Showing papers in "Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science in 2007"

Investigation of Fluid Flow and Steel Cleanliness in the Continuous Casting Strand

Abstract: Fluid flow in the mold region of the continuous slab caster at Panzhihua Steel is investigated with 0.6-scale water model experiments, industrial measurements, and numerical simulations. In the water model, multiphase fluid flow in the submerged entry nozzle (SEN) and the mold with gas injection is investigated. Top surface level fluctuations, pressure at the jet impingement point, and the flow pattern in the mold are measured with changing submergence depth, SEN geometry, mold width, water flow rate, and argon gas flow rate. In the industrial investigation, the top surface shape and slag thickness are measured, and steel cleanliness including inclusions and the total oxygen (TO) content are quantified and analyzed, comparing the old and new nozzle designs. Three kinds of fluid flow pattern are observed in the SEN: “bubbly flow,” “annular flow,” and an intermediate critical flow structure. The annular flow structure induces detrimental asymmetrical flow and worse level fluctuations in the mold. The SEN flow structure depends on the liquid flow rate, the gas flow rate, and the liquid height in the tundish. The gas flow rate should be decreased at low casting speed in order to maintain stable bubbly flow, which produces desirable symmetrical flow. Two main flow patterns are observed in the mold: single roll and double roll. The single-roll flow pattern is generated by large gas injection, small SEN submergence depth, and low casting speed. To maintain a stable double-roll flow pattern, which is often optimal, the argon should be kept safely below a critical level. The chosen optimal nozzle had 45-mm inner bore diameter, downward 15 deg port angle, 2.27 port-to-bore area ratio, and a recessed bottom. The pointed-bottom SEN generates smaller level fluctuations at the meniscus, larger impingement pressure, deeper impingement, and more inclusion entrapment in the strand than the recess-bottom SEN. Mass balances of inclusions in the steel slag from slag and slab measurements show that around 20 pct of the alumina inclusions are removed from the steel into the mold slag. However, entrainment of the mold slag itself is a critical problem. Inclusions in the steel slabs increase twofold during ladle changes and tenfold during the start and end of a sequence. All of the findings in the current study are important for controlling slag entrainment.

Experimental Investigation of the Viscosities in CaO-SiO2-MgO-Al2O3 and CaO-SiO2-MgO-Al2O3-TiO2 Slags

Abstract: The viscosities of high alumina blast furnace slags were experimentally determined by the rotating cylinder method using the Brookfield digital viscometer model LVDV-II+ pro Two different slag systems were considered for the current study, the CaO-SiO2-MgO-Al2O3 quaternary and the CaO-SiO2-MgO-Al2O3-TiO2 quinary system Experiments were conducted in the temperature range of 1650 to 1873 K The effects of temperature, basicity, TiO2, and silica activity of slags on viscosity were studied The viscosity decreases with basicity for high alumina blast furnace slags with basicity in the range of 046 to 08 At high basicity (∼08), slag viscosity decreases even with a small amount of TiO2 (∼2 pct) addition in the slag With an increase in silica activity in the range of 01 to 04, the slag viscosity increases, the increases being steeper below the liquidus temperature

Efficient Melt Stirring Using Pulse Sequences of a Rotating Magnetic Field: Part I. Flow Field in a Liquid Metal Column

Abstract: The use of a pulsed, rotating magnetic field (RMF) is presented as an auspicious method for obtaining an intensive stirring and mixing in a pool of liquid metal; the RMF pulses within a sequence have been applied with a constant or alternating direction. The resulting flow structure in a cylindrical liquid metal column has been explored by numerical simulations and by model experiments, using the ternary alloy GaInSn. Ultrasonic Doppler velocimetry (UDV) has been used to determine profiles of the vertical velocity. Both the numerical results and the velocity measurements demonstrate the capability of the proposed stirring regimes for overcoming the limited mixing character of conventional rotary stirring. The application of a time-modulated RMF offers considerable potential for providing an optimal flow pattern in a solidifying melt, for reasons of a well-aimed modification of casting properties.

