA and HIDEAKI SUITO
Carbon distribution ratios between CaO-Al2O3 slags and Fe-0.0003 to 0.8 mass pct Al-0.2 to 5.6 mass pct C alloys were measured at 1873K in an Al2O3, CaO, or graphite crucible. The carbon distribution ratios were dependent on the oxygen potential, determined by the Al(Al2O3) equilibrium, not by the C/Co (Pco = 1 atm) equilibrium. The (mass pct C)/ac ratios were proportional to the activity of Al in logarithmic form with a slope of 2/3, indicating that carbon in slag is dissolved as C2- ion. Solubilities of carbon in CaO-Al2O3 slags were also measured at 1873K under the CO-CO2-Ar gas mixtures in an Al2O3 or graphite crucible. It was found that C2- ion is present in the range of log PO2(atm) < -15 and CO2-3 ion in the range of log PO2(atm) > -7.

Representation of Mixed Reactive Gases on Free Energy (Ellingham-Richardson) Diagrams
C.V. ROBINO
Free energy vs 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.

Influence of Phosphorus Addition on the Surface Tension of Liquid Iron and Segregation of Phosphorus on the Surface of Fe-P Alloy
X.M. XUE, H.G. JIANG, Z.T. SUI, B.Z. DING, and Z.Q. HU
This article presents a study of the surface tension and phosphorus surface segregation in Fe-P alloys. The surface tension was measured by the sessile drop technique. The result of the dynamic surface tension for the low phosphorus content alloys shows that the alloy surface vaporization has a clear effect on the surface tension and causes a positive surface tension temperature coefficient. However, from this article, it is evident that phosphorus in liquid iron acts as a surface active element similar to arsenic. The surface segregation was determined using Auger electron spectroscopy. The result on the surface analysis of as-solidified sample indicates that the adsorption of impurity elements, such as oxygen, carbon, and nitrogen, can conceal phosphorus segregation on the free surface. Phosphorus segregation was also examined in the samples as-cleaned by Ar+ and then treated 30 minutes at 650°C. Phosphorus was found to segregate extensively on the surface of the alloys. On the basis of the analysis of the published data, the surface active intensity sequence of some nonmetallic elements was arrayed, and the surface active intensity of fluorine and boron in liquid iron was estimated.

Communication: The Influence of Oxygen Pressure and P2O5 on the Surface Tension of Liquid Iron Oxide at 1435°C
P.K. BHATTACHARYYA and D.R. GASKELL

Communication: Vacuum Evaporation of KCl-NaCl Salts: Part I. Thermodynamic Modeling of Vapor Pressures of Solid and Liquid Solutions
LILY L. WANG and TERRY C. WALLACE, SR.

SOLIDIFICATION

Thermomechanics of the Cooling Stage in Casting Processes: Three-Dimensional Finite Element Analysis and Experimental Validation
M. BELLET, F. DECULTIEUX, M. MÉNAÏ, F. BAY, C. LEVAILLANT, J.-L. CHENOT, P. SCHMIDT, and I.L. SVENSSON
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.

Scaling of Intragranular Dendritic Microstructure in Ingot Solidification
D.BOUCHARD and J.S. KIRKALDY
Analytic scaling formulas of complete constitutional generality for forced velocity cells and dendrites were in earlier research perfected for in 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.

The Design of Feed Systems for Thin-Walled Zinc High-Pressure Die Castings
M.T. MURRAY and J.R. GRIFFITHS
A number of methods have been devised for the design of feed systems in high-pressure die castings to optimize their mechanical properties. Most of these result in a decrease in the included gas porosity by having runners which decrease in area along the flow direction. Although the influence of the ingate design on matters such as surface quality of the casting and erosion of the die are well known, the ingate is not usually a critical part of design for optimizing mechanical properties. Nevertheless, it is known that the mechanical properties of pressure die castings made of zinc alloy 3 (Zn-4.1Al-0.05 Mg) can be improved by controlling ingate size and molten metal injection velocity. In this article, we describe a method for designing the ingate so as to optimize the mechanical properties.

Heat-Flow-Based Analysis of Surface Crack Formation During the Start-Up of the Direct Chill Casting Process: Part I. Development of the Inverse Heat-Transfer Model
J.B. WISKEL and S.L. COCKCROFT
A method for determining the temperature dependence of heat transfer in the direct chill (DC) water regime of the DC casting process has been developed. The technique uses as input the data acquired from one embedded thermocouple and involves the application of one-dimensional (1-D) and two-dimensional (2-D) finite element based heat conduction models in succession. The technique has been verified using hypothetical temperature data obtained from a transient casting simulation conducted with a known, idealized, heat flux profile. The results of the comparison indicate that the technique converges to the applied heat flux profile in approximately 12 seconds process simulation time; thus, it is suitable for investigation of the flow of heat during the start-up phase of the process. The accuracy of the technique was found to be satisfactory with thermocouples placed up to a depth of approximately 10 mm below the face of the ingot. The analysis of industrial thermocouple data, the suggested mechanism for crack formation during the start-up, and the remedial action are presented in part II.

Heat-Flow-Based Analysis of Surface Crack Formation During the Start-Up of the Direct Chill Casting Process: Part II. Experimental Study of an AA5182 Rolling Ingot
J.B. WISKEL and S.L. COCKCROFT
The flow of heat during the start-up of the direct chill (DC) casting process has been studied with the aim of determining the factors that make this phase of the process prone to face crack generation. Measurements have been made on an AA5182 rolling ingot instrumented with embedded thermocouples placed at key locations in the vicinity of the ingot face near its base. The resulting temperature data have been input to a two-dimensional (2-D) inverse heat-transfer model, developed in part I of this two part study, in order to calculate heat flux vs surface temperature curves in the direct water impingement regime. The findings indicate that the flow of heat is influenced by changing surface morphology and water flow conditions during the start-up phase. A finite element based simulation of the cast start, employing the calculated flux/surface temperature relations, reveals that the ingot shell at the point of water contact reaches a maximum thickness early in the casting process. The location of this maximum was found to coincide with the position where surface cracks are routinely found to initiate. Further, this maximum was found to also coincide with position at which the rate of deflection of the base of the ingot ("butt-curl") begins to slow. Based on the heat-flow analysis, it is believed that the face cracks form due to an excessive shell thickness during transient start-up conditions and that their occurrence could be reduced by an optimal combination of water flow rate and casting speed during start-up.

SOLID STATE REACTIONS

Communication: Discussion of "The Effect of Steel Chemistry on the Formation of Fe-Zn Intermetallic Compounds of Galvanneal-Coated Steel Sheets"
LUCILLE A. GIANNUZZI, PAUL R. HOWELL, and WILLIAM R. BITLER

Communication: Author's Reply
C.S. LIN and M. MESHII

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