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Chapter 11 TECHNOLOGIC OPPORTUNITIES FOR TITANIUM The successf ul introduction of titanium over the past three decades as a new structural material was based on its properties (i.e., it is light, strong, ductile, and corrosion resistant) and on the relative abundance of its ores throughout the world. Titanium's next important advances may well be based on property and processing improvements achievable by introducing advanced metal-winning techniques and the processing of metal to yield preferred fine-grained microstructures, which give improved properties. Means also appear to be at hand for incorporating a number of near-net-shape (NNS) technologies into the production scheme for making titanium end items. The latter technologies are attractive because they would permit economical parts production and the production of parts with preferred microstructures. Other practices that may become available in the future appear to be those that give improved economy in the production of titanium and provide improved properties by way of refining microstructures. This chapter considers those processes that the panel believes are most likely to attain significant industrial importance in the not too distant future (5 to 10 years). Metal-Winning Processes The foremost development in titanium metal-winning, already reduced to semicommercial practice by the D-H Titanium Company (and, separately, in pilot production during the past decade by TIMET), is the electrolytic reduction of the tetrachloride to metal. Available details were presented in Chapter 5. Electrolytic winning of titanium is only one of many new advances. Others include the improved and innovative techniques being incorporated in new or renovated Kroll and Hunter processing plants for sponge titanium production. The improved techniques are largely still proprietary but include changes such as greatly increased batch sizes. Continuous processes for producing sponge, and even processes f or continuously producing consolidated metal (ingot), are now in the research stage. The improved metal-winning processes that are now available (e.g., larger batch sizes and electrolytic processing) or that are being researched, are a ma jar key to extended growth in the applications of titanium. 153
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1S4 Sponge and Alloy Metal Consolidation and Processing Larger ingots of titanium and titanium alloys (e.g., up to 40,000 pounds at RMI) will be possible in the near future as new furnaces for the melting of titanium now on the drawing boards or under construction are completed and begin operation. More exact melting-parameter controls for the production of more homogeneous ingots are featured in the new equipment. Larger ingot availability provides not only an economy-of-size improvement for usual mill products, but also the possibility of applying titanium to end-items that were previously marginal due to ingot size limitations. Notable advances in metal melting are the electron-beam and plasma-arc furnaces that are now or soon will be in place for the reclamation and consolidation of titanium scrap forms that f orderly had little recycling value. The technology for such furnac~ng may not be regarded as an industrial technological breakthrough, but the application of these furnaces to the melt-processing step for titanium is a technologic opportunity. Further advances in this area (e.g., the continued development of producing square- or rectangular-section slab ingots suitable for direct rolling to flat-rolled products) are sure to come. The Japanese have produced a rectangular-section ingot by plasma-arc remelting . The Soviet s have produced small slab ingots by electroslag remelting techniques. There is indeed opportunity for the U. S . titanium industry to develop similar and improved capabilitie s . The opportunities f or technological advancement in metal working do not involve equipment so much as techniques; however, the new generation of metal working equipment (e.g., computer-controlled rotary forging equipment) will afford numerous advantages. Improvements in metal processing to achieve preferred microstructures tailored to fit particular applications for optimized properties (e.g., for needed combinations of high strength, toughness, creep, fatigue, and f ormability ~ are known today but not widely applied . Noteworthy t echniques were described in Chap ter 7 . Yttrium and Rare Earth Additions In 1974, the RMI Company pioneered the pilot production of titanium alloys containing 0.005 to 0.03 weight percent yttrium that refined the microstructure and significantly improved the yield of salable mill products from ingots. Because of the reluctance among certain users to accept yttrium additions in titanium, production was discontinued. Increased yield equates with increased productivity, a highly desirable improvement worth pursuing. The apparent advantages and disadvantages of yttrium additions to titanium alloys present a fairly complex picture . It is an area worth studying, not only for itself, but