Prediction of Fatigue Performance in Aluminum Shape Castings Containing Defects

Abstract: Fatigue properties of cast aluminum components are controlled by maximum defect size in the material. The larger the maximum defect size, the lower the fatigue strength and life. In the presence of casting defects, crack initiation can be ignored and fatigue life is mainly spent in crack propagation. Therefore, fatigue life of aluminum castings can be predicted by long or short crack growth models. The main problem is defining a starting defect size from readily available data, such as twodimensional (2-D) pore size measurements on metallographic sections. In this article, an extremevalue statistics (EVS) method was used to estimate the maximum defect size in 319 castings from conventional metallographic data. The maximum defect size predicted by EVS agrees quite well with the initiation defect sizes measured from fracture surfaces, and the predicted fatigue life is in reasonable agreement with the experimental data.

Modeling the Effect of Finite-Rate Hydrogen Diffusion on Porosity Formation in Aluminum Alloys

Abstract: A volume-averaged model for finite-rate diffusion of hydrogen in the melt is developed to predict pore formation during the solidification of aluminum alloys. The calculation of the micro-/macro-scale gas species transport in the melt is coupled with a model for the feeding flow and pressure field. The rate of pore growth is shown to be proportional to the local level of gas supersaturation in the melt, as well as various microstructural parameters. Parametric studies of one-dimensional solidification under an imposed temperature gradient and cooling rate illustrate that the model captures important phenomena observed in porosity formation in aluminum alloys. The transition from gas to shrinkage dominated porosity and the effects of different solubilities of hydrogen in the eutectic solid, capillary pressures at pore nucleation, and pore number densities are investigated in detail. Comparisons between predicted porosity percentages and previous experimental measurements show good correspondence, although some uncertainties remain regarding the extent of impingement of solid on the pores.

Understanding Bead Hump Formation in Gas Metal Arc Welding Using a Numerical Simulation

Abstract: Three-dimensional numerical simulations were conducted to study temperature distributions and fluid flows during formation of humped beads in high speed gas metal arc welding (GMAW). Based on simulation and experimental results, the physical mechanisms associated with humping phenomenon were investigated and two conditions responsible for hump formation were identified: the formation of thin liquid channel induced by surface tension pinching force and premature solidification of the melt in the thin channel that divides the weld pool into a front and rear portion. A strong backward fluid flow that produced an accumulation of melt at the rear of the weld pool increased the size of humps. Although surface tension was shown to be important in hump formation, Marangoni flow induced by negative surface tension gradients was not significant for hump formation. The simulation results clarified the fluid flow associated with two different hump shapes. Experimental welds without bead humping were made at a lower travel speed and were also simulated to illustrate the differences in heat and fluid flow from humped beads.

Formation Mechanism of Spinel-Type Inclusions in High-Alloyed Stainless Steel Melts

Abstract: Fundamental thermodynamics of the relationship between high-alloyed stainless steel melts (Fe-20 mass pct Cr-13 mass pct Ni-3 mass pct Si) and the inclusions were investigated. The formation mechanism of the inclusions containing the spinel crystals was developed based on the experimental results and from the compositions of the inclusions in the steel samples taken during plant operations. The molar content of alumina in the inclusions was found to be linearly proportional to the increase of aluminum content, indicating that the inclusions could contain alumina even with less than about 200 ppm aluminum in the steel melt, e.g., steel melts that were mainly deoxidized by silicon. Furthermore, the composition of the inclusions is shown to be a function of the activity of the deoxidizers such as aluminum and silicon in the steel melt. From the analysis of the plant samples, it was found that the contents of MgO and Al2O3 in the calcium silicate type inclusions increased continuously as the steel melt transfers from the argon oxygen decarburization (AOD) converter to the tundish. This composition change in the inclusions originated from the reduction of MgO and Al2O3 in the slags or refractories by silicon in the steel melt. Increases of MgO and Al2O3 contents were prominent in tundish samples, and thus, the spinel phase could be crystallized in the calcium silicate inclusion matrix in the tundish; and finally the spinel crystals grew during cooling of the steel melt through the continuous casting (CC) mold and in the slabs. On the other hand, manganese silicate type inclusions containing chromium oxide were observed after tapping of the molten steel to the ladle. The MnO and Cr2O3 in these inclusions was initially reduced by silicon in the steel melt in the ladle treatment (LT) process, followed by further reduction by aluminum through the LT to the CC mold. The fractions of inclusions containing spinel crystals in cast slabs were negligible at the alumina content of less than about 20 mass pct, while they critically increased at alumina contents greater than about 20 mass pct.

