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1997 TMS Annual Meeting: Tuesday Session

CAST SHOP TECHNOLOGY: Session III: Grain Refinement, Alloy Properties and Thermal Analysis

Sponsored by: LMD Aluminum Committee
Program Organizer: Wolfgang A. Schneider, VAW aluminium AG, Research and Development, Georg-von-Boeselager-Str.25, D-53117 Bonn, Germany

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Room: 230B

Session Chairperson: Stuart R. Thistlethwaite, London & Scandinavian Metallurgical Co. Ltd., Fullerton Road, Rotherham, South Yorkshire, S60 1DL, England

8:30 am


The use of AlTiB grain refiners for a precise control of the metallurgical cast structure has become an integral requirement in the manufacture of present day products. A permanent high standard in the quality and consistency of grain refiners is essential especially when considering their radical impact on the production result. This paper will discuss the consistency of grain refiners in terms of the composition, the production route and the required grain refiner properties. Particular attention will be given to an in-depth comprehension of the AlTiB microstructure. The above will be illustrated by microstructures and further supported by a selection of SPC data, process capabilities and (particle) distribution figures taken from various grain refiners. Information concerning recent advancements in the characterisation and process development of AlTiB grain refiners will also be incorporated in this paper.

8:50 am

EFFECTS OF TRANSITION METALS ON THE POTENCY OF TiBAl GRAIN REFINERS: A. Green, M.A. Kearns, London & Scandinavian Metallurgical Co. Ltd., Fullerton Road, Rotherham, South Yorkshire, S60 1DL, England

TiBAl grain refiners are known to be susceptible to fading and poisoning phenomena which can limit their effectiveness in some practical situations.The long-term fading behaviour of good and bad TiBAl grain refiners in 99.7% Al is presented as a function of temperature in the presence of Zr and other transition metals. It is shown that fade occurs more rapidly at higher temperatures and that the effects of Zr and other transition metals display a complex behaviour as a function of temperature. The observations are explained in terms of recent theories on the behaviour of TiBAl grain refiners which propose that potent TiB2 nuclei have TiAl3 layers present on certain facets. It is shown that the reported behaviour is consistent with interactions occurring between transition metals and the potent aluminide layer. Results are discussed in terms of key stages in the production of TiBAl grain refiners and lessons for the use of TiBAl in the aluminium industry are highlighted.

9:10 am

DEVELOPMENT OF AN IMPROVED AlTiC MASTER ALLOY FOR THE GRAIN REFINEMENT OF ALUMINIUM: W. Reif, Institute of Material Science, Technical University Berlin, Str. des 17.Juni 135, D-10621 Berlin, Germany; A. Green, London & Scandinavian Metallurgical Co. Ltd., Fullerton Road, Rotherham, South Yorkshire, S60 1DL, England; P.C. van Wiggen, KBM Master Alloys B.V., Klosterlaan 2, 9936 TE Delfzijl, The Netherlands; W.Schneider, VAW aluminium AG, Research and Development, Georg-von-Boeselager-Str.25, D-53117 Bonn, Germany; D. Brandner, Hoogovens Aluminium-Walzprodukte GmbH, Carl-Spaeter-Str.10, D-56070 Koblenz, Germany

The commercial AlTiB master alloys for grain refinement of aluminium contain TiB2 particles, which can be coarse and have the tendency to agglomerate in the melt. As a result of this, quality problems in different products occur.In Zr and Cr containing alloys TiB2 interacts with these elements leading to inhomogenous grain structure. In order to avoid the above mentioned disadvantages of the grain refinement with TiB2, a co-operative research programme has been carried out, to develop an improved AlTiC grain refiner. The main objectives of the project were: Development of an efficient AlTiC master alloy. Fundamental research to understand the mechanism of an AlTiC grain refiner. Evaluation of test methods for determination of the grain refining efficiency and agglomeration behaviour of TiC as standard test methods. Production scale testing of the developed AlTiC master alloy. The paper presents the results with respect to the above mentioned objectives.

