Materials Week '97: Monday AM Session

Focusing on physical metallurgy and materials, Materials Week '97, which incorporates the TMS Fall Meeting, features a wide array of technical symposia sponsored by The Minerals, Metals & Materials Society (TMS) and ASM International. The meeting will be held September 14-18 in Indianapolis, Indiana. The following session will be held Monday morning, September 15.

PROCESS AND MATERIAL BEHAVIOR MODELING SYMPOSIUM: Session I

Session Chair: S. Lee Semiatan, WL/MLLN, Wright Patterson AFB, OH

THERMAL STRESS DEVELOPMENT DURING THE VACUUM ARC REMELTING (VAR) PROCESS: M.K. Alam, Dept. of Mechanical Engineering, Stocker Engineering Center, Ohio University, Athens, OH 45701; S.L. Semiatin, Materials Directorate, Wright Laboratory, WL/ML, WPAFB, OH 45433

The development of temperature nonuniformities and the resulting thermal stresses during the VAR process were modeled using the commercial finite element method (FEM) code ABAQUS. Solutions were obtained for different values of ingot diameter, crucible-ingot interface heat transfer coefficients, and lengths of the solidified ingot. The stress solutions were obtained by using an elasto-plastic model with temperature dependent thermo-mechanical properties. Model predictions revealed that the maximum tensile thermal stresses are developed at the bottom of the ingot. The magnitude of the stresses increased with increase in ingot diameter and the interface heat transfer coefficients. The predicted development of large tensile stresses correlates well with observations of thermal cracking during VAR of near-gamma titanium aluminide alloy ingots.

8:55 am

PREDICTION OF DISTORTION IN SUPERALLOY COMPONENTS AFTER HEAT TREATMENT: T.C. Tszeng, W.T. Wu, Scientific Forming Technologies Corporation, Columbus, OH 43202

Unacceptable distortion or residual stresses are common problems in heat treating high-temperature structural materials such as superalloys. The ability to predict component distortion relies upon an accurate calculation of complex thermal, mechanical, and metallurgical changes during the heating and quenching cycles in the processes. Among other considerations in such a mathematical model, creep deformation is an important factor. In this presentation, progress in the incorporation of creep deformation in the modeling of several heat treating processes for superalloy components is summarized. Specifically, modifications to the computer program DEFORM to model solution treatment (heating, soaking, and quenching) and stress relief of Inconel 718 components are described. The calculated results are analyzed with reference to the prediction of component distortion after heat treatment. This study is partially funded by a US Air Force/Navy SBIR Award (Contract No. F33615-95-C-5238).

9:20 am

MICROSTRUCTURE/PROPERTY CONSIDERATIONS IN HIGH TEMPERATURE FORGING OF AEROENGINE DISK COMPONENTS: P.S. Follansbee, M.F. Henry, J. Nic, Physical Metallurgy Laboratory, General Electric Corporate R&D Center, Schenectady, NY 12301

The manufacture of modern aeroengine Ti and Ni-base disks places unique demands on cost and quality. Of all disk manufacturing operations, forging is the most amenable to simulation, which has become an essential tool in minimizing cost and assuring quality. In this presentation, we will review several important features of high temperature forging of disk components. We discuss some of the metallurgical features affecting forging of high gamma prime Ni-base components. We will emphasize the difficulty in prescribing constitutive behavior of these materials under high temperature conditions due to evolving microstructural conditions - particularly grain growth and grain refinement via dynamic recrystallization. The suitability of state variable approaches for these applications will be discussed.

9:45 am

CONSTITUTIVE RELATIONSHIP FOR SUPERPLASTICITY IN -TiAl ALLOYS: R.S. Mishra, A.K. Mukherjee, Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616

Several exciting developments are taking place in processing of -TiAl alloys for enhanced superplasticity. Multi-axial isothermal forging can produce 0.1-0.3 mm grain sizes in -TiAl alloys. These ultra-fine grained -TiAl alloys have exhibited low temperature superplasticity and provide the possibility of high strain rate superplasticity. Such possibilities are discussed with the help of constitutive equations for optimal superplasticity in -TiAl alloys. A three-dimensional 'superplasticity map' has been developed for -TiAl alloys to depict the optimum strain rate-temperature grain size domain for superplasticity. The use of these constitutive equations to develop a 'processing map' to produce ultra-fine grained -TiAl will be presented.

