Contents
Cover
Half Title page
Title page
Copyright page
Foreword
Overview and Titanium Alloys I
Cost Effective Synthesis, Processing and Applications of Light-Weight Metallic Materials
Abstract
Introduction
Synthesis/Processing
Applications
Conclusions
Acknowledgments
References
Prospects for Cost Reduction of Titanium Via Electrolysis
Abstract
Introduction
Ti Electrolysis
Electrolysis Modes
Summary
References
Implementation of Advanced Metal Technologies to Reduce the Cost of Aerospace Systems
Abstract
Introduction
Metal Processing Improvements
Breakthrough Metal Processing
Conclusions
References
Recent Developments in the Manufacturing of Low Cost Titanium Alloys
Abstract
Introduction
Manufacturing Cost Factors of Titanium Alloys
Automotive Applications
Armor Applications
Conclusions
References
Lightweight Metals
Ale-Finite Element Simulation of “U” Shape Aluminum Profile Extrusion
Abstract
Introduction
Equations of ALE Finite Element Method
ALE Simulation of “U” Shape Profile Extrusion
Results and Analysis
Conclusion
Reference
Effect of Reinforcement in Mg Alloy Fabricated by Powder Metallurgy Method
Abstract
Introduction
Experimental Procedures
Result and Discussion
Mechanical Properties
Conclusion
Reference:
Titanium Alloys
Using Superplastic Forming as a Means of Achieving Cost Benefits as Well as Enhancing Aircraft Performance
Abstract
Introduction
Discussion
Conclusion
References
Superplastic Behavior of Fine Grained Ti-6Al-4V
Abstract
Introduction
Experimental Procedure
Results and Discussion
Conclusions
References
Experimental Study on Titanium Alloy Superplasticity Performance and Processing Parameters
Abstract
1 Introduction
2 methods and principles of strain rate sensitivity exponent measurement
3 Experimental Study
4 Conclusions
Reference:
Enhanced Superplastic Forming of Ti-6Al-4V
Abstract
Introduction
Experimental procedure
Results
Discussion
Conclusions
References
Microstructure Evolution in Hydrogenated Ti-6Al-4V Alloys
Abstract
Introduction
Experimental
Results and Discussion
Conclusions
References
Kinetics of Decomposition of Martensite in Ti-6Al-4V-xH Alloys
Abstract
Introduction
Experimental
Results & Discussion
Conclusions
Acknowledgements
References
Fabrication of Cost Affordable Components for US Army Systems
Abstract
Introduction
Experimental
Results and Discussion
Conclusions
Acknowledgements
References
High Integrity, Low Cost Titanium Powder Metallurgy Components
Abstract
Introduction
Materials and Experimental Procedures
Results
Discussion
Conclusions
Acknowledgements
References:
Titanium Powder Injection Molding - A Cost Effective Alternative
Abstract
Introduction
Titanium Powder Metallurgy
Acknowledgements
References
Cold Spray Process for Cost-Sensitive Applications
Abstract
Introduction
Technical Concept and Brief Description of the Cold Spray Method
Advantages and Applications
Economics Benefits. Savings for Cost-Sensitive Applications
Conclusion
References
Laser Induced In-Situ Formation of Ti/TiN Composite
Abstract
Introduction
Experimental Procedures
Results and Discussion
Conclusion
Acknowledgement
References
Intermetallics
Dispersion Strengthening of Ti-48Al-2Cr-2Nb Alloy with the Al2Y4O9 Particles
Abstract
Introduction
Experimental
Results & Discussion
Conclusions
References
Processing and Properties of Titanium Aluminide-Ceramic Particulate Composite Materials
Abstract
Introduction
Experimental Techniques
Results and Discussion
Conclusions
Acknowledgements:
References:
Fabrication of Tinicu Shape Memory Alloy From Elemental Powders
Abstract
1. Introduction
2. Experimental details
3. Results
4. Conclusions
Reference
Thermal Stability of Alumina Mold Against Molten Ti–Al Alloys
Abstract
Introduction
Experimental Procedures
Results and discussions
Conclusions
Reference
In-Situ Synthesis of Al-Ti-C Master Alloy Grain Refiners by Different Methods
Abstract
1. Introduction
2. Experimental procedures
3. Results and discussions
4. Conclusions
Reference:
Fabrication of Cu-Based Functional Parts by Direct Laser Sintering
Abstract
Introduction
Experimental Setup and Procedure
Results and Discussions
Conclusions
Acknowledgments
Reference:
Synthesis of Ti5Si3 and Ti5Si3-2Mo Alloys
Abstract
Introduction
Experimental
Results and Discussion
Conclusions
References
Light Magnesium Constructions for Transportation Applications.
