Titanium Overview
Innovations in Titanium Powder Processing Vladimir S. Moxson, Oleg N. Senkov, and F.H. Froes One way of reducing the cost of titanium components is to use near-net shape powdermetallurgy techniques. This article describes a number of new approaches to producing components using the powder-metallurgy method for the aerospace, industrial, and consumer marketplaces. INTRODUCTION Titanium and its alloys are among the key advanced materials for performance improvements in aerospace and terrestrial systems. However, the high cost of producing titanium-based materials as compared to competing materials has led to investigations of potentially lower cost processes, such as net-shape powder metallurgy (P/M) techniques.1–4 There are two basic approaches to producing titanium powders—the blended elemental technique and the pre-alloyed method.2,3 The former approach uses essentially pure titanium and a master alloy (e.g., 60:40 Al:V) to produce alloys such as Ti-6Al-4V. With the pre-alloyed method, the alloying additions are already in the material prior to powder production. The blended-elemental powder costs less, but, in most cases, has inferior mechanical properties.2,3 On the other hand, the pre-alloyed powder has a higher cost and mechanical properties that can be at cast and wrought (ingot metallurgy) levels.2,3 The main source of powder for the blended-elemental P/M approach is titanium sponge fines,1–5 a by-product (about one percent at –100 mesh) of the Kroll magnesium process6 or the Hunter sodium process.7 The sponge fines (or powder) typically contain 0.12– 0.15 wt.% chloride. This inherent chloride content prevents the production of 100% dense compacts, which results in inferior weldability and reduced initiation-related properties, such as S-N fatigue.2,8 Sponge fines with less than 150
Figure 1. CP titanium MHR powder.
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ppm chloride can also be produced by an electrolytic process.5,9 Recently, there has been some renewed interest in this approach using an improved process.2,10,11 Commercially pure (CP) titanium powders with chloride levels less than 10 ppm can be obtained by crushing hydrogenated ingot material, followed by dehydrogenation (HDH). Alloy powder can also be produced by HDH with appropriate starting stock.12 Pre-alloyed powders from alloys such as Ti-6Al-4V can be produced by melting prealloyed stock by rotating an electrode (the plasma rotating electrode process2,3) or by gas atomization.2,3,13 POWDER PRODUCTION One new approach to powder production is the metal-hydride reduction (MHR) method,1,14 which combines reasonable cost (less than $10 per pound in large quantity lots) and high performance. Currently, MHR powder is produced in the Polema Tulachermet metallurgical plant in Tula, Russia. In this process, the titanium powder is produced from titanium dioxide by a reduction reaction with calcium hydride TiO 2 + 2CaH2 = Ti + 2CaO + H2
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The chemical reaction is performed at 1,100–1,200°C. To produce an alloyed powder directly, a mixture of oxides of the appropriate elements is used. This method of titanium-powder production does not include the intermediate production of titanium tetrachloride, and as a result, the powders contain a very low amount of chloride. If necessary, the oxygen concentration in the powders can be decreased to less than 0.1 wt.% by eliminating a passivating operation, which is used to decrease the possibility of powder flammability. The powders produced by this method contain high concentrations of hydrogen (0.4 wt.%), which could allow better sintering and microstructure modification in the powder products through thermohydrogen processing.15,16 The hydrogen is easily reduced to less than 10 ppm by vacuum sintering or annealing. Powders of CP titanium and Ti-6Al4V have been produced by the calciumreduction reaction method (Figure 1). The powder particles have a spongy morphology and consist of small particles welded together during the reductionreaction process. Under U.S. Army Research Laboratory Small Business Innovative Research
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c d Figure 2. Optical micrographs of (a) CP titanium and (b) Ti-6Al-4V turnings. Also shown are SEM micrographs of corresponding powders for (c) CP titanium and (b) Ti-6Al-4V after HDH.
