1997 TMS Annual Meeting: Thursday Session

CAST SHOP TECHNOLOGY: Session VIII: D.C. Casting

Session Chairperson: Robert B.Wagstaff, Wagstaff, N.3910 Flora Rd., Spokane, WA 99216

8:30 am

COMPREHENSIVE EVALUATION OF THREE REFRACTORY MATERIALS FOR HOT-TOP BILLET CASTING SYSTEMS AT GOLDENDALE ALUMINUM COMPANY: J. Martin Ekenes, Consultant, N.3418 Arden Road, Otis Orchards, WA 99027; Larry Bennett, Goldendale Aluminum Company, Craig Johnson, Permatech Inc.

The life of refractory components in billet hot-top casting systems is often the factor limiting mold package life. Evaluation of new materials in commercial casting operations requires careful planning in order to assure integrity in data collection. A full scale test spanning a five month period was conducted at Goldendale Aluminum Company comparing two silica based precision castable refractories and a graphite reinforced calcium-silica board. Comprehensive records facilitated testing a variety of hypotheses. This paper describes plan development, implementation, data collection, analysis, and the conclusion drawn.

8:50 am

CALCIUM CONTAMINATION OF MOLTEN Al-Mg ALLOYS BY CALCIUM CARBONATE POWDER: Dennis D. Yancey, Dept. of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139; David H. DeYoung, Aluminum Company of America, Molten Metal Processing Center, Alcoa Technical Center, Alcoa Center, PA 15069

A bench-scale study has been conducted to determine the effects of melt temperature, alloy composition, and powdered calcium carbonate quantity on Ca contamination of molten Al-Mg alloys. Calcium carbonate powder is often used in ingot plants as a "caulking" material in troughs, joints, etc. to prevent molten metal leaks. Results of the study show that: (i) at melt temperatures above 1425°F, calcium carbonate markedly increases the concentration of Ca in molten Al-4.5 wt% Mg alloy; (ii) Mg is required for the reaction of CaCO3 with the aluminum melt to occur; (iii) for a given quantity of calcium carbonate, within the temperature range of 1400-1550°F, increase in melt temperature increases Ca contamination; (iv) for a given temperature of 1450°F or 1550°F and within 0.008-0.36 wt% CaCO3, increase in the quantity of calcium carbonate markedly increases both the rate of Ca contamination and the maximum concentration of Ca obtained in the melt for a 5 hour interaction period. The decomposition of calcium carbonate and the associated Ca contamination of molten Al-Mg alloy appears to be a kinetically-driven phenomenon.

9:10 am

INVESTIGATION OF THE EFFECT OF DIFFERENT METAL FEEDING SYSTEMS ON CAST STRUCTURES USING A WATER MODEL: A. Buchholz, S. Engler, M.-S. Ji, W. Schmitz, Foundry Institute, RWTH Aachen, Intzestr.5, D-52056 Aachen, Germany

Forced convection imposed by the feeding system plays an important role in the solidification of continuous casting processes. Convection modifies heat and mass transport and thus has a significant impact on the cast structure. A better understanding of the fluid dynamics promises an improvement of continuous casting processes. To investigate flow patterns and the influence of different inlet systems on cast structures a water model was designed maintaining dynamic similarity between model and real casting equipment. Sump profiles were derived from etch lines achieved from real castings. The flow patterns were studied for different metal distributors comprising horizontal, vertical, inclined spouts and diffusor bag. The effect of the different flow patterns is correlated with results of metallographic analysis. The experiments are reviewed by some aspects of numerical simulation. The results show that physical modelling remains an instructive tool to study the dynamic behaviour of transient flow phenomena.