Abstract
Introduction
Experimental procedures
Experimental Results
Discussion
Conclusions
Acknowledgements
References
Author Index
Subject Index
HIGH PERFORMANCE METALLIC MATERIALS FOR COST SENSITIVE APPLICATIONS
A Publication of The Minerals, Metals & Materials Society
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© 2002
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FOREWORD
These are the proceedings of the symposium on “High Performance Metallic Materials For Cost Sensitive Applications” held as part of the TMS Annual Meeting in Seattle, WA, February 18–21, 2002. The editors of these proceedings were also the organizers of the symposium, with Dr. E. Chen as the chief organizer.
The three most important industries in driving technological changes, national security considerations, and economic advances are information/communication systems (computers), biotechnology and advanced materials and processes. Of these three, advanced materials and processes are the most far-reaching and vital to advances in the other two fields. Advanced materials are characterized by enhanced mechanical and physical characteristics compared to traditional materials, such as aluminum and steel, currently manufactured in large-volume assembly line production. These attributes either allow for significant improvements in product or device performance or, of perhaps even greater importance, allow for new technologies that are unachievable using the traditional materials. However, there is another feature of advanced materials – they are almost always more expensive than conventional materials. It is this aspect of lightweight metallic materials, which is addressed in the papers contained in these proceedings.
The opening overview paper, by the editors, attempts to present a broad view of the role of advanced processing in the development of light weight metallic materials with the focus on the cost-effectiveness of the processes discussed. The remainder of the papers are divided into the three categories of lightweight metals, titanium, and intermetallics, and the processing routes presented include vapor deposition, laser processing, plasma spraying, gas atomizing, cold spray, rapid solidification, injection molding, mechanical alloying, extrusion, superplastic forming, joining, and casting.
The organizers of this symposium, would like to thank everyone who helped make the event a success and particularly the session chairmen, the presenters, the authors of the 25 papers reproduced here and Steve Kendall of TMS who coordinated these proceedings.
F.H. (Sam) Froes, E.Y. Chen, R.R. Boyer, E.M. Taleff,
L. Lu, D.L. Zhang, C.M. Ward-Close and D. Eliezer.
The low density metallic materials aluminum, magnesium, and titanium are important in many segments of the world economy, ranging from aerospace to sports equipment. The importance of cost is strongly dependent on the industry being considered: in the construction and automobile industries, cost is extremely important, while in the aerospace and medical industries, performance is emphasized over cost. This paper provides an overview of the synthesis, processing, microstructures, mechanical properties, and applications of these lightweight materials and discusses the importance of cost-effective processing.
As performance demands increase, so do the demands for increased mechanical performance, such as increased strength and fracture toughness, at reduced overall weight. Reduced weight can be most efficiently realized by the use of light (low-density) metals, such as aluminum, magnesium, and titanium (Figure 1) [1]. Significant improvements in the mechanical performance of light metals are being made via ingot metallurgy processing, but even more substantial developments require “far from equilibrium” synthesis, with the attributes shown in Table I [2–5]. When cost is of primary importance (Figure 2) then this factor is emphasized over performance.
Figure 1. Effect of property improvement on structural weight in aerospace applications.
Figure 2. Relative importance of cost and performance in advanced material use industries (Courtesy of Congressional Office of Technology Assessment).