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155 also for the background insights on the several ways in which fine microstructure s contribute to a number of the technologic opportunities discussed in this chapter. Accordingly, the panel recommends research and development on the following aspects of yttrium and rare earth grain-refining and deoxidizing additions to titanium alloys: 1. Mechanisms for effects, and their control, on properties produced by yttrium and rare earth additions. 2. Effects of these additions on mechanical properties. Optimization of the variables and establishment of specifications that will ensure the safe and full exploitation of this potential opportunity. Near-Net-Shape (NNS) Proces sing The high cost of titanium and its alloys is due primarily to three f actors: high sponge cost; relatively poor yield f ram sponge to mill products, particularly for high-strength alloys; and large secondary fabrication losses, especially in military and commercial aerospace applicat ions . The average ratio of buy weight to fly weight for aircraft applications is estimated to be about 6 :1; some individual parts achieve a low 1.5:1 ratio whereas others approach 20:1. Since military and commercial aerospace uses that involve these high fabrication losses account f or about 75 percent of total aerospace titanium sales, processing that will improve yields has enormous potential for reducing cost and f or effectively increasing titanium supply without installing new sponge capacity. If it is assumed that U.S. sponge availability is 30,000 tons per year, 22,500 would go to aerospace. If the purchase to end-use ratio for this sponge could be reduced from the estimated average of 6:1 to 3 :1, effective capacity would be increased by 11,250 tons per year . Recognizing these possibilities early, the Manufacturing Technology Division of the Air Force ~ Materials Laboratory since 1972 has been actively developing fabrication technology to produce preforms near to the f inal shape to reduce the enormous f inishing losses and cost s of producing aircraft components for both airframes and engines. They have been joined in this effort by the Naval Air Systems Command. The panel commends these organizations for their foresight and for the excellent development programs they continue to sponsor. Today, numerous complex critical parts are flying in advanced aircraf t that give promi se of achieving important savings in dollars and resources.
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156 Programs sponsored by these organizations cover a broad front including: precision casting and precision powder metallurgy molding, both consolidated to theoretical density by hot isostatic pressing; isothermal shape rolling; diffusion bonding; and the combination of superplastic forming and diffusion bonding. Each of these areas represents a ma jor technological opportunity for titans um and should be pursued vigorously. Near-net-shape processing programs sponsored by the Air Force Materials Laboratory and Naval Air Systems Command will be discussed only in general terms in this report since detailed reports are available from the agencies and their contractors. Much progress has been made and reference will be made here only to outstanding problems and opportunities . Superplastic and Diffusion Bonding Fine-grained titanium alloys are superplastic at low strain rates within critical temperature ranges, generally near the beta transus where deformation occurs primarily by grain boundary shearing. It probably is not coincidental that alloys can be diffusion bonded under the same time-temperature conditions. The foregoing combination of processes has permitted a near-net-shape technology to be developed that permits the joining of the two operations into a single superplastic f arming ( SPF ) and diffusion bonding (DB) process, designated SPF/DB. Ti-6Al-4V in the fine-grained equiaxed condition commonly is processed using SPF/DB. Processing generally is done at about 930 to 960°C, with about equal quantities of alpha and beta phases present. Forming and bonding generally take place in a vacuum or an inert atmosphere. Oxide films on the surfaces of the titanium forging under these circumstances dissolve into the underlying metal because of the high solubility of oxygen in titanium. Any mating titanium surfaces will be diffusion bonded with a joint that will have base metal characteristics because of recrystallization across the bond interface. In producing complex parts by the SPF/DB process, the components are assembled in a die in the form of sheet, plate, ring, or other simple geometric shapes. Parts to be diffusion bonded are pressed together by mating dies at rather low pressures, about 150 psi. Argon pressure then is applied to the sheet surfaces to be expanded into the enclosed die. The result is a formed and bonded part that requires no machining and is ready for service. Thus, although the SPF/DB process employs rather expensive starting materials compared to conventional forming, the high yield permits considerable cost and weight saving. Accordingly, SPF/DB is expected to be used extensively in modern airframes like that for the B-1 a ~ rcraft.
