Sponsored by: LMD Aluminum Committee
Program Organizer: Julian V. Copenhaver, Technical/Quality Manager, NSA A Division of Southwire, PO Box 500, Hawesville, KY 42348
Tuesday, AM Room: A10
February 6, 1996 Location: Anaheim Convention Center
Session Chairperson: Dr. Andreas N. Alexandrou, Worcester Polytechnic Institute, 100 Institute Road, Worcester, Massachusetts 01609-2280
SEMI-SOLID PROCESSING: NEW ADVANCES IN NET SHAPE FORMING: Dr. Stuart B. Brown, Failure Analysis Associates, Three Speen Street, Framingham, MA 01701
Net shape and near net shape manufacturing using semi-solid metals has been undergoing a remarkable surge in activity. A number of new companies and initiatives have begun in semi-solid metal and component production. All exploit the well-known advantages of semi-solids, e.g., lower temperatures, reduced porosity, and improved mechanical properties than comparable cast metals. The flow behavior of semi-solids is still incompletely characterized, however, and must be understood to fully exploit these advantages. This presentation summarizes the last decade of research into the flow of semi-solid metals and discusses the implications of this research for net shape manufacturing using semi-solids. This research includes the effect of temperature, liquid/solid fraction, particle morphology, flow history, and methods for producing the semi-solid state. The presentation closes with a discussion of some particularly exciting new semi-solid processes.
SEMI-SOLID FORMING OF LIGHT METALS: NEW CHANCES FOR LIGHT WEIGHT STRUCTURES IN TRANSPORTATION APPLICATIONS: Dr. G. Hirt, R. Cramer, A. Winkelmann, T. Witulski, M. Ziligen, EFU Gesellschaft für Ur-/Umformtechnik mbH, Jägerheusstrasse 22, D-52152 Simmerath, Germany
Near net shape Semi-solid Forming of Light Metals offers the chance to achieve high strength, complex shaped components at economic cost levels. Possible applications include unsprung masses (i.e., steering knuckle) as well as thin-walled body structures or pressure-tight components in the brake system. The paper will present the actual status of R & D work carried out by EFU in the field of raw material production, reheating and semi-solid forming: Pilot scale equipment has been used to produce thixotropic billets from the following alloys: AlSi7Mg (356); AlMgSi1 (6082); AlCuMg2 (2024); AlSi18CuMgNi; and Al-Metal Matrix Composites. The typical billet diameters are 3 inch and 4.5 inch (length up to 3.5 m); For inductive reheating of these billets a computer controlled system has been developed, which allows closed loop control of the heating cycle by using a special inductive sensor to monitor the billet condition (liquid fraction, softness, melt loss). Recent experiments demonstrate that this control system allows a flexible response, i.e., in case of short term interruptions caused by the forming system. Various demonstrator parts have been produced from all alloys mentioned above and Magnesium alloy AZ 91. The demonstrator parts include a steering knuckle and a thin-walled rib-type structure (average wall thickness 2.5 mm, outer dimensions 500 mm x 150 mm). In the case of alloy AlMgSi1, mechanical properties which have been determined from specimens taken from semi-solid formed parts are close to the requirements according to DIN 1749 standard for forged parts. This implies an advantage of 6xxx alloys, compared to other thixoformed alloys (356, AZ 91). In SSM-Forming of MMC's, the mechanical properties are according to the specification after conventional processing. In this case the major advantages are material saving and reduced machining due to near net shape semi-solid forming.
SEMI-SOLID METAL (SSM) CASTING: APPLICATIONS AND CASE STORIES IN ALUMINUM AND MAGNESIUM: Dr. Kenneth P. Young, Buhler Incorporated, CH-9240 Uzwil, Switzerland
A semi-solid (SSM) casting process based around real time controlled horizontal cold chamber die casting machines is described. SSM casting has been successfully applied to both aluminum and magnesium alloys with the largest shot weight to date being 11.5 kilogram (25 pounds). SSM casting offers a unique combination of near net shape and premium quality mechanical performance. Aluminum components generally require either partial (T5) or full (T6) heat treatment, providing attractive combinations of strength and ductility. In the case of magnesium alloys, the ability to completely eliminate liquid metal handling from the casting floor allows SSM casting to compete with magnesium cold chamber die casting. In these instances, less massive gating than for aluminum is required and normal trimming operations can be considered. A series of case studies are presented together with projections for future developments. As raw material costs decrease and secondary recycling becomes viable, it is expected that SSM casting will be considered for a much wider range of applications.
RHEOMOLDING A ONE-STEP PROCESS FOR PRODUCING SEMI-SOLID METAL CASTINGS WITH LOWEST POROSITY: Dr. K. K. Wang, Nan Wang, Hsuan Peng, Sibley School of Mechanical and Aerospace Engineering, Cornell University, Upson Hall, Ithaca, NY 14853
Rheomolding is a new idea of combining rheocasting in an injection-molding-like apparatus to achieve net-shape manufacturing of semi-solid metal parts with lowest porosity. Instead of solid pellets or chips in conventional injection molding or thixomolding, as starting material, rheomolding begins with molten metal in a crucible. As the fluid enters the extruder, it is always subject to cooling along the barrel and begins to crystallize on the cold wall. The screw rotation and the moving fluids will break the dendritic crystals on the wall and form solid particles suspended in the remaining fluids. By adjusting the temperature profile along the barrel and screw rotating speed, one could achieve various degrees of solid fraction and perhaps manipulate particle sizes. A prototype Rheomolding machine has been built, and extensive experiments have been carried out with Sn-15% Pb alloy. In this paper, some details on temperature control, metallographical examinations, and resulting product quality and repeatability are discussed.