Table I. Attributes of “Far from Equilibrium” Synthesis.
|
1. Production of a Fine Dispersion of Second Phase Particles
2. Extension of Solubility Limits
3. Refinement of the Matrix Microstructure down to the Nanometer Range
4. Synthesis of Novel Crystalline Phases
5. Development of Amorphous (Glassy) Phases
6. Possibility of Alloying of Difficult to Alloy Elements
7. Inducement of Chemical Reactions at Low Temperatures
8. Scaleable Process |
Development of aluminum alloys for aerospace applications has changed from an emphasis on high strength to concern over other characteristics, including cost and durability [2]. For example, Alcoa developed the T77 temper for alloy 7150 (Al-Zn-Mg-Cu) to achieve good corrosion resistance with no sacrifice of strength. A major concern for aerospace applications is the need for lower cost materials because of the gap between airplane cost per seat and yield per passenger. Castings, Al-Li alloys, composite concepts (including lamellar ARALL and GLARE), superplastic forming, and Al-Mg-Sc alloys are all contributing to the mis of available materials. The automobile industry is also looking towards cost reduction for aluminum materials, with some recent emphasis given to production by continuous casting with consistent properties and performance. While the major concern in the automobile industry is cost, in the sporting equipment market even the dedicated amateur will bear relatively high costs if a product enhances his or her performance. Such markets may be amenable to I/M processing by severe plastic deformation (SPD), which can produce both an extremely fine microstructure and solubility extension. The high hardnesses achieved on subsequent aging in an Al-Fe alloy processed by SPD is shown in Figure 3 [6].
Figure 3. Microhardness of a Al-Fe alloy subjected to severe plastic deformation (SPD) and aged at 100°C [6].
Emphasis in the titanium arena has been on reducing cost, including lower cost production of sponge and use of lower cost Al-Fe master alloys rather than the traditional Al-V master alloys (i.e. for the work-horse Ti-6Al-4V alloy).
The major segment of P/M is concerned with production of low-cost, near-net-shape components. A important new development is in the use of powder injection molding (PIM) processes based upon earlier plastics technologies. This approach is particularly applicable to small, complex parts. A schematic of the process is shown in Figure 4 [7] and a part produced from titanium powder is illustrated in Figure 5 [8].
Figure 4. Schematic diagram of the Powder Injection Molding (PIM) process [7].
Figure 5. Titanium alloy watchcase produced using the PIM Process (Courtesy Hitachi Metals Precision/Casio Computer Co).
Semi-solid processing in which near-net-shape castings are fabricated at a temperature between the solidus and liquidus (i.e. semi-solid state) is growing in commercial importance [9]. While the cost of SSP components is a little higher than castings, the properties of materials produced by this newer technology are intermediate between those of cast and forged products (Figure 6).
Figure 6. Mechanical property behavior of forge, semi-solid, and cast material.
The majority of parts fabricated to date are from casting alloys (A356/A357), with some alloy development in progress to enhance mechanical properties while maintaining good processing characteristics.
The thixomat process (Figure 7) is rapidly growing in commercial importance for fabrication of complex magnesium components, such as the one shown in Figure 8.
Figure 7. Schematic of the Thixomat Process.
Figure 8. Complex magnesium thin walled magnesium housing for anti-skid brake system weighing 142 grams (Courtesy Thixomat).
A recent breakthrough at Cambridge University, called the FFC process, could have far reaching consequences for the titanium industry. Termed electro-deoxidation, the process uses molten salt electrolysis to remove oxygen from solid metal oxide. The result is pure titanium metal with near zero oxygen and nitrogen levels. Electrolytic extraction has been tried before for titanium, of course, from solutions of titanium dioxide or titanium tetrachloride in molten salts. But has never been demonstrated to be economic, except for the production of high purity electronic grade titanium. The FFC process differs from these previous routes in that neither the titanium dioxide nor the metal are ever dissolved in the electrolyte. The process is now being scaled up by the UK technology company, QinetiQ (formally DERA) (Figure 9) and is proving to be faster, more energy efficient and more environmentally friendly than the existing Kroll process and it uses cheaper ingredients, Figure 10.
Figure 9. QinetiQ FFC titanium pilot plant
Figure 10. Starting material and products of the QinetiQ FCC process.
The technique allows extension of solubility limits, production of novel phases, and much more refined microstructures than are possible using the I/M technique. The greatly increased chemistry/microstructure “window” can lead to enhanced mechanical and physical properties.
The five families of aluminum alloys being explored using the RS approach are the high strength and corrosion resistant alloys based on traditional 7000-series alloys; low density Al-Li alloys with increased lithium levels over those possible using the I/M approach, dispersoids-strengthened alloys for elevated temperature service based on low solubility/low diffusion rate additions, such as the transition metals (Fe, Mo, Ni) and rare earth elements (Ce); wear resistant high-Si alloys; and recycled alloys that are normally impossible to recycle (i.e. avoidance of detrimental segregation).