Characterization of Poorly-Crystalline Ferric Arsenate Precipitated from Equimolar Fe(III)-As(V) Solutions in the pH Range 2 to 8

Abstract: The neutralization of equimolar (0.1 M) Fe(III)-As(V) acidic sulfate or nitrate solutions at 22 °C (295 K) over the pH range 2 to 8 yielded a predominantly poorly-crystalline ferric arsenate that resembles its scorodite precursor: FeAsO4 · (2 + x)H2O (where 0 < x < 1). The X-ray powder diffraction (XRD) pattern of it consists of two broad peaks similar to those of two-line ferrihydrite, but clearly different. In addition to ferric arsenate, a small fraction of two-line ferrihydrite was found to be present in the precipitate, increasing in significance with the pH, from around 5 pct at pH 2 to 4 to around 30 pct at pH 8. A field emission gun–transmission electron microscope (FEG-TEM) analysis and a collection of X-ray chemical maps revealed a nanocrystalline structure that is relatively chemically homogeneous in the acidic domain (a constant iron-to-arsenic ratio at 0.98) but becomes progressively disordered and nonuniform at the high pH end. An aqueous phase arsenic concentration was found to increase with the pH and to vary at a fixed pH with the type of solution (SO4 vs NO3) and initial arsenic concentration used. Such variation in solubility appears to be linked to nanodomain structural differences.

Phosphorus partition between liquid steel and CaO-SiO2-P2O5-MgO slag containing low FeO

Abstract: CaO-SiO2-FeOx-P2O5-MgO bearing slags are typical in the basic oxygen steelmaking (BOS) process. The partition ratio of phosphorus between slag and steel is an index of the phosphorus holding capacity of the slag, which determines the phosphorus content achievable in the finished steel. The influences of FeO concentration and basicity on the equilibrium phosphorus partition ratios were experimentally determined at temperatures of 1873 and 1923 K, for conditions of MgO saturation. The partition ratio initially increased with basicity but attained a constant value beyond basicity of 2.5. An increase in FeO concentration up to approximately 13 to 14 mass pct was beneficial for phosphorus partition.

Fe Solubility in the Zn-Rich Corner of the Zn-Al-Fe System for Use in Continuous Galvanizing and Galvannealing

Abstract: The concentration of Al and Fe in molten zinc saturated with delta (∼Fe2Zn10Al) or eta (∼Fe2Al5Zn) intermetallic precipitates has been measured by melt equilibration for temperatures in the range of 450 °C to 480 °C. This information has generated a thermodynamic model for the phase equilibrium in the zinc-rich corner of the Zn-Al-Fe system for the range of compositions and temperatures of commercial interest in continuous galvanizing and galvannealing. The model is constrained by the reported activities of Al in the molten zinc phase and permits small compositional variance of the intermetallic solids. The resultant isothermal phase diagrams are compared with the work of others.

Preparation of Zirconium Metal by the Electrochemical Reduction of Zirconium Oxide

Abstract: An electrolytic production technique based on the electrochemical reduction of zirconium oxide was developed. Various factors affecting the reduction process were investigated. These factors include the cell voltage, the electrolysis time, the composition of the molten bath, and the temperature of the bath. The novel cell design succeeded in the production of zirconium powder or sponge with less than 400 ppm oxygen in a semicontinuous manner.