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157 The superplastic def ormation characteristic of fine-grained Ti-6Al-4V permits tensile strains of as much as 500 percent to be achieved, which ~ s sufficient to permit pressing of sheet parts to fill complex die cavities . This high superplastic capability also is suf f icient to permi t forging billets to fill closed dies and form near-net-shape disc forgings in a single step. Isothermal forming is a variation of superplastic forming. In this, the material is formed in hot dies approaching the temperature of the work piece. Isothermal forming is generally practiced by press-forging techniques. Isothermal shape rolling is accomplished in a similar manner, but the problems of maintaining the rolls at the working temperature are much greater than in isothermal press f orging. The temperatures employed in isothermal forming are about the same as those used in superplastic forming (i.e., near the beta transus). Fine-grained, equiaxed microstructures are preferred to acicular microstructures. The strain rates for isothermal forging are about one-tenth to one one-hundredth those used in conventional forging but are higher than those used in superplastic forming. The low strain rates employed allow considerable relaxation to take place during the forming operation even though actual dynamic recrystallization may not occur as in superplastic f arming . Thus, high strains may be accomplished in a si ngle f arming operation. The main problem is die materials. For an alpha-beta alloy like Ti-6Al-4V, molybdenum alloy dies may be required whereas for a beta alloy like Ti-lOV-2Fe-3Al, superalloy dies are satisfactory. Isothermal forging of f ers considerable economic advantage over conventional forging because the initial billets are comparable in cost, but isothermal forging results in a near-net-shape product with little or no scrap. In contrast, isothermal forging requires much greater die cost and press t ime . The trade-of f between the two processes varies f ram one case to another, but much greater use is expected for isothermal f orging in the future. Isothermal shape rolling also might be expected to be used more commonly f or gaining similar economic advantages in cases where rolled shape s can be applied in volume . Precision Casting The precision casting of complex titanium shapes has progressed rapidly during the past decade, particularly when the castings are subsequently hot isostatically pressed to close internal porosity. In principle, the properties of optimized castings should approach those of wrought products in most categories and have promise of even exceeding one or more of these properties, one example being elevated temperature creep. However, more validation testing of individual castings is required. Moreover, the problem of achieving optimized mechanical properties of castings, even after hot isostatic pressing, requires further study before proper design allowables can be established.
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158 Additional major problems in precision casting result from surface contamination due to mold-metal reactions; from defects such as porosity, cold shuts, and inclusions; and from the economical reprocessing of casting scrap, at least in some parts of the industry. Research and development in each of these areas promises valuable payoffs. Precision Powder Metallurgy Molding of Complex Shapes Precision powder metallurgy molding, like precision casting, shows promise for cost reduction and materials savings. It, too, has serious problems that justify a further research and development effort. Precision powder metallurgy has suffered most from the lack of high-quality, reasonable-cost powders. Only pilot-scale titanium powder production has been attained to date, and 1981 prices of alloy powder have exceeded $40 per pound ( for micron-sized, free-flowing spheres of precise chemistry, f ree f row the ever-present dirt that would be incorporated as embri ttling inc. fusions--the kind of powder required by present precision molding techniques). Such high prices would seem to preclude any significant future for the production of complex-shape, precision, titanium alloy molded parts, particularly in competition with precision-cast part s. When hot isostatically pressed, the properties of both precision-cast and precision-molded parts approach (their proponents maintain that they can equal and occasionally even exceed ~ wro ught properties. Unalloyed titanium powder, blended with master alloy part icles, is a possible lower cost alternative to alloy spheres, particularly for some applications. A major opportunity and challenge for titanium powder metallurgy, therefore, is the development of much lower cost, commercial ly pure titanium alloy powders. In considering the application of titanium powder metallurgy, it is important to realize that titanium and its alloys are almost unique among metals in their suitability for powder processing. For example, titanium pa rt s can be f armed by dif fusion bonding to provide important material savings. Yet diffusion bonding is not used with any other metal. To cite an exemplary case, the surface f ilm of A12O3 on aluminum is insoluble in aluminum at any temperature; therefore diffusion bonding will not take place. Complex oxides on alloy steels and superalloys also prevent diffusion bonding. In the case of titanium, thin surf ace oxides and nitrides dissolve readily in titanium above 500°C; therefore, parts pressed together in a vacuum or an inert gas atmosphere at or above that temperature will deform to provide intimate surf ace contact and will diffusion bond across the interface to provide joints as strong and ductile as the parts themselves. Indeed, when properly prepared, joints are indistinguishable from the base metal.
