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Session Chairperson: Dan J. Thoma, Los Alamos National Laboratory, Materials Science Technology Division, MS G770, Los Alamos, NM 87545
OPTIMIZATION OF THE ANGULAR WIDENING OF A CAST PLATE: J. Stetina, F. Kavicka, Technical University of Brno, Mechanical Engineering Dept., Technicka 2, 616 69 Brno, CZECH REPUBLIC; J.M. Khodadadi, Mechanical Engineering Dept., Auburn University, Auburn, AL 36849-5341
A 2-D numerical model was used to analyze the temperature field of a vertically cast 1700x600x40mm plate with different angles of widening in its cross section (0 to 4.4°). From the side of the widening, the plate was either isolated (using Plastizol) or non-isolated. A non-isolated plate has an optimum angular widening of 3° and the widening of 2° lies on the boundary of the origins of internal defects (e.g. shrinkages). The isolated plate has an optimum angle of widening of 2°. Using the calculations it is possible to predict that the plate itself will have the same internal quality with a widening of 3° after crystallization without isolation as with 2° of widening with the isolation. The knowledge gained in this study can lead to the reducing of the volume of liquid metal needed and to the lowering of costs for working the surface. (Authors are grateful to the US-Czech Science and Tech. Program for support of this research.)
TEXTURE EVOLUTION IN -TiAl SHEETS PRODUCED VIA THE MELT OVERFLOW RAPID SOLIDIFICATION PROCESS: M.L. Weaver, Center for Nonlinear and Nonequilibrium Aeroscience, Florida A&M University, 1800 E. Paul Dirac Rd., Rm. A337, Tallahassee, FL 32306-4005; H. Garmestani, Dept. of Mechanical Engineering, FAMU-FSU, College of Engineering, Florida A&M University, Tallahassee, FL 32306; G. Das, United Technologies, Pratt & Whitney, P.O. Box 109600, West Palm Beach, FL 33410-9600
The production of -TiAl foils by conventional ingot metallurgy involves casting ingots, hot forging ingots into billets, followed by several hot rolling, heat treatment, and surface grinding sequences. These methods, however, can result in production losses in excess of 50%. Recently, researchers at NASA-Langley and Ribtec have developed the Plasma Melt Overflow Process which combines plasma arc skull melting in a water cooled copper crucible with Melt Overflow Rapid Solidification Technology (MORST) allowing near-net-shape processing of strips that can be hot pack rolled to foil gauge with minimal production losses. Preliminary results indicate that foils produced via this method exhibit finer more uniform microstructures, higher strength, and higher ductility than comparable ingot metallurgy foils. In this investigation texture evolution has been evaluated in -TiAl alloys sheets produced via this technique. The results are compared with recent studies of texture in conventionally processed -TiAl.
LIQUID METAL-SOLID METAL REACTIONS WITH INTERMEDIATE INTERMETALLIC COMPOUND FORMATION: Robert J. Hanrahan Jr., Dan J. Thoma, Loren A. Jacobson, Los Alamos National Laboratory, Materials Science Technology Division-MST-6, TA 3, MS G770, Los Alamos, NM 87545
The study of liquid metal reactions have in most cases involved systems where the solid is dissolved by the liquid. The case where an intermetallic compound forms between the two metals has not been thoroughly investigated. As an example of this class of system we have chosen to study the reaction of Beryllium (solid)--with Rare Earths (liquid). Although most of the Be-RE phase diagrams are poorly defined in terms of solubility and the location of the eutectics, all have been characterized to the extent of having a single intermetallic compound with a melting point in excess of 1500°C and two simple eutectics with minimal solubility range in the solid phases. The intermetallic in each case is the same structure, Mbe13. The reaction kinetics, measured using the thickness of the intermetallic layer at various times, can be described by a parabolic model suggesting that the reaction is controlled by diffusion through the intermetallic. Variations in kinetics are observed between the various systems which are analyzed considering factors such as melting point of the liquid (eutectic temperature), Be solubility in the liquid, and compositional range of the intermetallic.
