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A D V A N C E D M A T E R I A L S & P R O C E S S E S | F E B R U A R Y / M A R C H 2 0 1 8 2 0 (Douglas DC-7), but in small amounts compared withmilitary aircraft. Military spending dropped in the 1990s, forcing consolidation of the titanium industry. By 2000, only three U.S. integrated ti- tanium producers remained: Titanium Metals Corp. (TIMET), Allegheny Tech- nologies Inc., (ATI), and RTI Internation- al Metals and its subsidiary Reactive Metals Inc. (RMI). In the USSR, state-run VSMPO (whichmade ingot)mergedwith AVISMA (which made sponge), and was revamping its operations away from the production of titanium for submarines. This required greater emphasis on qual- ity control so its products could receive certification for aerospace use domesti- cally and in the West. Realignment of the titanium indus- try was virtually complete by 2000, but the subsequent sharp downturn in the aircraft industry added further volatili- ty. The first decade of the 21st century has seen sharp highs and lows, brought about by disparate factors including the use of polymeric composites and titani- um in the Airbus A380 and Boeing 787 Dreamliner (both systems use more than 15% titanium); China’s huge in- crease in production of sponge, ingot, and mill products; and the perturbation in the credit market brought about by the real estate collapse of 2008. TITANIUM OCCURRENCE Titanium mineral occurs in nature as ilmenite (FeTiO 3 ), rutile (tetragonal TiO 2 ), anatase (tetragonal TiO 2 ), brook- ite (rhombic TiO 2 ), perovskite (CaTiO 3 ), sphene (CaTiSiO 5 ), and also geikielite (MgTiO 3 ). Although ilmenite is the most common, rutile is the most important source of raw material, especially for metal production. Some deposits of anatase and perovskite are rich enough to be of commercial interest, but the abundance and availability of high- grade deposits of ilmenite and rutile have postponed development of these minerals. Ilmenite is found in alluvial sands and hard rock deposits. The color of ore after concentrating is black. Con- centrate, with a density of 4-5 g/cm 3 , is processed to pigment-grade TiO 2 and metal. Titanium metal is won from the TITANIUM: THE BEGINNING As the ninth most abundant ele- ment, titanium is plentiful in the earth’s crust at approximately 0.62%. The two most common forms are ilmenite (FeTiO 3 ), which is present in igneous rocks, and rutile (TiO 2 ), which is found in beach sands. While converting the ore into metal is a time-consuming, ex- pensive process, it is worth the effort, because the metal offers a useful com- bination of metallurgical and physi- cal characteristics. It is the fourth most abundant structural metal after alumi- num, iron, and magnesium. Mineralogist William Gregor re- ported in a German scientific publica- tion [10, 11] in 1791 that he discovered a magnetic black sand resembling gun- powder (which was ilmenite) in Corn- wall, England. Rutile was discovered a few years later by Martin Klaproth, a German chemist, which he named af- ter the Titans, the precursors to the Olympian gods. Titanium readily com- bines with carbon, producing stable carbide, and when heated, it combines with oxygen and nitrogen. By decom- posing refractory materials and absorb- ing oxygen, titanium becomes impure, resulting in increased hardness and brittleness. Titanium is so sensitive to oxygen that only 0.1 wt% increases the hardness of commercially pure (CP) ti- tanium by a factor of about three. These difficulties prevented the pro- duction of a metallic form until 1910 when Matthew Hunter at Rensselaer Polytech- nic Institute reduced titanium tetrachlo- ride with sodium in a steel canister similar to a bomb casing, producing only a very small amount of metal. A production breakthrough came in the 1930s when William Kroll, a metallurgist in Luxem- bourg, developed a sponge process that could be scaled up for industrial use. Kroll came to the U.S. in 1940 just months be- fore Germany invaded Luxembourg. After World War II ended, he joined the U.S. Bu- reau of Mines (now U.S. Department of En- ergy) in Albany, Oregon, where he brought the process to its full potential [10, 11] . An ex- pensive arms race between the U.S. and the former USSR created the need to solve most of titanium’s complex metallurgical problems, bringing it to maturity as an aerospace metal. Sponge (so termed for its appear- ance) is titanium refined from ore. In the process, oxide ore is first converted to chloride using chlorine gas in the pres- ence of carbon (coke). This is followed by the Kroll sponge process in which titani- um tetrachloride is reduced by molten magnesium (or alternatively reduction by sodium in the parallel Hunter process) to metal in an inert argon atmosphere to prevent titanium binding with oxygen. The next step is vacuum distillation, or leaching, to remove magnesium chloride and excess magnesiummetal. The sponge is sheared and crushed prior to being melted into an ingot using a conven- tional ingot metallurgy (IM) approach in an inert atmosphere or vacuum, which is forged into billet or subsequently formed into mill products such as plate, bar, and sheet. Tight control is required during each step of the process due to titanium’s extreme reactivity at higher temperatures, in addition to working in an oxidizing environment with surface conditioning at intermediate stages. By 1950, the U.S. government wanted airplanes with superior perfor- mance to those of the Soviet Union. Be- cause steels used inaircraft had reached their performance limits, the superi- or strength-to-weight ratio of titanium offered a solution. Components could be as strong as steel, but 45% lighter weight. Thus the U.S. government be- gan building a titanium industry from the ground up to put the Kroll process into large-scale use. DuPont, Dow, Na- tional Lead, Crucible Steel, Union Car- bide, and other companies entered the business. In the UK, ICI Metals (later IMI) began producing sponge in 1948 and ingots in the early 1950s. The Soviet Union started producing sponge and in- gots in the mid-1950s. With the help of Kroll, Japan began sponge production in 1954 for industrial use. Thyssen and Krupp and Deutsche Edelstahlwerke GmbH also entered the field [10,11] .