JOM • May 2000
theoretical density are shown in Figure 5. Powder particles were welded to each other during fabrication, forming a honeycomb-like structure with continuous pores. The irregular shape of the particles allows the synthesis of a monolithic skeleton with good strength. Medical Applications
Porous Titanium Plates
Fully dense articles of Ti-6Al-4V were produced from chloride-free HDH titanium powders by blending with an Al40V master alloy. The blended powders were cold isostatically pressed at 360 MPa, vacuum sintered at 1,260°C for 4 h, and hot isostatically pressed at 927°C for 0.5 h. The tensile properties of the synthesized articles are equivalent to conventional wrought Ti-6Al-4V.2,3 Significantly improved fatigue properties at the same levels as exhibited by wrought materials have been achieved with P/M parts produced by a comMaster Alloy Sponge Fines
Final Product
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Direct Powder Rolling Sinter
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Re-Roll
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Technology has been developed to produce cost-effective porous titanium plates that can be used as a mechanical support for electrolyte membranes in high differential pressure, solid-polymer electrolyte, water-electrolysis cell technology and similar applications. The structures of sintered metal powders are ideal for this application because they provide a unique combination of fine pores to allow the water to set the electrode and, within the same structure, larger pores to carry the product gas away from the electrodes. Porous plates of 5–10 mm thickness and porous foils of 2.5–25 µm thickness have been produced by rolling a compacted powder billet. The fabrication procedure (Figure 4) includes direct powder rolling, sintering, re-rolling, and annealing. A scanning electron microscopy (SEM) micrograph of plates produced with 50%
Fully Dense, Blended Elemental P/M Shapes
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TITANIUM P/M COMPONENTS Environmental Applications Low-cost, corrosion-resistant parts, including nuts and washers and membrane retainers (Figure 3), have been made from CP titanium powder by a press-and-sinter method. This P/M technology has two competitive advantages. First, the cost of compacts made from powder is 40–50% lower than wrought titanium (bar stock, billet, plate, etc). Second, standard powder metal presses can be used to achieve high production volumes and near-net shapes. Nuts and washers are routinely made in volumes of 5,000–500,000 pieces per run, primarily for the chemical industry. In addition, C collars, used as parts of baskets for electroplating, have been produced for a number of years, with an annual output of around 10,000 pieces.
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(SBIR) sponsorship, ADMA Products, in conjunction with the University of Idaho, has been exploring a number of new processes for producing low-cost titanium powders. One approach, which has the potential of producing the lowest cost powder available, converts cleaned titanium machine turnings to powder using the hydride-dehydride (HDH) approach. Examples of CP titanium and Ti-6Al-4V turnings and the corresponding powders are shown in Figure 2. Mechanical properties are currently being developed from compacts fabricated by this approach. Assuming success, a legitimate question is the continued availability of turnings, both directly (i.e., because they are being used in large quantities) and indirectly (i.e., because near-net shapes requiring minimal machining are being produced). In this situation, mixed crushed turnings and lowcost sponge would be starting stock. Swarf, another low-cost starting stock, was too heavily contaminated for successful powder production. There are other powder-making sources in development, but these have not progressed beyond the research/pilot-plant stage. A few years ago, the Zakarpatye metallurgical plant in Ukraine was producing high volumes of titanium powder (more than 500 t/y) from titanium sponge less than 2–5 mm in size, mainly for the steel industry. Finer sponge was wet ground into –100 mesh powder and sold for under $2.00/lb. A similar process is currently used by the Titanium Institute in Zaporozye, Ukraine, and titanium sponge is supplied by the Zaporozye Titanium Magnesium Works. The AVISMA Titanium Magnesium Work, located in Berezniki, Russia, is currently the only titanium-powder pro-
Titanium is an excellent material for medical implants because of its high chemical stability in the human body, high specific strength, and low density. The MHR titanium powders have been used to produce several parts for medical application. One example is a porous dental implant—a stomatological system intended to fix dentures in which a partial or complete defect exists in the dental row (Figure 6). Requirements for an artificial foot are high specific strength, light weight, good corrosion resistance, and excellent durability. It should be adaptable to the exoskeleton and a variety of modular systems to respond to variations in patient activity levels. A flexible pyramid adapter that is strong enough to withstand the weight of the user is used to fit the artificial foot to the artificial shank. Because of its critical role, the pyramid adapter is generally made of heavy highstrength stainless steel. However, the pyramid adapters can alternatively be fabricated from Ti-6Al-4V using a P/M cold press plus vacuum sinter technique. These adapters are lighter and have a higher specific strength (Figure 7).
