Sponsored by: SMD Non-Ferrous Metals Committe, EPD Pyrometallurgy and Process Fundamentals Committees
Program Organizers: Debabrata Saha, Metals Industries Group, Air Products and Chemicals, Inc., Allentown, PA 18195; Dr. William E. Frazier, Naval Warfare Center, Warminster, PA 18974; William P. Imrie, Bechtel Group, San Francisco, CA 94105; Prof. David G. Robertson, Department of Metallurgical Engineering University of Missouri, Rolla, MO 65401
Tuesday, AM Room: B8
February 6, 1996 Location: Anaheim Convention Center
Session Chairpersons: Debabrata Saha, Metals Industries Group, Air Products and Chemicals, Inc., Allentown, PA 18195; David G. Robertson, Department of Metallurgical Engineering, University of Missouri, Rolla, MO 65401
GAS-METAL REACTIONS DURING DEBINDING AND SINTERING OF INJECTION MOLDED NONFERROUS METALS: Randall M. German, R. Tandon, A. Griffo, Penn State University, University Park, PA 16802
Injection molding of mixed elemental powders is the basis for forming sintered copper, steel, titanium, cobalt alloys, tungsten heavy alloys and various heat sink materials, including W-Cu and Mo-Cu. An organic binder (wax-polymer) is used to form a powder slurry that allows injection molding. However, during heating to the sintering temperature it is necessary to extract all of the binder. This involves careful control of the process pathway, including temperature, time, heating rate, and process atmosphere. For various powders it is possible to employ highly reducing atmospheres, but for other systems the processing is difficult because of undesirable thermochemical reactions, example hydride formation. Research results are presented on several material systems and the thermal processing options deemed necessary for full binder removal prior to reaction, pore closure, or system densification. Several examples of late stage pore generation via impurity reactions are used as illustrations of the difficulties in effective debinding.
EFFECT OF ATMOSPHERE COMPOSITION ON SINTERING OF BRONZE COMPONENTS: D.Garg, K.R. Berger, D.J. Bowe, J.G. Marsden, Air Products and Chemicals, Inc., Allentown, PA 18195
Premixed bronze powders are widely used to produce a wide variety of P/M components including bearings and bushings by the powder metal industry. The physical and mechanical properties of sintered bronze components such as dimensions, strength, microstructural characteristics, and sintered density are well known to change with sintering furnace design, particle size distribution in powders, green density, preheating and sintering time and temperature, and protective atmosphere flow rate. However, not much is known about the effect of protective atmosphere composition on physical and mechanical properties of sintered bronze components. This paper describes effect of the protective atmosphere composition on physical and mechanical properties of sintered bronze components.
INTERACTIONS BETWEEN CHEMICAL REACTION, PORE DIFFUSION AND HEAT GENERATION DURING THE ZINC SULFIDE EXOTHERMIC OXIDATION: F. Patisson, M. Galant Francois, D. Ablitzer, LSG2M Ecole des Mines, Parc de Saurupt,54042 Nancy Cedex, France
During gas-solid exothermic reactions, heat generation makes temperature rise and modifies the rate of the various mass transfer processes, namely the chemical reaction itself, the pore diffusion and the external transfer. We have investigated this thermal effect during the oxidation of zinc sulfide porous pellets in a thermobalance furnace, between 550 and 900deg.C and under O2-N2 atmosphere. The results have been analyzed by using, firstly, the law of additive reaction times derived from the isothermal Grainy Pellet Model, and, secondly, a new numerical model taking into account non-isothermal conditions. Both models can satisfactorily simulate conversion versus time, but the numerical model also depicts the temperature evolution within the pellet. Finally, it is shown that owing to the heat generation the zinc sulfide oxidation takes place under diffusion control (3/4 internal, 1/4 external) in all the cases studied. During the course of reaction, the pellet undergoes strong temperature variations, with thermal gradients confined to a shell oxide layer.
MECHANISMS OF FORMATION OF HYDROGEN POROSITY IN 7X50 AND 2X24 ALUMINUM ALLOYS; EFFECTS ON MECHANICAL BEHAVIOR: Fernand Marquis, Dept. of Metallurgical Engineering, South Dakota School of Mines and Technology, Rapid City, SD 57701-3369
In this investigation, we consider the mechanisms of formation of two types of porosity: primary and secondary, their characterization and their effects on fracture, fatigue and failure behavior of these alloys. The mechanisms of formation of three forms of primary or cast porosity: (a) macroscopic shrinkage porosity (MSP); (b) microscopic interdendritic porosity (MIP); and (c) hydrogen gas porosity (HGP) or primary hydrogen porosity ( PHP) are discussed and each of these forms of porosity is characterized. The mechanisms of formation of four forms of secondary or thermal/thermomechanical porosity: (a) high temperature oxidation (HTO), internal hydrogen precipitation (IHP), or secondary hydrogen precipitation (SHP); (b) preheating blistering porosity (PBP); (c) eutectic melting porosity (EMP); and, (d) dissolution microvoid formation (DMF) are discussed and each of these forms of porosity is characterized. The evolution of the primary and secondary types of porosity (with special emphasis on PHP and IHP) during commercial and laboratory processing and their effects on the fracture and failure behavior of both types of alloys, with special focus on fatigue crack initiation, was investigated. Methodologies for the decrease of the total porosity: volume fraction and size, its redistribution and the minimization of its effects on the mechanical behavior of both types of alloys are discussed.
