1997 TMS Annual Meeting: Wednesday Session

CAST SHOP TECHNOLOGY: Session VI: Solidification of Cast Alloys

Session Chairperson: John Grandfield, Comalco Research Centre, P.O.Box 316, Thomastown, Victoria 3074, Australia

8:30 am

MODELLING OF SOLIDIFICATION IN Al-ALLOYS: Nigel Saunders, Thermotech Ltd., The Surrey Research Park, Guildford GU2 5YG, and IRC in Materials for High Performance Applications, University of Birmingham, Birmingham B15 2TT, United Kingdom

Solidification in Al-alloys is a complex process usually occuring under non-equilibrium conditions with the possible formation of metastable low melting point eutectics and non-equilibrium phases. However, it has recently been shown that it is possible to use thermodynamic phase diagram calculations to simulate the non-equilibrium solidification of complex Al-alloys under so-called 'Scheil' conditions. Excellent results can be obtained for features such as fraction solid transformed as a function of temperature, phase formation, latent enthalpy and Cp of solidification etc. All of these features are important input into modelling of solidification processing particularly for software packages which simulate heat flow during casting. The paper will present the results of 'Scheil' simulations for a wide range of commercial Al-alloys showing comparison with experimental results for fraction solid transformed and phase formation. The use of subsequently derived Cp and heat evolution parameters in casting simulation packages will be discussed and examples of its application shown.

8:50 am

THE APPLICATION OF EFFICIENT PARALLEL PROCESSING TO FINITE ELEMENT MODELING OF FILLING, SOLIDIFICATION, AND DEFECT PREDICTION FOR ULTRA-LARGE SHAPE CASTINGS: David Snyder, Aluminum Company of America, Alcoa Technical Center, 100 Technical Drive, Alcoa Center, PA 15069; David Waite, Consultant, Coralville, IA; Alpesh Amin, HP/Convex Corporation, Atlanta, GA

Finite element modeling has become widely to used simulate the filling, solidification, and defect formation in casting processes. For extremely large, complex parts, typical of automotive structural applications, the high cost of tooling makes process modeling an extremely useful tool. However, as the size and complexity of casting part geometry increase, the computational requirements for modeling become formidable. To address the challenge for modeling ultra-large shape castings, a finite element code was developed to efficiently take advantage of shared-memory parallel computing systems. This paper gives a brief description of the mathematical models which describe the casting processes. The implementation of the models and the numerical issues regarding memory architecture, parsing of computations, cpu synchronization, and overhead minimization for parallel processing are discussed. Examples are presented for filling and solidification analyses of permanent mold, tilt-pour, and die casting models run on an 8-CPU HP/Convex SPP-1000 computer.

THE FOLLOWING PRESENTATION IS WITHDRAWN
9:10 am

IN-SITU TEMPERATURE MEASUREMENTS IN LOW PRESSURE PERMANENT MOLD CASTING: Florence Paray, Joe Gruzleski, Department of Mining and Metallurgical Engineering, McGill University, 3450 University Street, Montreal, Quebec, Canada H3A 2A7; Joe Clements, Grenville Castings Ltd., Merrickville, ONT, Canada K0G 1N0; Bahadir Kulunk, Timminco Metals, Research and Development Center, Haley, ONT, Canada K0J 1Y0

The low pressure casting process has an increasing popularity as it allows a rapid production of components close to the final "near net shape" with a very good casting yield. Depending on the property requirements of the final product, it is often necessary to control casting soundness. Internal porosity can cause a loss of pressure tightness, a critical factor in parts such as engine blocks and manifolds which are required to keep various gases and fluids. The development of criteria functions should allow the prediction of the thermal conditions required to maintain porosity below some critical predetermined level. In order to determine criteria functions, it is necessary to acquire thermal data during casting solidification. A low pressure casting machine and die were instrumented to obtain "in situ" thermal analysis curves during the solidification of flat plates of thickness varying from 1/8" to 3/4" in Sr modified and unmodified 6290, 356 and 319 alloys. The paper describes that effort and some of the results obtained.

9:30 am

A TECHNIQUE FOR THE ESTIMATION OF INSTANTANEOUS HEAT TRANSFER AT THE MOLD/METAL INTERFACE DURING CASTING: Michael Trovant, Stavros Argyropoulos, Department of Metallurgy & Materials Science, University of Toronto, 184 College Street, Toronto, Ontario, Canada M5S 3E4

Many current advancements responsible for improving the accuracy of solidification algorithms have resulted from the incorporation of additional complex casting related phenomena into the numerical calculation scheme. More recently, however, the proper specification of boundary conditions has been found to play a critical role in limiting model inaccuracy. Of particular interest to casting are the temperature boundary conditions, which when adopted by many models appear to be the most troublesome. Temperature boundary conditions can vary significantly with time and position along the face of the casting and accurate heat transfer coefficients are notoriously difficult to obtain experimentally for all points on the mold/metal interface, especially when the influence of thermal contraction is acknowledged. A novel technique which minimizes the error associated with selecting boundary conditions without experimentation is proposed. Advanced knowledge of the air gap formation and its correlation to the heat transfer coefficient at the mold/metal interface is used to formulate a coupled mathematical model which determines the increase of the air gap and predicts the instantaneous cooling conditions at a given mold wall. The objective is to allow the modeler to estimate effect of thermal contraction on the heat loss at mold/metal interfaces without resorting to experimentation.