The attractive elevated-temperature mechanical properties, which can be achieved in a RS aluminum alloy are shown in Figure 11.
Figure 11. Variation of ultimate tensile strength (UTS) with temperature for a rapidly solidified Al-Cr-Mn-Cu alloy containing dispersoids of a quasicrystalline phase [10].
Cast magnesium alloys may exhibit less-than-desirable strength, ductility, and creep behavior, and their non-protective oxide skin may lead to severe corrosion problems. Using RS, high strength/corrosion-resistant magnesium alloys containing rare earth additions (Y, Nd, Ce) have been developed.
The major concentration on RS terminal titanium alloys to date has been to enhance elevated temperature capability beyond IM alloy levels (i.e. >700°C) through dispersion hardening. The additions of Er and other rare earth elements produces dispersoids, which resist coarsening at least up to 800°C. However, much further optimization is required, particularly in increasing the volume fraction of second phase particles.
In combination with the near-net-shape advantages offered by the PM approach, RS may offer some advantages for the processing of the intermetallic gamma titanium aluminide alloys over the I/M approach.
In SD, finely divided molten metal droplets, produced by disintegration of a stream of molten metal using high energy inert gases, impinge on a substrate before they completely solidify. This allows some of the characteristics of RS to be achieved in combination with a near-net-shape capability.
Mechanical behavior can be enhanced in conventional aluminum alloys as a result of refining the constituent particle size (Figure 12). The spray deposition of titanium-based alloys has been investigated only recently, with some early successes.
Figure 12. Enhancement in mechanical properties of a spray formed 7xxx alloy.
Mechanical alloying (Figure 13) is a process in which heavy working of powder particles results in intimate alloying by repeated welding and fracturing. This process has the same attributes as RS - extension of solubility limits, production of novel structures, and refinement of the microstructure (down to the nanostructure range). Additionally, MA effectively produces dispersions of second phase particles.
Figure 13. Schematic of mechanical alloying process.
High-temperature aluminum alloys with Ti additions, low-density alloys with Mg additions, and extra-low-density alloys with Li additions have been developed through MA. The mechanical property combination which can be obtained in an MA Al-base alloy is shown in Table II. The MA Al-Li alloys exhibit minimal degradation of properties when stressed in the transverse direction and are characterized by excellent general corrosion resistance, which is 100x better than that exhibited by the IM alloy.
Table II. Room Temperature Mechanical Properties of MA Aluminum Alloys (Longitudinal).
MA has been employed in the development of “supercorroding” magnesium alloys for submarine applications, such as a heat source in diver suits, and as a hydrogen gas generator.
MA of titanium alloys has resulted in supersaturated solid solutions and metastable crystalline and glassy phases, as well as nanometer-sized grain structures.
Work on MA of the titanium aluminides indicates that this is an interesting fabrication method for both the alpha-2 and gamma families of alloys, resulting in the formation of surprisingly stable, nanosized grains, which are stable even after compaction by hot isostatic pressing (HIP’ing), Figure 14.
Figure 14. Stability of grain structure in a hot isostatically pressed gamma titanium aluminide.
In plasma processing, solid feed-stock is converted to the vapor phase at elevated temperatures due to the highly concentrated enthalpy. Chemical reactions occur and a solid product nucleates and grows at a lower temperature. A number of studies have led to the formation of nanometer sized metallic powders (Figure 15).
Figure 15. Nanosized metallic powders produced by plasma processing.
Production of alloys directly from the vapor phase allows even greater flexibility in constitutional and microstructural development than RS and MA. For example, the strength of a vapor deposited Al-7.5Cr-1.5Fe alloy is significantly higher than the strength of RS alloys up to at least 250°C, a result of the fine micro structure produced by the vapor deposition technique. Either monolithic material or alternating layers of two or more metals (Figure 16) can be produced, the latter with novel mechanical properties.
Figure 16. Schematic of apparatus for production of vapor quenched multi-layers and hardness versus layer thickness for a Al/Fe laminate.
The electron beam vapor quenching technique has been used to produce titanium alloys which are not possible by IM or even RS. One example is the production of low density Ti-Mg alloys. Mg boils below the melting point of titanium making production of a liquid alloy impossible by conventional methods.