SEMI-SOLID FORMING OF HYPEREUTECTIC Al-Si ALLOYS: Ittipon Diewwanit, Dr. Merton C. Flemings, Massachusetts Institute of Technology, Department of Materials Science and Engineering, Building 8-407, 77 Massachusetts Avenue, Cambridge, MA 02139-4307
Hypereutectic aluminum-silicon alloys, containing up to about 18% Si, are commercially cast but present difficulties due to their high liquidus temperature and wide freezing range. This paper describes research on fundamentals of semi-solid forming of these high silicon alloys. Viscosities of semi-solid alloys containing up to 50% Si are measured in a Searle type viscometer. Structure and viscosity are related to processing conditions. Behavior of the alloys with respect to structure and formability is similar to that of hypoeutectic alloys. The research demonstrates the feasibility of part forming of alloys containing up to 50% silicon, at temperatures as low as the Al-Si eutectic, 577 celsius.
10:10 am BREAK
ORIENTATIONAL RELATIONSHIPS IN RHEOCAST Al-6.5wt%Si: S. Sannes, SINTEF Materials Technology, N-7034 Trondheim, Norway; Dr. Lars Arnberg, Department of Metallurgy, Norwegian Institute of Technology, Alfred Getz vei 2b, N-7034 Trondheim, Norway; Merton C. Flemings, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139-4037
The orientational relationships between particles in agglomerates of semisolid Al-6.5wt%Si have been investigated. The alloy has been sheared at a rate of 900 s-1 for two hours at a fraction solid of 0.2 and then quenched. The orientational relationships between particles in the agglomerates have been investigated on polished samples from electron beam scattering patterns in a scanning electron microscope. The results show that all particles belonging to the same agglomerate are related either by low angle grain boundaries with misorientations of less than 10 or by misorientations of about 60 around the <111> axes which corresponds to coherent twins. All particles in the microstructure can be identified as belonging to an agglomerate. No preferred orientational relationship between different agglomerates has been observed.
SEMISOLID PROCESSING OF SPRAY CAST ALLOYS: EXPERIMENTS AND MODELING: E. Tzimas, Dr. Antonios Zavaliangos, Assistant Professor, A. Lawley, Department of Materials Engineering, Drexel University, 32nd and Chestnut Streets, Philadelphia, PA 19104
Metallic systems which possess a fine, equiaxed, non-dendritic microstructure behave thixotropically and can be formed to net shape in the semisolid state. Semisolid processing provides several advantages over conventional casting, forging, and P/M techniques, but has not been accepted widely until recently. A combined theoretical-numerical-experimental program is under way at Drexel University to assess quantitatively the potential of thixoforming, as applied to spray cast materials. Here we present experimental results on the flow/deformation behavior of semisolid spray cast binary Al-4.5w/oCu and 2014 aluminum alloys at different temperatures (590-640C) and strain rates (0.1-10/sec) under simple unconstrained compression. The resistance of both alloys to flow increases initially with strain. During deformation the solid skeleton that is formed initially by the solid grains appears to be destroyed. This results in a significant decrease of the flow resistance which finally reaches steady state. The steady state stress in the binary alloy is significantly lower than in 2014. This difference is attributed to the presence of unmelted secondary phases in 2014. Similarities with the behavior of low cohesion fully saturated soils are noted and utilized establishing a modeling framework for semisolid processing.
THREE DIMENSIONAL FLOW OF THIXOTROPIC MATERIALS IN A CHANNEL WITH SUDDEN EXPANSION USING AN IMPLICIT SEGREGATED FINITE ELEMENT SOLUTION: Dr. Andreas N. Alexandrou, Associate Professor, Director, Aerospace Program, Department of Mechanical Engineering, Worcester Polytechnic Institute, 100 Institute Road, Worcester, MA 01609
The importance of processing of metal and metal matrix composites in semi-solid state is increasing quite rapidly. This manufacturing approach offers distinct advantages over similar methods of near-net-shape making, such as better control of the microstructure and thus mechanical properties and better control of the processing conditions due to the "fluid"-like flow of the material. The effective use of this process presumes good knowledge of the material behavior. Unfortunately, the constitutive behavior is quite complex and depends not only on the volume solid fraction but also on the processing conditions. Currently, the only mathematically sound theory attempting to describe their rheology is based on an internal variable approach. In this paper we are taking an alternative phenomenological approach where the behavior of semi-solids is expressed through a continuous shear thinning-thickening Bingham fluid constitutive relation whose construction relies on the generalization of available experimental data. This constitutive relation is used to study the flow in a three-dimensional channel with a sudden expansion. Results will be presented for different geometries and flow parameters.
HOW PROCESS DESIGN AFFECTS QUALITY OF SEMI-SOLID METALWORKING PARTS: P.A.A. Khan, F. R. Dax, Concurrent Technologies Corporation, 1450 Scalp Avenue, Johnstown, PA 15904
PANEL DISCUSSION: SEMI-SOLID PROCESSING TECHNOLOGIES, CHALLENGES AND FUTURE PROSPECTS: Moderator: Prof. A. Alexandrou
The contributing authors and audience will be invited to join in a panel
discussion on the future of Semi-solid Processing in light of recent
developments in the field.
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