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b Figure 3. Low-cost, corrosion-resistant parts made via a press-and-sinter method. Shown are (a) nuts and washers and (b) membrane retainers 240 mm outer diameter, 152 mm internal diameter, and 8 mm thickness.
ducer in Russia. Three methods of titanium sponge production are used: sponge fines generated as a by-product by screening to about –100 mesh size; a special dry grinding in argon of fine sponge (less than 2 mm); and, most recently, as part of the magnesium reduction process directly in a retort by adding proprietary ingredients to break up the sponge during the reduction process. Also, the Ust-Kamenogorsk Titanium Magnesium Works in Kazakhstan generates titanium powder as a by-product (i.e., about 1% of their titanium sponge output).
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Anneal
Figure 4. Equipment for producing rolled titanium porous plates from powder.
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Figure 5. An SEM micrograph of a porous (50%) titanium plate used as a support for electrolytic membranes.
bined cold and hot isostatic pressing from titanium powders with chloride levels of less than 0.02 wt.%.3,8 Thus, it is reasonable to expect that P/M parts made from the chloride-free MHR powder will also have mechanical properties comparable to those of the wrought material, and work is planned in this area. Armored Vehicle Components In phase II of the Army-funded SBIR program mentioned previously, large (up to 113 kg) complex components will be produced using a low-cost, loose powder sintering approach. The goal is to lighten armored vehicles (e.g., the MIA2 main battle tank and the Crusader).17 Sheet Foil from TiAl Because of their low density and elevated-temperature mechanical properties, TiAl-based alloys have strong potential for use in advanced gas-turbine engines and airframe components exposed to high temperatures (e.g., heat shields for re-entrant space vehicles).18,19 Ambient temperature behavior (especially ductility, fatigue, fracture toughness, etc.) remains a concern; high cost is also a barrier to widespread use. Currently TiAl-based flat products, such as sheet and foil, are available from only one off-shore source at very high prices.20
Using a patented process21 and other proprietary procedures, very thinsheeted foils have been produced with a good surface finish, minimizing or even eliminating the need to grind. The process used to produce TiAl foils as a flat section with a density no less than 25% of theoretical is sintered from the powder of a low-ductility reactive alloy. This is welded by diffusion welding with cover foils, made from ductile reactive metal, that hermetically seal inner and surface pores and then assembled with two heat-resistant sheets in the laminated package. Cover foils are made from metal that belongs to the same metal system as the sintered powder. A package is then made from a sinter metal with parting agents. After outgassing and package sealing, the assembly is hot rolled at 982–1,343°C with a reduction of 4–20%. This reduction cycle is repeated until the desired thickness and density is achieved. The rolled sintered material is separated from the package. Hot isostatic pressing may be used at any step to facilitate compaction. After hot isostatic pressing, a fully dense material is obtained with tensile properties at cast and wrought levels.22
Figure 7. A pyramid adapter for an artificial foot made from CP titanium powder.
Golf-Club Head Components
References
In the past few years, the design of sports equipment has become much more sophisticated, with advanced materials playing a major role in improved equipment and better performance by the user.23–26 Titanium has been used in various equipment, but has made the most impact in golf club heads,25,26 because weight distribution in the club head is so important in maximizing the opportunity for the golfer to hit a good shot.27 Also, titanium has a very high strength-to-density ratio. While many of the titanium components have been produced by casting approaches,28 the P/M technique also plays a part.
1. V.S. Moxson, O.N. Senkov, and F.H. Froes, Int. J. of PM, 34 (5) (1998), p. 45. 2. F.H. Froes and D. Eylon, Int. Mats. Reviews, 35 (3) (1990), p. 162. 3. F.H. Froes and C. Suryanarayana, Reviews in Particulate Materials, 1 (1993), p. 223. 4. S. Abkowitz et al., P/M in Aerospace, Defense and Demanding Applications–1993, ed. F.H. Froes (Princeton, NJ: MPIJ, 1993), p. 241. 5. F.H. Hayes et al., JOM, 36 (6) (1984), p. 70. 6. W.J. Kroll, Trans. Electrochem. Soc., 78 (1940), p. 35. 7. M.A. Hunter, J. Amer. Chem. Soc., 32 (1910), p. 330. 8. S. Abkowitz and D. Rowell, JOM, 38 (8) (1986), p. 36. 9. G. Cobel, F. Fisher, and L.E. Snyder, Titanium ’80, Science and Technology, vol. 3, ed. H. Kimura and O. Izumi (Warrendale, PA: TMS, 1980), p. 1969. 10. U. Ginatta et al., Proc. 6th World Titanium Conference, ed. P. Lacombe, R. Tricot, and G. Beranger (Nanterre Cedex, France: Societé Française de Metallurgie, 1989), p. 753. 11. M. Ginatta, in this issue. 12. P.P. Alexander, U.S. patent 2,427,338 (1945). 13. C.F. Yolton, “Gas Atomized Titanium Powder,” Rapidly Solidified Materials (Warrendale, PA: TMS, 1985), p. 97. 14. B.A. Borok, Trans. of Central Research Institute for Ferrous Metallurgy (Moscow, Russia), 43 (1965), p. 69. 15. O.N. Senkov, J.J. Jonas, and F.H. Froes, JOM, 48 (7) (1996), p. 42. 16. O.N. Senkov and F.H. Froes, Hydrogen Energy, 24 (1999), p. 565. 17. J.S. Montgomery et al., JOM, 49 (5) (1997), p. 45. 18. F.H. Froes, D. Eliezer, and C. Suryanarayana, J. Matls. Sci., 27 (1992), p. 5113. 19. Y-W Kim, in this issue. 20. C.F. Yolton, U. Habel, and H. Clemens, Advanced Particulate Materials and Processes, ed. F.H. Froes and J. Hebeisen (Princeton, NJ: APMI, 1997), p. 161. 21. V.S. Moxson and A.E. Shapiro, U.S. patent 5,903,813 (11 May 1999). 22. G. Das, P&W Aircraft, West Palm Beach, private communication (February 2000). 23. F.H. Froes, JOM, 49 (2) (1997), p. 15. 24. C. Shira and F.H. Froes, JOM, 49 (5) (1997), p. 35. 25. F.H. Froes, JOM, 50 (9) (1998), p. 15. 26. C. Shira and F.H. Froes, “Advanced Materials in Golf Clubs” (Paper presented at the Conf. on the Engineering of Sport, Sydney, Australia, 9–12 June, 2000). 27. F.H. Froes, JOM, 51 (6) (1999), p. 18. 28. F.H. Froes, Light Metals Age, 55 (1&2) (1997), p. 40.
ACKNOWLEDGEMENTS The authors acknowledge Marlane Martonick for manuscript preparation. The assistance of graduate students J. Qazi and M. Cavusoglu in conducting experimental work is greatly appreciated. We recognize the help of J.C. Hebeisen and P. Tylus, BodycoteIMT, in hot isostatic pressing. Paul Bartolotta, Gopal Das, Rob Leholm, and M.G.H. Wells are acknowledged for their interest, experimental assistance, and financial support of some aspects of the work reported.
Vladimir Moxson is with ADMA. O.N. Senkov and F.H. Froes are with the Institute for Materials and Advanced Processes, University of Idaho.
a b Figure 6. The details of an intrabony porous dental implant showing (a) a schematic of the installation technique and (b) an SEM micrograph of the porous part with in-grown tissue.
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For more information, contact F.H. Froes, Institute for Materials and Advanced Processes, University of Idaho, Mines Building, Room 321, Moscow, Idaho 83844-3026; (208) 885-7989; fax (208) 885-4009; e-mail
[email protected].
JOM • May 2000