9:50 am BREAK
THE FLAMMABILITY OF ALLUMINUM IN OXYGEN: D. Bruce Wilson, Quantos Consulting, Mesilla Park, NM 88047; Joel M. Stoltzfus, NASA/White Sands Test Facility, Las Cruces, NM 88004-9544
The reaction of oxygen and solid aluminum produces the well-known surface coating of aluminum oxide, A1203. Thermodynamic properties and kinetic mechanism of this reaction is extensively documented. Less well-defined are the corresponding liquid phase reaction and reactions which would be characterized as "burning" of the aluminum. This paper reviews the solid aluminum-oxygen reaction as a foundation for discussing the flammability of aluminum in oxygen enriched atmospheres. Appropriate thermodynamic analysis and kinetic reaction mechanisms are developed. Available models of aluminum burning are reviewed and evaluated. The appropriate information for design of engineering applications of aluminum in oxygen atmospheres is summarized.
EFFECT OF ATMOSPHERE COMPOSITION ON HOMOGENIZING Al-Li AND Li-Mg ALLOYS: Z. Zurecki, Air Products and Chemicals, Inc., Allentown, PA 18195-1501
The selection of atmosphere composition is important for controlling surface defects of ingots while homogenizing reactive aluminum alloys. An improper selection of annealing atmosphere can result in surface blistering in aluminum-magnesium and sub-surface depletion of lithium in aluminum-lithium alloys. To determine atmosphere compositions required for homogenizing aluminum-magnesium and aluminum-lithium alloys with acceptable surface quality, a number of exploratory experiments were carried out in nitrogen and carbon dioxide based atmospheres including nitrogen or carbon dioxide and mixed with reactive impurities or additives such as oxygen, moisture, and sulfur hexafluoride. This paper will describe the influence of these atmospheres on the surface quality of aluminum-magnesium and aluminum-lithium alloy ingots.
THE EFFECTS OF OXYGEN CONCENTRATION ON THE SURFACE QUALITY AND OTHER PROCESSING VARIABLES FOR ANNEALING ALUMINUM AND ITS ALLOYS: G.R. White, T. Philips, H.S. Nayer, BOC Gases, Murray Hill, NJ 07974
Traditional atmospheres used for aluminum annealing have been air, exothermic gas, and cryogenic purity nitrogen. Air and exothermic gas have several problems associated with them such as surface discoloration, toxic emissions, and explosion hazards. High oxygen levels can cause magnesium streaking and oil staining, and therefore, pure nitrogen has been the atmosphere of choice for high quality aluminum coils. A comparative study of the surface quality of key aluminum alloys annealed in nitrogen with different amounts of oxygen content was made. The atmosphere composition inside the furnace was analyzed during the purge and heat cycle to determine its effects on purge times and exhaust emissions. The aluminum coils after annealing were visually inspected for surface staining, followed by lab analysis of the surface oxides. The study revealed that a small concentration of oxygen is acceptable, and did not adversely effect the surface quality or any of the other processing variables.
CHEMICAL REACTION AND HYDRODYNAMIC SCIENCE ISSUES FOR GAS-NONFERROUS LIQUID METAL REACTIONS: R. Mutharasan, M.J. Koczak, S. Kalidindi, Drexel University, Dept. of Materials Engineering, Philadelphia, PA 19104
The phenomena of gas-molten metal reactions for the production of metal matrix composites consist typically of a combination of one or more gas transport steps and a liquid gas reaction step. Reaction products include such strength enhancers, grain refiners, or high thermal conductivity phases as TiC, AIN, SiC, Si3N4, TiN in aluminum alloys. Nitrides, carbides and oxides can be produced at various volume fractions and in fairly fine sizes (500 nm to 5 um) in laboratory melts by Reactive Gas Injection method (RGI). Gas injection methodology to produce the reinforcements is attractive because it can potentially be scaled to production scales in excess of 10,000 kg/h. For continuous production, the reactant molten metal can be continuously fed into the crucible where the reactive gas is injected. The molten metal phase reactant may also be involved in a controlling transport step, depending on its concentration and viscosity. It is well known that the reaction product reinforcement size and shape of these phases have significant impact on the physical, mechanical and thermal properties. The current state of knowledge is unable to predict the kinetics and growth of solid reaction products obtained. The primary goal is to develop a fundamental framework for the understanding and prediction of the particle size in such reaction systems. In addition, the role of gas transport, liquid hydrodynamics and decomposition are important issues. A reaction-transport model is being developed using a reaction, transport, and thermodynamic framework, both at macro and at atomistic level to assess gas - liquid interactions.
VALIDITY OF THE KINETIC LANGMUIR'S LAWS FOR THE VOLATIZATION OF METALLIC ELEMENTS IN VACUUM METALLURGY: A NUMERICAL APPROACH: J. P. Bellot, H. Duval, D. Ablitzer, Ecole des Mines, Parc de Saurupt, 54042 Nancy Cedex, France
During the non-ferrous metal processing, the melting step under vacuum or
under rarefied gas leads to volatile metal losses. It is the case of the
metallurgical processing of the nickel-based, titanium-based or zirconium-based
alloys where the evaporation losses don't allow a sufficient control of the
alloy chemical composition. A theoretical approach has been carried out
involving both a continuum description in the liquid phase and a molecular
approach in the rarefied gas phase. The finite volume method is used in order
to solve the coupled transport equations in the liquid, whereas the Boltzmann
equation is modelled by using a Monte Carlo method. At the liquid metal and
metallic vapor interface, the volatilization and recondensation fluxes link up
the two calculations. Simulation results are given for the melting and refining
of reactive metals such as titanium alloys where the control of the chemical
composition remains a major difficulty. The validity of the kinetic Langmuir's
law is discussed for different operating conditions.
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