9:30 am

THE DEVELOPMENT OF A COMMERCIAL Al-3% Ti-0.15% C GRAIN REFINING MASTER ALLOY: A.J.Whitehead, S.A.Danilak, Shieldalloy Metallurgical Corporation, Newfield, NJ 08344; Douglas A. Granger, Aluminum Company of America, Alcoa Technical Center, Alcoa Center, PA 15069

An Al-3% Ti-0.15% C master alloy has been developed and is now being used for ingot grain refinement in Alcoa. A description is given of the development of the high ratio Al-6% Ti-0.02% C master alloy and the progression from this alloy to the more acceptable lower ratio Al-3% Ti-0.15% C alloy. Acceptance for commercial use came only after extensive metallurgical characterization and evaluation of the grain refining performance, including the impact of alloy type and the presence of tramp elements. Details of the production, testing and characterization of this new grain refining master alloy are discussed.

9:50 am

THE GRAIN REFINEMENT OF Al-Si FOUNDRY ALLOYS: J.A. Spittle, J.M. Keeble, IRC for Materials in High Performance Applications, Department of Materials Engineering, University of Wales Swansea, Swansea SA2 8PP, United Kingdom

Whereas small concentrations of Si have been shown to enhance the grain refinement of aluminium by addition of an Al-Ti-B master alloy grain refiner, increasing Si contents in excess of 2-3% result in a continuous increase in primary aluminium solid solution grain size. Two explanations of these observations have been proposed to date based on the influence of Si on either the nucleation or growth of the aluminium primary crystals. Neither of these explanations appears to fit all the available grain size data. In an attempt to further clarify the origin of the Si coarsening effect, grain size studies have been performed on Al-Si and Al-Zn alloys as a function of solute content. It appears that the coarsening is a result of the influence of the Si content on aluminium grain nucleation. A coarsening mechanism is suggested based on the coupled influence of Si level on melt undercooling and primary phase freezing range.

10:10 am BREAK

10:20 am

MODIFICATION OF SILICON IN EUTECTIC AND HYPER-EUTECTIC Al-Si ALLOYS: Ben Heshmatpour, Shieldalloy Metallurgical Corporation, 12 West Boulevard, P.O. Box 768, Newfield, NJ 08344

Refining of eutectic silicon in hyper-eutectic and eutectic Al-Si alloys is accomplished by using phosphorus-bearing additives. Commercially available copper-phosphorus (CuP) in a variety of forms and concentrations is widely used for this application. Large addition rates are needed for effective silicon modification via CuP. The recently developed ferro-phosphorus (FeP) based tableted product provides significant performance and cost advantages while requiring much smaller addition rates, lower alloy temperature, and short contact times. This paper compares the results for refinement of A390.1, B390.1, and 339.1 alloys using CuP and tableted FeP.

10:40 am

EFFECTS OF RESIDUAL TRANSITION METAL IMPURITIES ON ELECTRICAL CONDUCTIVITY AND GRAIN REFINEMENT OF EC GRADE ALUMINIUM: R. Cook, M.A. Kearns, P.S. Cooper, London & Scandinavian Metallurgical Co. Ltd., Fullerton Road, Rotherham, South Yorkshire, S60 1DL, England

Removal of transition metal impurities is a key step in production of high conductivity EC grade Aluminium. Titanium and Vanadium in particular are generally removed by adding an excess of Boron to precipitate stable borides before decanting the treated metal. It is nevertheless advantageous to add sufficient grain refiner to avoid hot cracking of the cast bar without jeopardising electrical conductivity. We report here a study of the effects of residual Vanadium on the efficiency of different grain refining additives and electrical conductivity of the product. It is shown that Vanadium must be below a threshold figure to give adequate grain refinement at levels which do not compromise conductivity. The effects of residual Fe and Si impurities on grain refinement and conductivity are also described and their role is discussed in terms of constitutional supercooling effects. The relevance of the results to the manufacture of EC grade wire is discussed.