10:10 am

SUPERPLASTIC DEFORMATION AND CAVITATION MODEL BASED ON GRAIN BOUNDARY SLIDING: A.K. Ghosh, Department of Materials Science and Engineering, The University of Michigan, Ann Arbor, MI 48109-2136

The importance of grain boundary sliding (GBS) in superplastic deformation is well known. It is also known that the extent of sliding is a function of grain boundary character and varies from one region to another within a test specimen, leading to local regions of stress concentration and grain rotation. Recently, an attempt was made to model the response to the stress concentration by considering glide-climb creep in the grain mantle region. Grain boundary migration occurs naturally as a consequence of this process, as long as accommodation processes are adequate. At low stresses, where triple point accommodation problems are minimal, cavity nucleation is a result of satisfying a normal stress criterion on grain boundary particles. A simplified polycrystal model is developed to track the opening of voids and their growth as a function of strain rate, which is consistent with the deformation model itself. Results based on such an initial model will be presented.

10:35 am

MODELING OF MECHANICAL AND MICROSTRUCTURAL BEHAVIOR OF SUPERPLASTIC 7475 ALUMINUM: W.L. Moore, M. Zelin, D.E. Ebersole, Concurrent Technologies Corp., 1450 Scalp Ave., Johnstown, PA 15904

Increased use of superplastic aluminum alloys for component fabrication depends on the increased ability of design engineers to predict post-formed mechanical and identify areas where inspection is critical. To this end, software enhancements to the MARC^TM analysis code have been developed as a key part of a program to optimize the superplastic forming process for aluminum aircraft assemblies. The program focuses on blowforming of 7475 aluminum in the superplastic condition, with particular attention on prediction of forming process parameters such as optimized forming schedules (including backpressure), thickness, stress and strain distributions, and microstructural changes (including cavitation and grain growth). The predictions of the process simulation, and their subsequent verification by comparison with the characteristics of formed components, are summarized.

11:00 am

ANALYSIS OF CAVITATION BEHAVIOR OF A SUPERPLASTICALLY DEFORMED NEAR-GAMMA TITANIUM ALUMINIDE ALLOY: C.M. Lombard, Materials Directorate, Wright Laboratory, WL/MLLM, WPAFB, OH 45433; A.K. Ghosh, Dept. of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109; S.L. Semiatin, Materials Directorate, Wright Laboratory, WL/ML, WPAFB, OH 45433

The uniaxial hot tension behavior of a near-gamma titanium aluminide alloy sheet (Ti-45.5Al-2Cr-2Nb) was determined in the as-rolled condition (initial grain size 3 to 5 µm) and rolled-and-heat treated (1177°C/4 hours or 1238°C/2 hours) conditions (initial grain size 10 to 12 µm). Microstructure evolution, cavitation rates, and failure modes were established via constant strain rate tests at 10^-4 sec^-1 to 10^-2 sec^-1 and test temperatures between 900 and 1200°C. Interrupted tests were also conducted in order to study cavity nucleation and growth. Cavity shapes and volume fractions were analyzed as a function of total strain. For all initial microstructural conditions, the failure mode was established as predominantly cavitation/fracture controlled. The cavitation behavior was interpreted using a theoretical analysis of the isothermal hot tension test under cavitating conditions.

11:25 am

STRIP CAST SIMULATION TO INVESTIGATE PHASE SELECTION IN 3xxx SERIES Al ALLOYS: L. Carroll¹, K.A.Q. Reilly¹, B. Cantor¹, P.V. Evans², ¹Oxford Centre for Advanced Materials and Composites, Dept. of Materials, University of Oxford, Parks Road, Oxford OX13PH, UK; ²Alcan International Limited, Banbury Laboratory, Southam Road, Banbury, Oxon OX167SP, UK

Electron beam surface processing is an experimental technique capable of generating steady-state growth velocities in the strip casting regime. Growth velocity is an important factor in determining intermetallic phase selection during solidification. In this work the influences of solidification rate and alloy content, including the addition of grain refiner, have been examined with respect to phase selection. Phase identification has been accomplished through a combination of phase extraction, x-ray diffraction, transmission electron microscopy and energy dispersive x-ray microanalysis. The aim of this work is to optimize alloy design in 3xxx series Al alloys in order to take full advantage of strip casting for production of beverage can body sheet.