POTENTIAL OF SQUEEZE CASTING ALUMINUM (Al) AND ALUMINUM-REINFORCED METAL MATRIX COMPOSITES (Al-MMC) FOR BOTH THE AUTOMOTIVE AND ARMY GROUND VEHICLE APPLICATIONS: Paul J. Huang, Army Research Laboratory, AMSRL-MA-B, APG, MD 21005; Robert Purgert, Precision Metal Forming Co., P.O. Box 25441, 9545 Midwest Ave., Garfield, OH 44125
The main driving forces for lightweight materials for the automotive industry and military applications are cost effectiveness, high-strength, and wear resistance. Precision Metal Forming Company (PMF) has developed a new and innovative squeeze casting process called metal compression forming (MCF). MCF integrates the deceptively simple concept of solidification of metal under direct pressure with closed die forging and low-pressure permanent-mold fill technologies. This hybrid process therefore combines the advantages of traditional direct squeeze casting and low-pressure permanent-mold casting. This study discusses property advantages attained with this process over traditional aluminum casting processes.
3:20 pm BREAK
KINETIC COMPETITION DURING DUPLEX PARTITIONLESS SOLIDIFICATION: Donald R. Allen, John H. Perepezko, Dept. of Mat. Science and Eng., University of Wisconsin - Madison, Madison, WI 53706
A structure composed of both partitionless face-centered cubic (fcc) and bodycentered cubic (bcc) phases has been observed following high undercooling solidification of nickel-vanadium (Ni-V) alloys for the composition range 4751.7 at.% V. Transmission electron microscopy analysis has identified regions in which fcc and bcc grains on the order of 100 nm in size coexist in a duplex structure. Splat-quenched foils were analyzed both in plan view and cross section. This structure has revealed a unique set of kinetic conditions that can allow for duplex solidification, since similar nucleation and growth rates are required for a range of compositions. A nucleation and growth kinetics analysis has been developed to describe the conditions under which this structure may form. The duplex partitionless structure previews an entirely new class of microstructures achievable during high undercooling solidification processing. The support of NASA (NAG8-1278) is gratefully acknowledged.
MICROSTRUCTURAL EVOLUTION IN Al-Cu-Si ALLOY MELT SPUN RIBBONS: K.-P. Cooper, J D. Ayers, H.N. Jones, R. Vardiman, Naval Research Laboratory, Code 6321, Washington, DC 20375-5343
Several Al-Cu-Si alloys were melt spun to produce stable, fine grain microstructures that would be amenable to superplastic deformation and consolidation. Scanning electron microscopy of the ribbon cross-sections showed two distinct alternating microstructural morphologies suggesting transitions in solidification behavior. One consisted of intimately interlocked Al and A12Cu () phases with dispersed Si. The other consisted of equiaxed dendritic Al with inter-dendritic and Si. The latter was found usually in the middle portion of the ribbon cross-section. The solidification mechanism for the interlocked structure is not understood, but its formation may have involved multiple nucleation of the phase or a degeneration of a more regular, but unknown, initial structure as the ribbon cooled to ambient temperature. The equiaxed dendritic structure probably arose as a result of independent nucleation events in the supercooled liquid ahead of the solid-liquid interface. Transmission electron microscopy revealed Al grain sizes varying from 1µm near the wheelside to 8µm with subgrains near the free surface and different shapes and sizes for and Si in the interlocked and dendritic regions.
HEAT TRANSFER AND SOLIDIFICATION IN UPCASTING OF COPPER: K. Harkki, L. Holappa, Helsinki University of Technology, Laboratory of Metallurgy, Vuorimiehentie 2, FIN-02150 Espoo Finland
An extensive study has been conducted to find out heat transfer and solidification behaviour in the copper upcasting mold. The casting direction in the upcasting process is vertically upwards and it has applications for copper and copper based alloys. The study combined industrial scale measurements, mathematical modeling and me/allographic examination of the cast rod samples. The industrial scale measurements involved temperature measurements with 14 thermocouples inserted in the copper jacket of the mold. Also temperature measurements of the cooling water were carried out. Axial heat flux profiles were determined quantitatively from the temperature measurements and they were used as boundary condition for the mathematical model calculations. The heat flux was observed to have a maximum value near the meniscus and to decrease rapidly with increasing distance up to the mold. Temperature profiles in the cast rod were simulated using a model based on finite element method. The results of the simulations were utilized for further understanding of the heat transfer and solidification process in the mold. Copper rod samples were examined to evaluate crystal growth of the cast copper rod. The results of the investigation work can be utilized to optimize the copper upcasting process.
