Titanium (Ti) is a transition metal in Group 4 (the titanium subgroup) of Period 4 of the Periodic Table, with the symbol Ti, atomic number 22, and relative atomic mass 47.867. Titanium is a hard, lustrous, silvery-white metal that exists in a hexagonal α phase and a cubic β phase. It has a relative density of 4.506, a melting point of 1,668 °C, and a boiling point of 3,287 °C. Its resistivity is 42 × 10⁻⁸ Ω·m (at 20 °C). In nature, it primarily occurs as rutile, perovskite, and ilmenite. Sponge titanium is produced by reducing titanium tetrachloride with magnesium at high temperatures, and titanium ingots are then obtained by melting the sponge using the electric arc method.
Titanium possesses exceptional properties, which have led to its widespread use. Its main applications include: aerospace, shipbuilding, chemical and petrochemical industries, transportation, defense, marine, power generation, construction, metallurgy, medical devices, sports equipment, consumer goods, and light industry.
Pure titanium is rarely used in practical applications because of its low strength, which makes it unsuitable as a structural material. Therefore, titanium alloys are predominantly used in practical applications. Liquid titanium can dissolve almost all metals, so it can form alloys with a wide variety of metals; aluminum is the primary alloying element in titanium alloys. The presence of impurities in titanium has a significant impact on its mechanical properties; in particular, interstitial impurities (oxygen, nitrogen, and carbon) can greatly increase titanium's strength while significantly reducing its ductility. The excellent mechanical properties of titanium alloys as structural materials are achieved through strict control of appropriate impurity levels or the addition of alloying elements.
Titanium alloys can be classified by application into heat-resistant alloys, high-strength alloys, corrosion-resistant alloys (such as titanium-molybdenum and titanium-palladium alloys), low-temperature alloys, and special-purpose alloys (such as titanium-iron hydrogen storage materials and titanium-nickel shape memory alloys).
The smelting process for titanium alloys is similar to that for titanium, except that the addition of different alloying elements to molten titanium is carefully controlled. Currently, Western countries such as the United States and Japan are employing new titanium alloy smelting technologies, including electron beam cold hearth furnaces and plasma cold hearth furnaces. Electron beam cold hearth furnaces have been successfully applied to the smelting of pure titanium and GR5 alloys, while plasma beam technology is the most effective method for smelting titanium alloys with complex compositions.Russia has developed a smelting technology similar to the cold-hearth furnace, known as "crust-slag-consumable-electrode smelting." In addition, cold-crucible melting technology has seen significant advancements in recent years. When combined with the centrifugal casting process for precision casting of titanium castings, a second-generation cold-crucible melting furnace is currently being manufactured. This furnace will greatly increase melting capacity, shorten melting time, enable fully suspended melting, and eliminate metal crust formation.
Another key factor limiting the widespread use of titanium alloys is that their low thermal conductivity, high hardness, low modulus of elasticity, and chemical reactivity at high temperatures make them difficult to machine, forge, and weld. The technical challenges associated with nickel and titanium alloys-and the resulting high costs-make it difficult to use them on a large scale; however, their range of excellent properties is simply too attractive for enhancing the performance of weapons and military equipment.
The increased use of titanium alloys in the aerospace sector is due not only to their inherent excellent properties but, more importantly, to rapid advances in precision forming technologies. The emergence of precision casting technology for large, monolithic, and complex titanium alloy components has opened new avenues for expanding the use of titanium alloys in aircraft fuselages and aircraft engines. The F-22 extensively utilizes precision-cast titanium alloy components (54 in total) in the vertical tail rudder actuator housing and other critical load-bearing areas, accounting for approximately 7.1% of the overall structural weight.
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