The extremely efficient EB process (85–90% of the electrical energy is converted to useable energy) provides a potential for very high temperature processing. Thus, reactions which do not normally occur can be run to completion and, in combination with rapid cooling, the products can be retained to ambient temperatures.
A number of synthesis/processing techniques have been briefly reviewed in the text above. Some of the newer techniques offer improved mechanical properties, and some have the potential for reduced cost. However, it is the ingot-metallurgy approach which is by far the dominant method of commercial use. A comparison of the techniques is given in Table III [11]. Table III. Comparison of the characteristics of a number of advanced processing/synthesis techniques.
Applications for advanced materials can occur in various segments of the economy. The processes used to synthesize these materials are generally more expensive than conventional ingot processes, so that use must be justified by increased system performance. The cost/performance increase is much easier to justify in some industries than others (Figure 1). For the automobile and aerospace industries, an important aspect of increased system performance is reduced weight and, hence, improved fuel economy (Figures 15 and 16).
Figure 17. Effect of weight reduction on fuel consumption for an automobile.
Figure 18. Lightweight magnesium wheel on the Ferrari 550 Maranello. Unsprung weight reduction significantly improves handling.
A major success story is the use of advanced materials on the Boeing 777, which has a greater number of advanced materials than any other Boeing Commercial Airplane [12]. These materials earned their way into the vehicle because of their contributions to weight reduction, cost, and maintainability/durability (Figure 19).
Figure 19. An example of reduced operating cost (fuel and maintenance) on the Boeing 777 by use of advanced materials
Much of the development effort in the aerospace is focused on cost reduction. The primary efforts for each alloy system include, for: a) titanium - laser forming, structural castings, SPF/DB, warm drawing and high speed machining; b) aluminum - friction stir welding (FSW), thick plate materials with improved properties, high speed machining and improved casting technology; and c) steels - corrosion resistant gear alloys and a high strength corrosion resistant (CRES) alloy to replace the high strength low alloy steels and. A high strength CRES alloy would significantly reduce the machining, heat treating and corrosion problems associated with the high strength low alloy steels.
These technologies are in various stages of maturity, with, for example, the primary goal for the titanium structural castings is to generate the required design allowables, to the FSW of aluminum still requiring a significant effort to determine, the most appropriate alloys, how and if they will be heat treated, and characterizing the properties for airframe applications. However, this technology has already been demonstrated and implemented for space applications, on the Delta series launch vehicles, with very significant cost savings. The Boeing Company became the first U.S. firm to use this technology in commercial production.
A 2014-T6 aluminum alloy isogrid propellant tank prototype weld joint is shown in Figure 20. FSJ has been used in production for this type of structure while reducing cost, improving quality, and increasing the weld strength. Thousands of feet of production welds have been made with no defects as determined by ultrasonic inspection. It is used on the Delta IV program.
Figure 20. Weld Joint in a Prototype Isogrid Propellant Tank Panel.
Both the speed and environmental impact/rideability of a high speed train, such as the Shinkasen Series 300, are enhanced by the use of aluminum to replace the steel used in the earlier Shinkansen Series 100, Figure 21 (13).
Figure 21. Japan Railway Shinkansen 100 (right), 300 (left) series trains. Note the lower profile of the newer vehicle.
The sporting equipment business is very significant at around US $20B per year in the USA alone. Just by taking two examples, the impact that advanced materials have made on sports performance can be clearly shown: golf and skiing [14–16]. Particularly with drivers, improved golf club heads can be fabricated from titanium (Figure 22) with a demonstrable performance enhancement, and the name of the game for “top shelf” skis is titanium (Figure 23).
Figure 22. Titanium driver golf club heads (Courtesy BIAM)
Figure 23. Titanium Volant skis.
This paper has discussed a number of innovative materials/synthesis techniques as applied to the light metals aluminum, magnesium, and titanium and to the intermetallic titanium aluminides, Some potential applications have been presented with the impact of performance and cost in various industries put into perspective.
The author appreciates useful discussions with E.G. Baburaj, J. Hebeisen, R.M. German, J. Liu, L. Öveçoglu, S. Patankar, O.N. Senkov and F. Sun. In addition, the author would like to acknowledge the assistance of Mrs. Marlane Martonick in manuscript preparation.