RISERING EFFECT ON THE MICROSTRUCTURE OF ASTM F-75 INVESTMENT CASTINGS: M. Castro, L.F. Ramirez, M. Herrera, H. Maucha, M. Mendez, J. Mendez, Centro de Investigacion y Estudios Avanzados del IPN-Unidad Saltillo, Carr. Saltillo-Monterrey Km. 13, 25000 Saltillo, Coahuila, Mexico
As-cast mechanical properties in dentritic alloys, as Co-based materials, are strongly dependent on the porosity level developed during the solidification process. High ductility levels can only be achieved when the porosity is completely eliminated. In such a high reactive materials, to avoid the problems conveyed by the high temperatures required to produce sound porosity-free castings, it is necessary to explore other solutions as those applied in conventional casting process for defect elimination. In this study, we report the effect of different size risers on the porosity formation in investment cast ASTMF-75 alloys. The liquid metal at 1430°C was poured into cylindrical ceramic molds preheated at 900°C. The morphology as well as the location of the porosity changed with the riser size. An intergranular porosity situated in the center section was observed when it was applied to the biggest size riser. When using the smallest size riser, an additional elongated, and peripherically situated porosity appeared. The observed changes in porosity are associated with the microstructural changes; while in the former case the grains are equiaxed, in the second case the grains are columnar. As the dendritic arm spacing showed that the cooling rate was very similar in both cases, the results are explained considering the nucleation mechanisms.
THE CONTROL OF DENDRITE TIP GROWTH RATE AS A WAY TO PROMOTE GOOD MICROSTRUCTURES IN DENRITIC ALLOYS: H. Mancha, F. Cepeda, M. Herrera, J. Mendez, M. Castro, M. Mendez, Centro de Investigacion y de Estudios Avanzados del IPN-Unidad Saltillo, Carr. Saltillo-Monterrey Km. 13-Apdo. Postal 663, 25000 Saltillo, Coahuila, Mexico
As-cast mechanical properties, in particular ductility, of equiaxed and dendritic alloys such as Co-based alloys and Al-based alloys are strongly dependent on size and spatial distribution of second phase particles. In this paper, we present the results of a mathematical modeling study on the solidification process of these type of alloys. The effect of cooling parameters and nucleating conditions, in the grain growth kinetics of denritic structured grains, are presented and compared with experimental results. High nucleation rates as well as low cooling rates produce small grain size with large intergranular second phase particles. On the opposite side, low nucleation rates and high cooling rates allow the formation of large grain size with small fractions of intergranular second phase particles and high volume fractions of intragranular smaller second phase particles. These intragranular particles have a different chemical composition to those located in grain boundaries. In the case of Co-based alloys, it seems to be necessary to complete elimination of large intergranular lamellar structured carbides to obtain the 8 percent elongation to fracture established by the ASTM F-75 international standard. The grain growth kinetics control, based on the control of the dendrite tip growth rate, offers a way to obtain grain microstructures that allow the best as-cast mechanical properties. The same principle is applicable to the Al-based alloys. The mathematical model predictions on volume fraction and spatial distribution of second phase particles are discussed and compared with the experimental results.
MODELLING QUENCHED IN SHORT-RANGE ORDER IN METALLIC ALLOYS: A. Varschavsky, E. Donoso, Universidad de Chile, Facultad de Ciencias Flsicas y Matematicas IDIEM, Casilla 1420, Santiago, Chile
Using a rate vacancy loss equation and an overall rate constant for order establishment, a model for describing the roles of quench temperature, quench rate and vacancy sink density is proposed. This model allows computation of the retained value of the first short-range order parameter of quenched binary alloys and the prediction that the most highly disordered state obtainable occurs with quenches from intermediate temperatures. Numerical results were tested against experimental data based on energetic analysis of differential scanning calorimetric traces pertaining to a reordering process towards the equilibrium state. Kinetic evaluations of these traces yield experimental estimates of quenched-in vacancy concentrations. Good agreement between modelled and experimental assessments was obtained in Cu at.% Zn alloy.
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