9:30 am

PHYSICAL (BY PARTICLE IMAGE CELOCIMETRY) AND MATHEMATICAL MODELING OF METAL DELIVERY DEVICES FOR EM AND DC CASTING OF ALUMINUM: Dong Xu, J.W. Evans, Dept. of Materials Science and Mineral Engineering, University of California, Berkeley CA 94720; Daniel P. Cook, Corporate Research & Development, Reynolds Metals Company, Richmond, VA 23261-7003

A half-section water model of a pilot scale caster (at RMC, Richmond, Virginia) has been constructed at UC Berkeley. A particle imaging velocimetry (PIV) system has been assembled to measure velocities in the model for various simulated nozzle and bag geometries. The PIV system is a relatively inexpensive system using an incandescent light source, rather than the usual high-power laser. A digital camera, microcomputer and software (Optical Flow Systems) track the frame-to-frame movement of neutral-density particles in the water and provide vector maps of the velocity. Measured velocities have been compared to velocities computed using the finite element package FIDAP and show good agreement.

9:50 am

INFLUENCE OF FLUID FLOW FIELD AND POURING TEMPERATURE ON THERMAL GRADIENTS IN THE MUSHY ZONE DURING LEVEL POUR CASTING OF BILLETS: G.-U. Grün, W. Schneider, VAW aluminium AG, Research and Development, Georg-von-Boeselager-Str.25, D-53117 Bonn, Germany

The occurrence of feathery crystals during experimental casting trials on a level pour casting unit yields a strong relationship as well to the pouring temperature as to the governing fluid flow pattern within the liquid melt pool of the billet. Three-dimensional simulations of the coupled fluid flow and heat transfer problem during the stationary phase of the level pour casting process are used to investigate the variations of the thermal field in the liquid part of the solidifying billet. The calculated flow field patterns and temperatures are discussed in dependency of casting velocity, pouring temperature and size of pouring gate. The resulting thermal gradients in the mushy zone over the circumference of the billet are correlated to microstructure analysis of the corresponding casting trials.

10:10 am BREAK

10:10 am

OPEN MOLD WITHOUT DISTORSION FOR ALUMINUM DC CASTING: MODELING AND TESTING THE OPTIMUM SHAPE AND MATERIAL: B. Hannart, Pechiney Rhenalu, Centr'Alp-BP24, F-38340 Voreppe, France; O. Bonnet, Pechiney CRV, Centr'Alp-BP27, F-38340 Voreppe, France; A. Noraz, Aluminium Pechiney, Centr'Alp-BP27, F-38340 Voreppe, France

The thermomechanical behaviour of open molds used for DC casting of aluminum slabs was analysed, and their design was optimized, in order to avoid thermal distorsion. The analysis used a thermomechanical finite element model, describing the mold geometry, the material properties, and the heat transfer between mold, molten metal and cooling water. The deformations observed during and after the casting were realistically reproduced, both in 3D and in 2D. The 2D model, allowing to test new designs in less than 10 minutes, was used to propose several solutions for reducing the residual distorsion of the mold. The different solutions were tested in industrial conditions at Aluminium Dunkerque, and mold distorsions were systematically measured. With the optimum choice of mold geometry and alloy, the residual distorsion was completely eliminated.

10:40 am

DIRECT CHILL CASTING OF ALUMINUM ALLOYS: INGOT DISTORSIONS AND MOLD DESIGN OPTIMIZATION: J.-M. Drezet, M. Rappaz, Laboratoire de Métallurgie Physique, Ecole Polytechnique Fédérale de Lausanne, MX-G, CH-1015 Lausanne, Switzerland