1. I.J. Polmear, Light Alloys-Metallurgy of the Light Metals, 3rd Edition, 1995, Halstead Press, John Wiley and Sons, New York, USA.
2. John Liu, J.T. Staley and W.H. Hunt, Jr., Third ASM Paris Conf, Eds. F.H. Froes, T. Khan and C.M. Ward-Close, ASM Int., Materials Park, OH, 1997, 91.
3. F.H. Froes and C. Suryanarayana: “Powder Processing of Titanium Alloys”, Book Series on Reviews in Particulate Materials, eds. A. Bose, R. German, and A. Lawley, (1993) MPIF, Princeton, NJ.
4. F.H. Froes, P Tsakiropoulos, C. Suryanarayana, and W. Baeslack: Light Materials for Transportation Systems, ed. N. Kim, Center for Advanced Aerospace Materials, Pohang Univ. of Sci. and Tech., Pohang, Korea, 1993, 27.
5. L. Öveçoglu, F.H. Froes, N. Srisukhumbowornchai and D. K. Mukhopadhyay “Grain Growth Behavior of Nanograined Gamma TiAl Compacted by Hot Isostatic Pressing,” ASM Int. Conf. HIP ‘96, eds. F.H. Froes, R. Widmer and J. Hebeisen, ASM Int. Materials Park, OH, 227.
6. O.N. Senkov et al, PRICM-3, Eds. M.A. Imam et al, TMS, Warrendal, PA (1998) 847.
7. R. M. German, Powder Metallurgy Science, 2nd Ed, 1994, MPIF, Princeton, NJ.
8. F.H. Froes and R. M. German, Metal Powder Report, June 2000, 2.
9. F.H. Froes, Light Metal Age, Oct. 1998, vol. 56, nos. 9 and 10, 56.
10. H. Kanahashi et al, PRICM-3, Eds. M.A. Imam et al, TMS, Warrendale, PA (1998) 2,091.
11. F.H. Froes, Light Metal Age, vol. 58, nos. 1&2, Feb. 2000, 72.
12. John Liu and F.H. (Sam) Froes, Synthesis/Processing of Lightweight Metallic Materials, eds. F.H. (Sam) Froes et al, TMS, Warrendale, OH, 1999, 103.
13. F.H. Froes, Light Metal Age, vol. 52, nos. 3&4, April 1994, 12.
14. F.H. Froes, JOM, vol.49, no. 2, Feb. 1997, 15.
15. F.H. Froes, JOM, vol. 51, no. 6, June 1999, 18.
16. F.H. Froes, MRS, Special Issue, vol. 23, no. 3, March 1998, 32.
Fused salt electrolysis is one of the more interesting techniques available for producing or refining titanium. The method is quite versatile, not only in control of metal purity, particle size, and distribution, but also in feedstock, cell configuration and operating mode. Three basic operating techniques are available: electrowinning, electrorefining, and electron mediated reaction. At present, only electrorefining is commercial, although electrowinning titanium was demonstrated on the industrial scale some 40 years ago and is again showing some promise. Current technologies and prospects for further progress are reviewed.
Ti, occurs naturally as a dioxide, rutile, or mixed oxide with iron, ilmenite. Ancient beach sand ores, naturally concentrated in rutile (TiO2), are mined for Ti metal production. Ilmenite deposits are mined for both their Ti and Fe values.
Because of its affinity for gases and most metals in the periodic table, Ti is quite difficult to win from its ores. Blast furnace reduction of ilmenite produces pig iron and a high titania slag (synthetic rutile) that is suitable for metal production. Ti is won from either form of oxide by chlorination in the presence of carbon at high temperature. The resulting tetrachloride is purified and reduced by Na, or Mg, to produce Ti metal under argon in stainless steel pots at about 900C. Mg is the more economical option. Na is used only to produce electronic grade Ti sponge for subsequent electrorefining to 99.99% - 99.9999% metallic purity crystal with gas totals of less than 100ppm. All of these processes are expensive.
Ti has such a rich suite of useful properties that even small changes in its cost structure could result in significant new applications. This paper critically reviews existing electrolysis technologies for potential for cost savings.
