1997 TMS Annual Meeting: Monday Session

APPLICATIONS OF SENSORS AND MODELING TO MATERIALS PROCESSING: Session II

Session Chairs: B.G. Thomas, Dept of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign, 1206 West Green Street, Urbana, IL 61801; B.Q. Li, Dept of Mechanical Engineering, Louisiana State University, Baton Rouge, LA 70803

2:00 pm

NUMERICAL ANALYSIS OF FLOATING ZONE REFINING PROCESSES: S.P. Song, B.Q. Li, Department of Mechanical Engineering, Louisiana State University, Baton Rouge, LA 70803

A numerical model is developed to represent complex electromagnetic, thermal and free surface deformation phenomena in floating zone refining and single crystal growth processes. The model is developed using a coupled boundary element and finite element method, with finite element meshes used for the melting zone region and boundary elements for the exterior region or free space. The free surface deformation model is developed using the weighted residual method. With the model, the complex transport and free surface phenomena in a floating, zone system are studied as a function of various operating conditions including applied current, frequency, inductor position and shape, surface tension, floating zone diameter and height. Model development and numerical results are presented.

2:25 pm

MODELING OF SOLIDIFICATION AND VELOCITY OF ATOMIZED MOLTEN DROPLET DURING ATOMIZATION AND SPRAY FORMING: Y.H. Su, C.-Y. A. Tsao, Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan, China

A mathematical model to describe the solidification behaviors of atomized droplets during flight, in terms of nucleation temperature, recalescence temperature, nucleation position, solid fraction at nucleation temperature, and droplet temperature and velocity, is formulated. The concept of transient nucleation is applied to model such short nucleation event. A maximum droplet velocity exists, beyond which droplet velocity shows an inflection phenomenon during the flight. For shorter flight distance, smaller droplet is faster to reach a given flight distance; however, for longer flight distance, the situation is reversed. Variations of the gas flow patterns have more effects on smaller droplet, and the effects are more significant at longer flight distance. A minimum surface heat transfer coefficient exists as the droplet flies. Prior to nucleation or recalescence, smaller droplet has lower temperature at a given flight distance, and has lower nucleation temperature. Medium size droplet flies over the shortest flight distance before the nucleation starts. Smaller droplet has larger solid fraction at the end of recalescence. Atomization gas has more effects on droplet momentum than on the heat content of the droplet.

2:50 pm

THE EFFECT OF FORCED COOLING A PERMANENT COMPOSITE MOLD ON AIR GAP FORMATION: D.R. Gunasegaram1, D. Celentano2, T.T. Nguyen1, 1CRC for Alloy and Solidification Technology (CAST) and CSIRO Division of Manufacturing Technology, Locked Bag 9, Preston 3072, Australia; 2International Center for Numerical Methods in Engineering, Edificio C-1 Campus Norte-UPC, Gran Capitan, s/n. 08034 Barcelona, Spain

It is well known that the air gap that forms between casting and mold during the solidification process of an aluminum alloy substantially alters the rate of heat transfer at this interface. This paper reports studies on the effect of force cooling a composite permanent mold on the initiation and growth of the air gap. Interesting comparisons are made with the case where no forced cooling is employed. The two experiments are simulated using a fully coupled thermo-mechanical model called VULCAN, a finite element code, and its temperature and displacement predictions are validated. The air gaps are measured using LVDTs. The alloy used is A356, and the mold comprises H13 steel and beryllium-copper. Air jets are used to force cool the mold component surrounding an isolated thick section of the casting. The inverse heat conduction problem is solved in order to obtain boundary conditions for VULCAN.

3:15 pm BREAK

3:25 pm

EFFECT OF TRANSVERSE DEPRESSIONS AND OSCILLATION MARKS ON HEAT TRANSFER IN THE CONTINUOUS CASTING MOLD: B.G. Thomas, D. Lui, B. Ho, G. Li, Y. Shang, Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign, 1206 West Green Street, Urbana, IL 61801

Results from mathematical models and plant experiments are combined to quantify the effect of transverse depressions and oscillation marks on heat transfer in the continuous casting mold. A heat transfer model has been developed to calculate transient heat conduction within the solidifying steel, coupled with the steady-state heat conduction with the continuous casting mold wall. The model features a detailed treatment of the interfacial gap between the shell and mold, including mass and momentum balances on the solid and liquid powder layers. The model predicts the solidified shell thickness down the mold, temperature in the mold and shell, thickness of the resolidified and liquid powder layers, heat flux distribution down the mold, mold water temperature rise, ideal taper of the mold walls, and other related phenomena. The important effect of non-uniform distribution of superheat is incorporated using the results from previous 3-D turbulent fluid flow calculations within the liquid cavity. Results from plant experiments confirm that transverse surface depressions and oscillation marks form at the meniscus and move down the mold. Measurements of mold thermocouple temperatures and breakout shell thickness were used to calibrate the models. The predicted local surface temperature fluctuations were consistent with transient mold temperature measurements. The results indicate that the surface depressions and oscillation marks are filled with mold flux, but still have a significant effect on decreasing heat transfer, especially locally. Insights are gained into the formation of associated surface cracks and breakouts.

3:50 pm

CHANGE OF PATTERN OF SOLIDIFICATION IN INGOTS CAST FROM THE BOTTOM, REPRESENTED BY A MATHEMATICAL MODEL AND ITS EFFECT ON THE APPARITION OF DEFECTS IN THE SEAMLESS TUBES OF C.V.G SIDOR: J. Rodriguez, A. Valles, C.V.G SIDOR, Puerto Ordaz, Venezuela, M. Rao, Universidad Nacional Experimental de Guayana, Puerto Ordaz, Venezuela

This paper shows a mathematical model based on finite elements, applied to bottom cast ingots of C.V.G Sidor Plant. The model was used as a strategy to determine the thermal effects that are produced when the solidification conditions are modified, without interfering with the production process. The modification consisted of placing a thermal insulant on top of mould. The results obtained by the model indicated that the solidification time increases with insulant on top of the mould. With these results a significant number of casts were run with and without insulant. Later by means of a metallurgical analysis it was determined that the ingots cast with insulant reduce the level of nonmetallic inclusions and the presence of internal blistering in seamless pipes.

4:15 pm

MATHEMATICAL MODELING AND EXPERIMENTAL MEASUREMENTS OF EXOTHERMIC PHENOMENA IN NON FERROUS SYSTEMS: S.A. Ferenczy, S.A. Argyropoulos, Dept of Metallurgy and Materials Science, University of Toronto, 184 College Street, Toronto, Ontario, Canada M5S 3E4

Microexothermic and macroexothermic phenomena have been indentified various non-ferrous systems, which from recent experimental and mathematical studies have been shown to enhance heat and mass transfer. This paper will present experimental results and computer simulations describing the transient exothermic dissolution of nickel cylinders into liquid aluminum. Axisymmetric heat, mass and momentum equations the SIMPLER algorithm was modified to incorporate phase change and the microexothermic macroexothermic events. Dissolution experiments were performed and the results compared to the mathematical simulation. The development of coupled temperature, concentration and momentum boundary layers are examined.