A Methodology to Predict the Effects of Quench Rates on Mechanical Properties of Cast Aluminum Alloys

Abstract: The mechanical properties of age-hardenable Al-Si-Mg alloys depend on the rate at which the alloys are cooled after the solutionizing heat treatment. Quench factor analysis, developed by Evancho and Staley, was able to quantify the effects of quenching rates on the as-aged properties of an aluminum alloy. This method has been previously used to successfully predict yield strength and hardness of wrought aluminum alloys. However, the quench factor data for aluminum castings is still rare in the literature. In this study, the time-temperature during cooling and hardness were used as the inputs for quench factor modeling. The experimental data were collected using the Jominy end quench method. Multiple linear regression analysis was performed on the experimental data to estimate the kinetic parameters during quenching. Time-temperature-property curves of cast aluminum alloy A356 were generated using the estimated kinetic parameters. Experimental verification was performed on a cast engine head. The predicted hardness agreed well with that experimentally measured. The methodology described in this article requires little experimental effort and can also be used to experimentally estimate the kinetic parameters during quenching for other aluminum alloys.

Coating Weight Model for the Continuous Hot-Dip Galvanizing Process

Abstract: A coating weight model was developed to describe the pressure and wall shear stress distributions as functions of slot gap (d) and impingement distance (Z), for the air knife wiping of the liquid zinc coatings in continuous hot dip galvanizing at ratios of Z/d ≤ 8. This model was then used in validation studies in order to predict the coating weight as a function of the process parameters. The model was based on improved correlations for pressure and shear stress developed by a combination of experimental and computation techniques, which has resulted in more accurate predictions of coating weight validated using industrial coil average coating weight data, particularly for coating weights of up to 75 g/m2. For this region, the maximum deviation between the predicted and measured coating weights was 8 pct. The coating weight model was further developed by incorporating a lumped heat-transfer analysis to predict the solidification “dry line” of the coating. For a typical continuous galvanizing process, the model predicts an 80 pct coating solid fraction for a coating weight of 130 g/m2 to occur at 15 m from the air knives, which agrees qualitatively with visual observations in continuous galvanizing lines.

Continuous Fuming of Zinc-Bearing Residues: Part II. The Submerged-Plasma Zinc-Fuming Process

Abstract: A new high-temperature submerged plasma zinc-fuming process has been developed for the treatment of zinc leach residues, electric arc furnace (EAF) dusts, and other zinc-containing waste materials. Continuous operation of this process requires high zinc-fuming rates while retaining vessel integrity through the formation of a stable freeze lining. A zinc-fuming process model using the FactSage thermodynamic databases and ChemApp thermodynamic software has been developed, which simultaneously describes chemical, thermal, and heat-transfer outcomes of this process. The model has been used to systematically investigate the potential effects of operating parameters such as feed composition, fuel/oxygen ratio, electrical power and fluxing parameters on bath temperature, heat loss, Zn concentration and content in slag, Cu concentration, and content in matte.

The Effects of Surface Roughness and Metal Temperature on the Heat-Transfer Coefficient at the Metal Mold Interface

Abstract: This article focused on the effects of surface roughness and temperature on the heat-transfer coefficient at the metal mold interface. The experimental work was carried out in a unique and versatile apparatus, which was instrumented with two types of sensors, thermocouples, and linear variable differential transformers (LVDTs). The monitoring of the two types of sensors was carried out simultaneously during solidification. The concurrent use of two independent sensors provided mutually supportive data, thereby strengthening the validity of the interpretations that were made. With this type of instrumentation, it was possible to measure temperature profiles in mold and casting, as well as the air gap at the metal mold interface. Commercial purity aluminum was cast against steel and high carbon iron molds. Each type of mold had a unique surface roughness value. Inverse heat-transfer analysis was used to estimate the heat-transfer coefficient and the heat flux at the metal mold interface. A significant drop in the heat-transfer coefficient was registered, which coincided with the time period of the air gap formation, detected by the LVDT. An equation of the form \(h\, = \,\frac{1} {{b^{\ast}A + c}}\, + \,d\) was found to provide excellent correlation between the heat-transfer coefficient and air gap size. In general, an increase in mold surface roughness results in a decrease in the heat-transfer coefficient at the metal mold interface. On the other hand, a rise in liquid metal temperature results in a higher heat-transfer coefficient.

Quench Sensitivity of an Al-7 Pct Si-0.6 Pct Mg Alloy: Characterization and Modeling

Abstract: The quench sensitivity of a cast Al-7 wt pct Si-0.6 wt pct Mg alloy was characterized by tensile tests and scanning electron microscopy. Specimens were cooled from the solution treatment temperature following 58 different cooling paths including interrupted and delayed quenches. Analysis of the microstructure showed that quench precipitates were Mg2Si (β), which nucleated heterogeneously on Si eutectic particles as well as in the aluminum matrix, presumably on dislocations. The quench sensitivity of the alloy’s yield strength was modeled by multiple C-curves, using an improved methodology for quench factor analysis. The three C-curves used in the model represented loss of solute by (1) diffusion of Si to eutectic particles, (2) precipitation of β on Si eutectic particles, and (3) precipitation of β in the matrix. The model yielded a R 2 of 0.994 and a root-mean-square error (RMSE) of 7.4 MPa. The model and the implications of the results are discussed in the article.

Experimental Measurements of Load Distributions on Friction Stir Weld Pin Tools

Abstract: An understanding of the forces acting on the pin of friction stir weld tools is critical to appropriate design, especially in materials with limited toughness such as polycrystalline cubic boron nitride. This article describes a study to measure the longitudinal force distribution on a friction stir weld pin tool. Total longitudinal forces were recorded on a dynamometer while welding 6061 aluminum with pins that varied in length and diameter. A model was developed that characterizes the pin force as a function of pin length and diameter. As the pin length approaches zero, the longitudinal force reaches an asymptote, which is apparently the longitudinal force due to the shoulder. The force due to the pin increases with pin length, but appears not to vary significantly with pin diameter. The force distribution on the pin appears to increase linearly with distance from the shoulder. Unexpected force variation was found at large pin lengths, a result which has yet to be explained.

Computational Modeling of Microstructure Evolution in Solidification of Aluminum Alloys

Abstract: In this article, a front tracking (FT) model and a modified cellular automaton (MCA) model are presented and their capabilities in modeling the microstructure evolution during solidification of aluminum alloys are demonstrated. The FT model is first validated by comparison with the predictions of the Lipton–Glicksman–Kurz (LGK) model. Calculations of the steady-state dendritic tip growth velocity and equilibrium liquid composition as a function of melt undercooling for an Al-4 wt pct Cu alloy exhibit good agreement between the FT simulations and the LGK predictions. The FT model is also used to simulate the secondary dendrite arm spacing as a function of local solidification time. The simulated results agree well with the experimental data. The MCA model is applied to simulate dendritic and nondendritic microstructure evolution in semisolid processing of an Al-Si alloy. The effect of fluid flow on dendritic growth is also examined. The solute profiles in equiaxed dendritic solidification of a ternary aluminum alloy are simulated as a function of cooling rate and compared with the prediction of the Scheil model. The MCA model is extended to the multiphase system for the simulation of eutectic solidification. A particular emphasis is made on the quantitative aspects of simulations.

Phosphorus Partition between Liquid Steel and CaO-SiO 2 -FeO x -P 2 O 5 -MgO Slag Containing 15 to 25 Pct FeO

Abstract: CaO-SiO2-FeO x -P2O5-MgO bearing slags are typically used in the basic oxygen steelmaking (BOS) process. The partition ratio of phosphorus between slag and steel is an index of the phosphorus holding capacity of the slag, which determines the phosphorus content achievable in the finished steel. The influence of factors such as FeO content and basicity on the phosphorus partition ratio was investigated at two different temperatures. The partition ratio initially increased with basicity but remained constant beyond a basicity of 2.5 to 2.6. An increase in the FeO content up to 18 to 20 mass pct was beneficial for the phosphorus partition at a basicity level of 2 to 3, but a higher concentration of FeO resulted in a decrease in the phosphorus partition ratio; the FeO concentration corresponding to this transition varied with basicity and temperature. At even lower basicities, however, the equilibrium phosphorus partition showed either no change, or a marginally decreasing trend, with an increase in the FeO content.

Roasting of La Oroya Zinc Ferrite with Na 2 CO 3

Abstract: The possibility of metals recovery from zinc ferrite residues using transformational roasting processes was examined by roasting zinc ferrite residue from Doe Run Peru’s La Oroya plant (Peru), containing 19.5 pct Zn, 26.6 pct Fe, 750 g/t In, and 520 g/t Ga, with Na2CO3 and leaching with 200 g/L H2SO4 solutions. The X-ray diffraction (XRD) and diagnostic leaching tests indicate that approximately 87 pct of the zinc in this residue is present as franklinite (ZnFe2O4), with the remaining zinc present as entrained ZnSO4 or unleached ZnO. Both preliminary and design of experiments (DOE) testing, using a 22 central composite design (CCD), were performed to test the effects of temperature and a Na2CO3 addition on metals extraction and on the formation of minerals during roasting, and the solubility of these minerals during leaching. Both methods of testing showed that zinc and iron extractions increased with increasing temperature and Na2CO3 additions over the range of conditions tested. Roasting at 950 °C and 80 pct Na2CO3 produced a roasted residue from which 99 pct of the Zn, 88 pct In, and 85 pct Ga could be recovered by leaching, but from which up to 81 pct Fe was also dissolved. Mineralogical analysis using XRD and scanning electron microscopy (SEM)/energy dispersive X-ray (EDX) analysis showed that, for these conditions, ZnFe2O4 decomposes in the presence of Na2CO3 to form ZnO and either α-NaFeO2 or β-NaFeO2. Some of the ZnO formed reacts with Na2CO3 and silicates in the residue to form Na2ZnSiO4 and some unreacted Na2CO3/Na2O/Na2SO4 was also identified after roasting using SEM/EDX. All these phases are dissolved in acid leaching, leaving unreacted ZnFe2O4 and precipitated PbSO4 as the only phases identified in the leach residues. These results indicate that NaFeO2 is formed preferentially to Fe2O3 during roasting and that the NaFeO2 formed during roasting is highly soluble in acidic solutions. The results were also compared with studies on the roasting of more ZnFe2O4-deficient electric arc furnace (EAF) dusts with Na2CO3 or NaOH and indicated that, although roasting with Na2CO3 required higher roasting temperatures to achieve high zinc extractions, much lower Na2CO3 additions are required and higher indium recoveries are possible, if the combination of Na2CO3 roasting and H2SO4 leaching is used.

Determination of Wetting Behavior, Spread Activation Energy, and Quench Severity of Bioquenchants

Abstract: An investigation was conducted to study the suitability of vegetable oils such as sunflower, coconut, groundnut, castor, cashewnut shell (CNS), and palm oils as quench media (bioquenchants) for industrial heat treatment by assessing their wetting behavior and severity of quenching. The relaxation of contact angle was sharp during the initial stages, and it became gradual as the system approached equilibrium. The equilibrium contact angle decreased with increase in the temperature of the substrate and decrease in the viscosity of the quench medium. A comparison of the relaxation of the contact angle at various temperatures indicated the significant difference in spreading of oils having varying viscosity. The spread activation energy was determined using the Arrhenius type of equation. Oils with higher viscosity resulted in lower cooling rates. The quench severity of various oil media was determined by estimating heat-transfer coefficients using the lumped capacitance method. Activation energy for spreading determined using the wetting behavior of oils at various temperatures was in good agreement with the severity of quenching assessed by cooling curve analysis. A high quench severity is associated with oils having low spread activation energy.

A New Lance Design for BOF Steelmaking

Abstract: This article summarizes the outcome of research work carried out to improve the performance of the oxygen lance in the LD steelmaking process. It is stated that the lack of control of the foamy slag and the augmentation of interfacial area creation between the slag and metal are the major hindrances in running the process effectively for improved turn-down steel quality. The ineffectiveness of the existing design in producing liquid metal droplets in the presence of slag foam is explained. In order to augment the droplet generation, a new oxygen lance with a central subsonic nozzle through which flow can be controlled has been introduced and its blowing performances studied using numerical and water model studies. The jet characteristics studied in the numerical simulations show no jet coalescence. The interferences of the jets with the bath have further been analyzed by hydrodynamic model studies. It has been found that the droplet generation rate improves significantly due to the presence of the central jet. Further, it has been observed that controlling the flow rate through the central hole can be used as an effective process control tool.