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159 Ti tanium powder metallurgy, therefore, is simply dif fusion bonding of small particles instead of large components. With hot isostatic pressure assuring interparticle contact' sound, 100-percent-dense, ductile' strong, titanium parts can be produced. As noted earlier, however, it is not possible to take maximum advantage of the potential of this process without the ready availability of low-cost, high-quality powder. Re search and development in powder preparation of both pure titanium and titanium alloys is therefore an identified need. Titanium Mill Products by Tonnage Powder Metallurgy (TPM) Bottlenecks in the titanium production cycle were reviewed in Chapter 8. A major general bottleneck singled out was the custom-job-shop nature of the titanium mill product industry. Is there any prospect of breaking this major bottleneck? To answer this, it is instructive to review here the basics of winning, melting, and mill processing that were outlined in Chapters 5 and 7. Kroll and Hunter processes win titanium from TiC14 crystal particles that agglomerate loosely into sponge. Electrolytic processes can produce much larger crystals. These agglomerates are compacted and double- OF triple-melted into large ingots that are custom-job-shopped through hot breakdown, hot rolling, and cold-hand-pack rolling to produce alloy sheet. Each operation is accompanied by elaborate conditioning that results in a reduction in the yield of salable sheet f ram the starting sponge to perhaps 50 percent. A near-net-shape alternative is t o process the as-won part icle agglomerates directly into rolled sheet, to compact them into extrusion billets for producing rods and tubing, or to press them into forging preforms. These powder metallurgy operations might be accomplished with substantial savings of time and energy. Minimum cap ital outlays, reduced energy use, and low inventory accumulation at each operation might result. Overall yields approaching 90 perc ent might be expected f ram the initial sponge using the tonnage powder metallurgy (TPM) technology envisioned. Recognizing the foregoing early in titanium's industrial evolution, du Pont undertook a major, multiyear, multimillion dollar program during the 19 50s to bypass the titanium melting operation by processing powder directly to mill products. At that time, du Pont was a major producer of t itanium sponge. The near-net-shape route to mill products was to be used to achieve the cost reduction du Pont felt was necessary for titanium to become an important metal to the process industries. This large effort failed because of a single, unanticipated technical problem that has been kept as a trade secret until released by du Pont for this NMAB report. The problem was that traces of chloride residual to the sponge that could not be removed except by vacuum melting. During fusion welding (of such TPM processed sheet), these chlorides volatized causing welding arc instability that adversely affected consistent weld quality.
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160 Even if this problem were solved, only part of the panel ventures to speculate that titanium tonnage powder metallurgy could ever supplant an important portion of titanium ingot metallurgy. If the du Pant data on its near success were not available, the potential would perhaps not merit more than passing mention in thi s report. However, those data (see Appendix K), plus the serious bottlenecks of current ingot metallurgy and custom-job-shop processing and the high costs associated with these processes, led the panel o consider whether or not the entrenched ingot metallurgy can ever prof itably be supplanted. This led to the panel' s recommendation that an appropriate government agency (perhaps the Air Force) sponsor a detailed study to calculate both the energy consumption and the complete manufacturing costs of mill products by conventional ingot metallurgy and to compare these data with those of tonnage powder metallurgy mill products using an assumed reasonable cost for tonnage powder (e.g., 100, 200, and 400 percent of sponge cost). If there i s little difference between the mill product costs of the two systems even with an optimistic assumption for the cost of tonnage powder, the established arc melting route wl 11 certainly prevail. If TPM appears to off er attractive savings, it is anticipated that, as soon as these estimates are published, one or more industrial organizations will carefully assess the potential and take appropriate action. Rapid Solidif icat ion tTechno logy ~ RST ~ RST is arguably the most exotic development in physical metallurgy since precipitation hardening. Employing cooling rates in excess of one million degrees Centigrade per second on exotic alloy compositions, ultrafine microstructures are achieved--amorphous and microcrystalline--with properties better than the best obtained by the major structural metals such as iron, aluminum, and titanium. One example, exotic in both composition and properties, is amorphous 50Ti-40Be-lOZr (atomic percent) which gave the following combination of properties (Tanner and Ray 1977~: Yield strength 328,000 psi Bend ductility zero T Density 4.13 g/cm3 RST drawbacks are, however, equally dramatic . In f act, it is possible that RST titanium may never graduate from the laboratory. At least a decade--probably two--of research and development will be required to prove its usefulness. Prudence suggests that it should not be ignored because, for example, RST aluminum is at least half a decade in development ahead of RST titanium and today shows some promising results. This advantage also may allow aluminum to gain back some markets lost to titanium because unmatched strength versus weight propert ies similar to those cited above promise important new applications for aluminum. RST titanium may have an even greater potential.
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161 REFERENCE Tanner, L. E . and R . Ray. Physical Properties of Tis o-Be4 o-Zr Scripta Met., 11, 1977: 783-789.
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Representative terms from entire chapter: