DOI: 10.5028/jatm.2009.01010717 Vinicius A. R. Henriques* Institute of Aeronautics and Space Titanium production for São José dos Campos - Brazil [email protected] aerospace applications *author for correspondence Abstract: Titanium parts are ideally suited for advanced aerospace systems because of their unique combination of high specific strength at both room temperature and moderately elevated temperature, in addition to excellent general corrosion resistance. The objective of this work is to present a review of titanium metallurgy focused on aerospace applications, including developments in the Brazilian production of titanium aimed at aerospace applications. The article includes an account of the evolution of titanium research in the Brazilian Institute (IAE/CTA) and the current state-of-art of titanium production in Brazil. Key words: Titanium, Aerospace industry, Powder metallurgy. INTRODUCTION Over the last decade, the focus of titanium alloys Titanium could also replace aluminum when the operating development has shifted from aerospace to industrial temperature exceeds around 130C, which is the normal applications. However, the titanium industry continues to maximum operating temperature for conventional depend on the aerospace market and this sector will aluminum. These conditions exist, for example, in the constitute a significant percentage of total consumption for nacelle and auxiliary power unity (APU) areas and wing years to come. The metallurgy of titanium and Ti-base anti-icing system for airframe structures. Steel and nickel- alloys has been intensely researched over the last 50 years. based alloys are obvious alternatives, but they have a Titanium has unique properties such as its high strength-to- density about 1.7 times that of titanium (Andersen, 1980, weight ratio, good resistance to many corrosive Donachie, 1988). environments and it can be used over a wide range of temperatures. Typical engineering applications of titanium Corrosion resistance can be a very important issue. The alloys include the manufacture of cryogenic devices and corrosion resistance of titanium is such that corrosion aerospace components. The high buy-to-fly ratio associated protective coatings or paints are not required: (paint is with many titanium components, combined with forging applied when titanium comes into contact with aluminum and machining difficulties, has led to a strong drive for near- or low alloy steel components to prevent galvanic corrosion net titanium manufacture. A very promising method of of the contact material). Much of the floor support structure attaining this goal is powder metallurgy (P/M) (Froes, under the galleys and lavatories is in a very corrosive 1980). environment that dictates the use of titanium to provide high structural durability (Andersen, 1980). The primary justifications for using titanium in the aerospace industry are (Boyer, 1994): Polymer matrix composite (PMC) compatibility is becoming a bigger issue with increased use of composite weight savings (primarily as a steel replacement); structures on aircraft. Titanium is galvanically compatible space limitation (replace Al alloys); with the carbon fibers in the composites, whereas operating temperature (Al, Ni, steel alloys replacement); aluminum (and low alloy steels) and carbon generate a corrosion resistance (replace Al and low alloy steels); significant galvanic potential. The choice of titanium in and these instances is related to the criticality of the structure (Boyer, 1994). composite compatibility (replace Al alloys). Titanium is not as widely used as aluminum or steel but it is Weight savings are due to the high strength-to-weight ratio. not a rare or precious metal. Titanium is the fourth most The lower density of titanium compared with steel permits abundant metal in the earth's crust (0.86 per cent by weight) weight savings, replacing steels even though they may be after aluminum, iron, and magnesium. However, titanium is higher strength. As the strength of titanium alloys is difficult to extract from its core, difficult to process, and significantly higher than Al alloys, weight savings can be difficult to manufacture. Just accounting for the extraction achieved when they their replace aluminum despite the 60 and processing costs to produce an ingot, titanium is ~30 per cent higher density, assuming that the component is not times more expensive than steel and ~6 times the cost of gage limited (Allen, 1996). aluminum (Hurless, 2002). ____________________________________ Received: 05/05/09 Historically, titanium has experienced anemic growth when Accepted: 26/05/09 compared with aluminum and stainless steel alloys. Journal of Aerospace Technology and Management V. 1, n. 1, Jan. - Jun. 2009 7 Henriques, V.A.R. The