9:50 am

MECHANISM OF DENDRITE FRAGMENTATION IN CASTINGS: Shan Liu, Shu-Zu Lu, A .Hellawell, Michigan Technological University, Dept. of Metallurgical and Materials Engineering, Houghton, MI 49931

A temperature gradient stage was designed with a controlled deceleration mechanism to simulate the columnar growth in castings/ingots where the growth velocities and temperature gradients at a dendritic front undergo a continuous decrease during solidification. Transparent materials of SCN-H2O and NH4Cl-H2O systems were used so that the solidification process with various decelerations could be directly observed and video recorded. It is observed that fragmentation of secondary arms is directly related to the deceleration and this provides an intrinsic resource for the formation of equiaxed grains in castings where convection may be available for transportation of the fragments to the open liquid. The responses of a dendritic array to the deceleration, including primary spacing adjustment and tip radius adjustment etc., are also observed and discussed.

10:10 am BREAK

10:20 am

THE VISUALIZATION OF THE PARTICLE CONTENT OF LIQUID ALUMINUM ALLOYS: N.D.G. Mountford, A. Simionescu, I.D. Sommerville, Department of Metallurgy & Materials Science, University of Toronto, Toronto, Ontario, Canada M5S 3E4

Aluminum alloys can contain heterogenous impurities which will affect the strength of the final product and result in either failure during manufacture or in service. Pulsed ultrasound, conveyed down metal guide-rods from energized piezo-electric crystals, is passed into the liquid metal and reflections from any impurities can be recorded by special counting devices. Small particles of the order of 10 to 15 µm can be resolved and their behaviour such as in settling recorded. Sequential tests using particles of identified size and distribution such as SiC and TiAl3 demonstrated the systems reproducibility. Such changes in composition resulting from imposed turbulent transfer with the associated pick up of very small oxide particles could also be shown. The effect of "sludge" formation such as that found in die-casting applications due to intermetallic phase precipitation could also be measured. This could be of value in process control of die casting systems. The developed system is adaptable to on line quality measurement and the recording of molten aluminum cleanliness in crucibles, ladles, furnace wells, launders and other transfer systems.

THE FOLLOWING PRESENTATION IS WITHDRAWN
10:40 am

FRACTAL ANALYSIS: POROSITY IN ALUMINUM CAST ALLOY: Bo-Tao Lee, Shu-Zu Lu, A. Hellawell, Michigan Technological University, Dept. of Metallurgical and Materials Engineering, Houghton, MI 49931

Porosity is a very common defect in Al-Si cast alloy. To examine the porosity, a fractal analysis was conducted to characterize the pores using two numbers, Fractal Dimension, D, and Shape Factor, . This analysis was based on the measurement of perimeters/areas of all individual pores using an image analyzer with changing magnifications. The result indicates that two types of pores, i.e., rough/shrinkage pores and smooth/hydrogen pores, can be distinguished by these fractal numbers. The shrinkage pores show a larger value of fractal dimensions whereas the hydrogen pores have a fractal dimension close to one. Combined with analysis for silicon particles, it is suggested numerically that shrinkage pores are more likely associated with the silicon phase that is not well modified, showing a rather coarse flake structure, and the hydrogen pores are in the regions where modification was well done with fine silicon particles.

11:00 am

NUMERICAL SIMULATION OF DIE FILLING IN SEMISOLID METAL PROCESSING: Andreas N. Alexandrou, François Bardinet, Mechanical Engineering Department, Worcester Polytechnic Institute, Worcester, MA 01609; Willem Loué, Pechiney CRV, Centre de Recherche de Voreppe BP27, 38340 Voreppe, France

Semisolid Metal Processing is gaining interest very rapidly. This manufacturing approach offers distinct advantages over other near-net-shape technologies, like a more homogenous microstructure and less porosity and thus excellent mechanical properties. A perfect control of the die filling during processing is however necessary, especially in the case of semisolid forming of aluminum, where a non-controlled die filling can lead to oxide inclusions. Numerical simulation would be a powerful tool, as it would allow to predict die filling and the optimization of die design. However, the constitutive behaviour of such semisolid metals is rather complex. Their non-Newtonian behaviour does not depend only on the volume fraction of liquid, but also on the metal´s history prior to processing and the processing conditions. In this paper, a Bingham power-law relation is presented, capable of describing correctly the rheological behaviour of the semisolid metal. This constitutive equation is then introduced in a modified version of the casting simulation package SIMULOR. Simulation results on the filling of a 2-D cavity under various conditions are shown. Issues related to die design of dies, using numerical simulation will also be addressed.

11:20 am

SOLIDIFICATION PROCESSING OF ALUMINUM CASTING ALLOY REINFORCED WITH CERAMIC MICROSPHERES FOR THIXOFORMING: P.D.D. Rodrigo, K. Xia, N. Setargew, Materials Group, Department of Mechanical and Manufacturing Engineering, University of Melbourne, Parkville, Victoria, Australia 3052; P. Fitzgerald, G. Withers, Cyco International, 1297 Nepean Highway, Cheltenham, Victoria, Australia 3192

A cast aluminum matrix composite based on an Al-Si-Mg alloy has been developed. The reinforcement was in the form of hollow microspheres, a ceramic by-product from coal power stations. The particles were incorporated into the aluminum alloy melt by mechanical stirring either in the semisolid state or full liquid state. The composite melt was cast from full liquid temperature to produce slugs for thixoforming. A variety of compositions, reinforcement volume fractions and processing parameters were used. The as-cast microstructure consisted of fine, equiaxed dendritic primary grains in a matrix of secondary grains and eutectic phases. The primary dendritic grains evolved to become non-dendritic after reheating the slugs to semisolid temperature. The slugs were subsequently thixoformed at a temperature corresponding to a primary solid fraction of about 50% into thin plates and alternator pulleys. Special wear tests on the pulleys showed that they were comparable to steel pulleys. The material is potentially suitable for those applications demanding good wear resistance, light weight and low cost.