On the Microstructure of a Freeze Lining of an Industrial Nonferrous Slag

Abstract: Labscale freeze layers of an industrial nonferrous slag with Al2O3-CaO-FeOx-MgO-SiO2-ZnO as main components are studied to explore the microstructure and the composition of an industrial freeze lining. The freeze layers were formed by submerging a watercooled probe into a liquid slag bath. The influence of submergence time, of heat input from the furnace, and of the rotational speed of the crucible is studied. The microstructures of the freeze layers are examined using scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and X-ray diffraction (XRD). Thermodynamic software is used to interpret the solidification microstructure. The results show that the freeze layer microstructure consists of different zones, depending on the local thermal history. These zones show different growth morphologies and different microstructure scales, from an amorphous matrix with small crystals to large columnar crystals. Furthermore, two microstructure types are observed, one with melilite columnar crystals and the other with olivine columnar crystals. These microstructure types appear for similar experimental conditions and are even observed within the same freeze layer. An increase in submergence time or in heat input from the slag bath does not seem to favor a particular microstructure type. A high rotational speed of the crucible resulting in a higher convection in the slag bath seems to favor the microstructure type with olivine columnar crystals.

Simulating the Residual Stress in an A356 Automotive Wheel and Its Impact on Fatigue Life

Abstract: Keeping the weight of unsprung rotating components low is critical for fuel efficiency in automobiles; therefore, cast aluminum alloys are the current material of choice for wheels. However, pores formed during solidification can combine with residual stresses and in-service loads to reduce the fatigue life of this safety critical part. In this study, a model of the residual stresses arising from the quench stage of a T6 heat treatment was developed. The resulting predictions were compared to residual strain measurements made on quenched wheels via a strain gage/sectioning technique. The predictions were shown to be sensitive to the alloy’s flow stress behavior, yet no data were available for the temperature- dependent and strain-rate-dependent inelastic behavior of A356 in the as-solutionized condition. Measurements of this behavior were made using a GLEEBLE 3500, and the data were incorporated into the model, significantly improving the correlation between model and experiment. In order to determine the influence of residual stress upon the final fatigue performance of the wheel during service, the change in stress level due to machining was first calculated. The residual stress was then compounded together with a service stress to determine the local stress at all points in the wheel during idealized operation. Finally, the fatigue behavior was predicted using a unified initiation and propagation model based on this local stress and an idealized pore size.

Kinetics and Reaction Mechanism of Soda Ash Roasting of Ilmenite Ore for the Extraction of Titanium Dioxide

Abstract: Ilmenite by virtue of being rich in iron often has small concentrations of impurities such as Mn3O4, Al2O3, CaO, Cr2O3, and MgO. Removal of iron and other impurities is essential for the production of pigment grade TiO2. An investigation of the removal of iron was undertaken by roasting the mineral ilmenite with soda ash in air above 873 K. A ternary phase was identified, which played an important role in the overall kinetics. The dependence of temperature, time, mixture composition, and the microstructure of the reaction layer were studied in order to establish the reaction mechanism. We report the results of overall kinetics of roasting reaction by analyzing the activation energy in the temperature range of 873–1173 K and also explain the reaction mechanism. The derived value of activation barrier compares well with the diffusion rates of Fe2+ and Fe3+ ions in the ilmenite lattice.

Multistage Fatigue Modeling of Cast A356-T6 and A380-F Aluminum Alloys

Abstract: This article presents a microstructure-based multistage fatigue (MSF) model extended from the model developed by McDowell et al.[1,2] to an A380-F aluminum alloy to consider microstructure-property relations of descending order, signifying deleterious effects of defects/discontinuities: (1) pores or oxides greater than 100 μm, (2) pores or oxides greater than 50 μm near the free surface, (3) a high porosity region with an area greater than 200 μm, and (4) oxide film of an area greater than 10,000 μm2. These microconstituents, inclusions, or discontinuities represent different casting features that may dominate fatigue life at stages of fatigue damage evolutions. The incubation life is estimated using a modified Coffin–Mansion law at the microscale based on the microplasticity at the discontinuity. The microstructurally small crack (MSC) and physically small crack (PSC) growth was modeled using the crack tip displacement as the driving force, which is affected by the porosity and dendrite cell size (DCS). When the fatigue damage evolves to several DCSs, cracks behave as long cracks with growth subject to the effective stress intensity factor in linear elastic fracture mechanics. Based on an understanding of the microstructures of A380-F and A356-T6 aluminum alloys, an engineering treatment of the MSF model was introduced for A380-F aluminum alloys by tailoring a few model parameters based on the mechanical properties of the alloy. The MSF model is used to predict the upper and lower bounds of the experimental fatigue strain life and stress life of the two cast aluminum alloys.