high cost of production limited the use of titanium to The reduction reaction is carried out in a large retort, in an applications requiring high performance or where life cycle inert atmosphere, where titanium tetrachloride is sprayed cost analyses justified its use. The aerospace and defense onto a high-temperature reactive surface of molten communities stimulated initial development of titanium magnesium. TiCl4 reacts to form a sintered, porous mass of alloys in the early 1950s. Aircraft development during the titanium, salt, and unreacted chemicals called “sponge” Cold War was performance-driven without much regard to (Kroll, 1937). After the reaction reaches its equilibrium cost. A radical example of this was the SR-71 Blackbird state, the residual impurities are eliminated by a vacuum reconnaissance aircraft, with over 90 per cent of the distillation stage. One end of the retort is cut off and the structure being titanium (Hurless, 2002). sponge is jacked out of the retort. It is then chopped and ground into chips of about one centimeter size. The chips The growth potential and cost-cutting initiatives of titanium are acid-leached, water-washed, and dried. In the final were severely limited by the dependence upon the cyclic stage, the chips are compressed and welded into an nature of the aerospace industry, with the market electrode, which in turn is melted into an ingot in a vacuum experiencing 4-5 year cycles of boom and bust. During arc furnace. Alloys of titanium are formed by adding chips boom periods, users were only interested in rapid access to of the alloying elements to the electrode as it is being materials, leaving reduced time for development of low compressed and welded into an electrode shape. The arc cost techniques; bust periods had few resources available melting removes volatile impurities and improves for low cost techniques (Hurless, 2002). Cost-conscious homogeneity. High purity alloys require an initial melt and markets, such as the automotive industry, are reluctant to two further remelts [Kroll, 1940]. commit to titanium because of the unpredictability of the cost fueled by the boom and bust economics. However, with The Kroll process has a number of serious disadvantages world production of 60 million vehicles annually, even [Collings, 1983]: 0.5kg of titanium in 50 per cent of the vehicles produced would increase titanium use by 30 per cent, helping to l It is a multi-step process. stabilize the cost and reduce dependence on the aerospace l Each individual step is a batch process. industry (Andersen, 1980). l The initial stage operates at high temperature in retorts with short working lifetimes. Every stage of titanium production except the mining of ore l It produces a sintered product with high levels of has a tremendous potential impact on the final cost of impurities. titanium products. Regarding ore mining, the cost of ore is l The second stage removes some of the impurities and highly dependent on demand, thus cost reductions will discharges a polluted waste stream. naturally result from a major increase in the demand for l The purification stage removes the rest of the impurities titanium products. When considering cost reduction, it is through multiple melts in high vacuum arc furnaces. strategic to focus on the early stages of production, where any cost advantages achieved would be carried through all All of these inherent disadvantages add up to a product that production stages. The cost of extracting titanium from the is so expensive it can compete with other metals only in ore is approximately 20 times that of steel on a 1-to-1 very specialized niche markets. weight basis, but roughly 11 times when accounting for the density advantage of titanium (less titanium would be Titanium alloys required to perform the same function as steel) [6]. Titanium exists in two crystallographic forms. At room Beyond that, machining costs for titanium are generally temperature (RT), unalloyed (commercially pure) titanium significantly higher than for the other materials (at least 10 has a hexagonal close-packed (hcp) crystal structure times higher than to machine Al). In that sense, titanium referred to as alpha (α) phase. At 883°C (1621°F), this alloy production using powder metallurgy (P/M), starting transforms to a body-centered cubic (bcc) structure known from the elemental or prealloyed powders is a interesting as beta (β) phase. The manipulation of these route considering its lower costs, versatility and also the crystallographic variations through alloying additions and facility it offers to manufacture parts with complex thermomechanical processing is the basis for the geometry and close to the final dimensions (Henriques development of a wide range of alloys and properties.