During the direct chill (DC) semi-continuous casting of aluminum alloys, the metal experiences high thermal stresses which are partially relaxed by deformation. This deformation is responsible for three main ingot distorsions: butt curl, butt swell and non-uniform rolling faces pull-in. These distortions are detrimental to the productivity of the process because they require butt sawing and more ingot scalping before rolling. On the other hand, residual stresses may induce longitudinal cracking of the cold ingots. Under pseudo steady-state conditions, i.e. after nearly one meter of casting, the solidified shell contracts towards the liquid pool (Pull-in). This contraction which amounts to about 9% at the lateral faces center is only 2% at the ingot corner. If a rectangular mold is employed, the resulting ingot is therefore concave ("bone shape"). To compensate for this non-uniform contraction of the ingot, the sides of the mold are designed with a convex shape, usually with three linear segments. Nevertheless, instead of producing flat rolling sheet ingots, such molds produce W-type ingot cross section. A comprehensive 3D mathematical model based upon the Abaqus software has been developed for the computation of the thermomechanical state of the solidifying strand during DC casting and subsequent cooling of rolling sheet ingots. Based upon a finite element formulation, the model determines the temperature distribution, the stresses and the associated deformations in the metal. This paper concentrates on the non-uniform contraction of the lateral faces and shows comparisons between computed and measured ingot cross-sections after complete cooling. Finally, the influence of the mold design on the final ingot cross-section is assessed and the use of an inverse method for mold design optimization is presented.

11:00 am

WATER COOLING IN DIRECT CHILL CASTING: PART 2, EFFECT ON BILLET HEAT FLOW AND SOLIDIFICATION John Grandfield, Comalco Research Centre, P.O.Box 316, Thomastown, Victoria 3074, Australia

Water cooling plays an important role during DC casting. Control of the water cooling is essential for good process performance. In some cases the ability of the water cooling to remove heat limits productivity, and scrap can be generated due to variation in water cooling. The considerable work conducted to date on water cooling in DC casting is reviewed. The boiling theory is covered in a companion paper. Published measurements of cooling intensity and the affect of water cooling on the temperature distribution during casting are analysed. Various mold water system designs are discussed. The effect of variables such as water flow rate, impact velocity, composition, temperature etc are presented. Practical implications for controlling water cooling and the casting process are suggested.

11:20 am

CONSTITUTIVE MODELING OF HIGH-STRENGTH ALUMINUM CASTING: Keh-Minn Chang, Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, WV 26506; Jerry Harris, Ravenswood Aluminum Corporation, Ravenswood, WV 26506

High strength aluminum 7xxx alloys are susceptible to cracking during casting, and computational simulation is adapted to obtain better understanding and satisfactory control. Constitutive model of thermomechanical properties of the as-cast ingot is necessary for calculation of thermal stress developed after solidification. Considering the potential influence of precipitation, ingot properties are evaluated by a continuous cooling method, which allows controlled cooling at different rates to test temperatures of interest. The results indicate that thermomechanical behaviour of cast ingot remarkably differs from that of final wrought product. The relationship of mechanical properties are correlated with the microstructure of cast ingots.

THE FOLLOWING PRESENTATION IS WITHDRAWN
11:40 am

3D ANALYSIS OF EM SEMI-LEVITATING AND FREE SURFACE PROBLEMS: M. Ramadan Ahmed, Aluminum Co. of Egypt, 48/50 Abd El-Khalek Sarwat St., Cairo, Egypt; S. El-Masry, Faculty of Engineering & Technology, Helwan, Egypt; Fawaz Moustafa, Ibrahim Moustafa, Egyptalum Company, Nag Hammady, Egypt

Developing an accurate mathematical model for the analysis of electromagnetic (EM) semi-levitating and free surface problems is still in the top of attentions for many researchers working in the field of the EM casting and others near-net-shape product applications. In this paper, a 3D iterative solution based on the Boundary Element technique coupled with a heat transfer analysis is derived for modeling an electromagnetic (EM) caster producing aluminum ingot installed in the Egyptalum Company. The main objective of this work is studying the effects of the EM screen location, the cooling system strategy and the melt withdrawal speed, and the effects of the estimated electrical setting parameters on the EM caster performance and its operation stability. The numerical results show that the location of the screen as well as the melt withdrawal speed have significant effects on the stability and the shape of the formed aluminum ingot. On the other hand, changing the supply frequency from 1500 Hz up to 2500 Hz increases the total electrical power consumed but it has no significant effect on the ingot shape. Finally, measurements showing a comparable reduction of 10% in the total electrical power consumed and a stable equilibrium free surface shape are recorded as the EM caster is tested, in the company, using those concluded setting parameters.