Continuous fuming of zinc-bearing residues: Part I. Model development

Abstract: The disposal of zinc-containing residues is an important sustainability issue currently facing the metallurgical industry. These residues can be treated by using zinc slag-fuming processes in which heavy metals are chemically reduced and evaporated from a molten slag bath. Continuous operation of these processes requires high zinc-fuming rates while retaining vessel integrity. To meet these challenges, a new generation of technologies is being developed that depends on the formation of a freeze lining on the internal reactor walls. A general zinc-fuming process model that simultaneously describes the chemical reactions, phase equilibria, and thermal- and heat-transfer outcomes of these processes has been developed using the FactSage thermodynamic databank system and the ChemApp programmer’s library for thermochemical applications. This simultaneous description of these processes allows the model to be used as part of a new approach to the design of freeze linings, namely, the use of slag engineering and composition adjustment to obtain optimum process efficiency and freeze lining behavior.

Electrical Resistivity of Metal Powder Aggregates

Abstract: An equation for calculating the electrical resistivity of a compressed powder mass, consisting of oxide-coated metal particles, has been derived. In addition to the intrinsic interest of the problem, the proposed equation is useful for describing the very early stages of electrical sintering processes. The problem is approached in a new way, relating the actual powder system to an equivalent system consisting of deforming spheres in a simple cubic packing, which is much easier to examine. The proposed equation was experimentally verified from measurements of the electrical resistivity for aluminum, bronze, iron, and nickel powders under pressure. The consistency between theoretical predictions and experimental results was reasonably good in all cases.

Electrochemical Deoxidation of Titanium Foam in Molten Calcium Chloride

Abstract: Titanium foam, prepared by using a patented powder-metallurgy–based process involving a powder blend that was molded, foamed, and sintered using a three-step thermal treatment, was deoxidized in a molten CaCl2 bath. The polarization experiments were carried out by cathodically polarizing the foam (working electrode) against a counter (graphite) electrode. Under constant potential (polarization) mode, the dominant mechanism of deoxidation was the ionization of oxygen, present in the foam, and its subsequent discharge, as CO2/CO, at the anode surface. More than ∼85 pct oxygen could be effectively removed by carrying out the electro-deoxidation experiments in fresh and pre-electrolyzed melt(s) at an electrolyte temperature of 950 °C. Scanning electron microscopy–energy dispersive X-ray (SEM-EDX) detection of the deoxidized foams did not show a presence of any inclusion(s) or secondary phase(s).

A Density Model for Multicomponent Liquids Based on the Modified Quasichemical Model: Application to the NaCl-KCl-MgCl 2 -CaCl 2 System

Abstract: A theoretical model based on the modified quasichemical model is presented for the density of multicomponent inorganic liquids such as molten salts. By introducing in the Gibbs energy of the liquid phase temperature-dependent molar volume expressions for the pure components and pressure-dependent excess parameters for the binary (and, if necessary, higher-order) interactions, it is possible to reproduce and eventually predict the molar volume and the density of the multicomponent liquid phase using standard interpolation methods. The model is applied to the NaCl-KCl-MgCl2-CaCl2 liquid solutions. No ternary pressure-dependent model parameters were required; the binary pressure-dependent parameters suffice to reproduce satisfactorily the experimental density data available for the NaCl-KCl-MgCl2, NaCl-KCl-CaCl2, NaCl-MgCl2-CaCl2, KCl-MgCl2-CaCl2, and NaCl-KCl-MgCl2-CaCl2 liquids. This is the first of two articles on the density model. In a subsequent article, the model is applied to the NaF-AlF3-CaF2-Al2O3 base electrolyte used for the electroreduction of alumina in Hall–Heroult cells.