Electrolysis from aqueous solutions fails because of hydrogen over-voltage. In a fused salt, electrolysis is straightforward in theory, not so easy in practice. Electrode potentials are readily calculated from the Nernst equation and can be measured using a counter electrode. The Nernst Equation, NFE = G, where N is the number of moles, F is the faraday constant, E is volts and G is the free energy of formation of the salt governs the process. Figure 1 illustrates the possible anode and cathode reaction voltages in a chloride salt bath.
Figure 1 Chloride electromotive series relative to divalent Ti, (1).
In this mode, Ti is extracted from a precursor compound such as TiCl4 by electrolysis in a two-step process. Example electrode reactions in a chloride bath containing TiCl2 would be:
(1) 
(2) 
During the 1950’s and 1960’s, TIMET (2) came tantalizingly close to making electrowinning work on the full industrial scale. They had the chemistry and process model right. However, anode support configuration was too complex to be workable and improvements in materials of construction were still needed. Nevertheless, TIMET demonstrated an ability to produce Ti crystal by electrolytic means on the industrial scale. Crystal quality was superior to Kroll sponge of the day in all respects.
TIMET designed a technically successful electro-winning cell using a grossly porous “membrane” —a coarse screen basket—at cathode potential. As shown schematically in Figure 2, the basket was integral with the cathode assembly and was withdrawn with each crystal deposit for stripping. Sodium chloride was charged into the cell and melted under an argon cover.
Figure 2 Schematic of TIMET electrowinning cell circa 1962.
TiCl4, being covalent, cannot be electrolyzed directly. It must first be partially reduced to the dichloride. TiCl2 is generated by contacting TiCl4 with an excess of Ti metal in the cathode basket, as discussed below. Trivalent Ti does not electrolyze effectively; concentrations of divalent Ti of at least a few percent are needed for efficient electrolysis. The ionizing reactions proceed according to:
(3) 
(4) 
(5) 
TIMET employed a refractory lined cell with graphite anodes arranged about the periphery in the anode compartment; chlorine generated was ducted to chlorinators treating rutile. Anode current entry was through the cell bottom and required continuous cooling to prevent leakage. This proved to be an Achilles heel as things turned out. Power failures led to leaks, even cell drainage.
Cathode current entry was through the removable cell top. The cathodes and feed pipe were enclosed in the centrally located cathode basket with the cathodes disposed about the TiCl4 feed pipe situated in the center of the cell. This entire assembly was of carbon steel.
The TIMET cell was started by turning on power and electrolyzing an amount of Na in the cathode basket before feeding any TiCl4. With a substantial amount of Na present, TiCl4 was fed to create a deposit of sponge for reaction with TiCl4 to generate TiCl2 according to equations 3–5. TiCl4 was introduced through a central feed pipe deep into the center of the cathode chamber such that TiCl4 bubbled upward among the cathodes and reacted with the Ti crystal accumulating there before being reduced by electrolysis. Reactions 3 – 5 remained in dynamic equilibrium within the cathode basket during the run.
A small amount of TiCl4 was entrained in the chlorine offgas from leakage or oxidation of TiCl2 on the anode. These gases were ducted to the chlorinators converting rutile to TiCl4 As the Ti deposit grew, current was increased to a set rate and continued until the desired amount of TiCl4 was fed.
At the end of each run the cathodes and basket were withdrawn, harvested, leached in dilute acid, dried, and packaged for melting. Current efficiency was high because molten salt is a poor conductor of electricity. Compared with massive sponge, electrolyzed crystal was relatively fine and discrete and required very little crushing; only light acidic rinsing was needed to remove entrained salt. Quality of the best crystal was <200 ppm O2 and < 40 ppm Fe. A hundred tons of crystal were made in nominal lots of 400 lbs. TIMET established the basic viability of electrowinning Ti from TiCl4 and was never troubled with the membrane blinding that defeated contemporary efforts elsewhere (2). Later, Ginatta (3) reported similar success.
Cost saving potential over Kroll sponge arises from process simplifications that eliminate the need for Mg or Na recycle circuits. Metal harvesting is simpler and purity is more controllable and predictable from the electrolytic process. It remains to get the engineering right. Modern refractory technologies should help.