11:00 am

EXPERIMENTAL MEASUREMENT OF ELECTRICAL CONDUCTIVITY OF ALUMINUM ALLOYS AT ELEVATED TEMPERATURES: Raphaël Craen, Nagy El-Kaddah, Department of Metallurgical & Materials Engineering, The University of Alabama, P.O. Box 870202, Tuscaloosa, AL 35487-0202; Willem Loué, Péchiney CRV, Parc Economique Centr'Alp-BP27, 38340 Voreppe, France

The knowledge of the electrical conductivity of aluminum alloys is critical for the analysis and computer simulation of induction heating and melting operations as well as electromagnetic casters. While accurate conductivity data are available for pure aluminum, there is a paucity of data for aluminum alloys, particularly at elevated temperature. This paper describes an eddy current technique for measuring the electrical conductivity of metallic specimens at high temperatures. In this technique, which is based on measurement of the electric energy dissipation (Joule Heating), the electrical conductivity is determined from measurement of the heating rate of the specimen. The measurement is made by subjecting an insulated cylindrical specimen to a uniform axial alternating magnetic field, and measuring temperature of the specimen during heating. The method requires no contact with the specimen, and is capable of providing electrical conductivity data to the melting point of the specimen with an error of less than five percent. Upon validating the technique using pure aluminum, measurements have been conducted on Al-Mg (5182) and Al-Li (8090) wrought alloys, and on foundry and rheocast Al-Si (357) cast ingots. The results show that electrical conductivities of Si and Mg alloys are about one half of pure aluminum, and the microstructure of Al-Si 357 alloy has little effect on the electrical conductivity of the alloy. The Li containing alloy exhibited a much lower conductivity than Si and Mg alloys. Expressions are presented for the conductivities of these alloys up to 450°C.

11:20 am

DETECTION OF SOLIDIFICATION REACTIONS USING HEAT PIPE TECHNOLOGY: M. Mahfoud, F. Mucciardi, J.E .Gruzleski, Department of Mining and Metallurgical Engineering, McGill University, 3450 University Street, Montreal, Quebec, Canada H3A 2A7

Thermal analysis is the measurement of changes in the temperature of a material as it is cooled from an elevated temperature. Usually the temperature changes such as those which occur during solidification are recorded as a function of the cooling time to detect various phase transformations. Although the use of thermal analysis to study the solidification of aluminum dates back 30 to 40 years, the physical techniques for performing thermal analysis have hardly evolved. One aspect of research at McGill has focused on developing a novel device based on heat pipe technology for performing thermal analysis of aluminum alloys. The device operates in-situ on a semi-continuous basis. In addition, the new device allows predefined cooling rates to be set and/or changed during the solidification process. This paper describes the use of the device for: a) measuring grain refining of 356 alloy using Al-5% Ti master alloy, b) determination of eutectic modification of 356 and 413 alloys, and c) detection of reactions involving the formation of copper and iron intermetallics in Al-Si foundry alloys.

11:40 am

COMPUTER-AIDED COOLING CURVE ANALYSIS (CA-CCA), APPLIED TO AN Al-Si SYSTEM: M.A. Ramirez A., J.C. Escobedo B., A.H. Castillejos E., A. Flores V., F.A. Acosta G., Centro de Investigación y de Estudios Avanzados del IPN, Unidad Saltillo, P.O.Box 663, 25000 Saltillo, Coahuila, México

The computer-aided cooling curve analysis is a new method for thermal analysis that can be used in foundries of low budget and with small investment. This method tries to simulate the classic differential thermal analysis method (DTA) by using only an acquisition data system coupled to numerical methods in micro-computers. The kind of information provided by this technique includes thermodynamical and thermophysical data, heat transfer parameters, solidification kinetics, and microstructure features. All this information is more than that obtained with the classic DTA, and it is necessary in order to get a real comprehension of the solidification process. CA-CCA has been applied to the study of an Al-Si system to try to characterize it and to predict microstructure, because it is possible with this technique to obtain the evolution of solid during the solidification process and the segregation behaviour of this system.

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