In another version of fused salt electrolysis, TiO2 is the feed stock. A diluent-halide salt containing fluoride is used to ionize Ti; O2 evolves off the anode combined with carbon. The cost of purifying natural or synthetic rutile for feedstock would be added to the cost of using and handling fluoride under environmental controls. The only known use of this procedure was in a patent application (4) to get around prior patent art.
A second procedure involving oxide was developed at Cambridge University, reported by Chen, Fray, and Farthing (5). They employed TiO2 pressed and sintered into pellets as the cathode in a fused salt bath of CaCl2 at 950C. Under an impressed cathodic potential, a small amount of oxygen is ionized such that the remaining oxide becomes oxygen deficient and electrically conductive. TiO2 simply converts to metal as the oxygen is ionized and removed—with the cathode becoming porous in the process. The applied potential was said to be kept under that needed to reduce Ca. The ionized oxygen reacts with CaCl2 in the bath to form the more stable CaO, which is soluble in CaCl2. At the anode, O2, CO, and CO2 are discharged with reformation of CaCl2. The rate-limiting step is oxygen diffusion out of the oxide and may limit cell productivity. British Titanium (BTi) was incorporated in 1998 to commercialize the “FFC-Cambridge” process. BTi funded DERA – Farnborough, UK to build a test plant to demonstrate one kg batches. On the one Kg scale, oxygen content is reported to be as low as 60 ppm. Further scale up is said to be in progress.
This novel procedure might benefit from less-than-pigment-purity TiO2. By mixing oxides, certain alloys can be produced directly to reduce certain alloy costs still further. Aside from the direct cost of electrolysis, the major support facility would be conventional sintering ovens. Graphite anodes would be consumable items. This process eliminates both the Cl2 and Mg circuits. With suitable purity of TiO2 and other controls, it could, in principle, capture some of the electronic market where the highest sponge grades now compete in the low-quality end of this market.
This mode refers to the procedure where Ti is electrolyzed off of the anode and goes into solution for electro-deposition on the cathode. Electrorefining was researched extensively by the U.S. Bureau of Mines in the 1950’s and 1960’s. The Bureau work proved the efficacy of electrolytic refining Ti. TIMET also researched the method and produced laboratory quantities of electrorefined crystal for many years. TIMET used a double-pass technique where a billet was the starting anode that was only partially consumed before the current was reversed to complete the run. Very low current densities were used.
In 1985, The Alta Group improved on the Bureau and TIMET processes and established the market for electrolytic grade Ti. Alta’s typical product today contains between one and five ppm total metallic impurities with less than about 100 ppm total gases (H, C, N, O). Total metallic impurities can be less than one part ppm with most individual metal impurities being in the ppb-ppt range by glow-discharge mass-spectrometry.
The electrode net reactions are:
(6) 
(7) 
Side reactions occur on each electrode.
(8) 
(9) 
To the extent reactions 8 and 9 occur, current efficiency is lost; in other words Ti electrorefining has a built-in “chemical short circuit.” Researchers at the Bureau of Mines (6) identified this problem early on and went to great lengths to integrate the average effective valence of the electrolysis bath into their process controls—it should be below 2.1, preferably below 2.05 for acceptable current efficiency.
Metals disposed near Ti in potential tend to electrolyze along with Ti and comprise the so-called “window elements.” This is a problem only when producing electronic grade material where metallic impurity levels of parts per billion or less are important. Al, Cr, Mn and Zr are common window elements. Operating under high over-voltage increases (high current density) window-element impurities in crystal product. Iron is another important impurity because of its high level in input sponge and its common presence in cell superstructure as well as ambient dust and rust in a mill environment.
Electronic grade Ti is used principally in the manufacture of computer chips for diffusion barriers, contacts, interconnects and anti-reflective coatings. About 90% of all Electronic Ti deposited is co-sputtered with nitrogen to produce TiN which is valued for its electrical conductivity, resistance to diffusion, and its optical characteristics. The total size of this market in lbs is miniscule relative to the Ti industry as a whole, but its commercial value in the production of computer chips is significant. Electrorefining is a mature process.
Guidance for improving the cost structure of electronic grade Ti comes from the Bureau of mines work—eliminate the chemical short circuit. An additional possibility is to move back a step in the overall process to electrowinning. For best economy, electrowinning from TiCl4 requires recycling chlorine on site.