Titanium : symbol Ti, atomic number 22. Titanium has a low density and is a strong, lustrous, corrosion-resistant (including sea water, aqua regia and chlorine) transition metal with a silver color.
The two most useful properties of Titanium are corrosion resistance and the highest strength-to-weight ratio of any metal.
Titanium and its alloys offer excellent corrosion resistance to acids, chlorides and salt; a wide continuous service temperature range, from liquid nitrogen (-322°F) to 1100°F; and the highest strength-to-weight ratio of any metal. Titanium is almost as resistant to corrosion as platinum, capable of withstanding attack by dilute sulfuric acid and hydrochloric acid as well as chlorine gas, chloride solutions, and most organic acids.
Titanium is about 45% lighter than steel, 60% more dense than aluminum and more than three times stronger than either of them. While expensive initially, titanium lowers life cycle costs because of its long service life and reduced (or non-existent) maintenance and repair costs. For example, the Navy replaced copper-nickel with titanium for seawater piping systems on its LDP-17 San Antonio Class of ships because it expects titanium to last the entire 40 to 50 year life of the ship.
Titanium is always bonded to other elements in nature. It is the ninth-most abundant element in the Earth’s crust (0.63% by mass) and the seventh-most abundant metal on the planet.
Titanium can be alloyed with iron, aluminum, vanadium, molybdenum, among other elements, to produce strong lightweight alloys for aerospace (jet engines, missiles, and spacecraft), military, industrial process (chemicals and petro-chemicals, desalination plants, pulp, and paper), automotive, agri-food, medical prostheses, orthopedic implants, dental and endodontic instruments and files, dental implants, sporting goods, jewelry, mobile phones, and other applications.
Titanium falls into a family of metals called reactive metals, which means that they have a strong affinity for oxygen. At room temperature, titanium reacts with oxygen to form titanium dioxide. This passive, impervious coating resists further interaction with the surrounding atmosphere, and it gives titanium its famous corrosion resistance. The oxide layer must be removed prior to welding because it melts at a much higher temperature than the base metal and because the oxide could enter the molten weld pool, create discontinuities and reduce weld integrity.
Common Grades of Titanium
Titanium is divided in four classes: commercially pure (CP, or unalloyed), alpha, alpha-beta and beta. Note that many companies and experts treat CP and alpha alloys as one group. The “alpha” and “beta” refer to phases of the metal’s crystalline structure at various temperatures. Adding oxygen, iron, aluminum, vanadium and other elements to the alloy can precisely control the crystal structure, and hence the alloy’s properties.
The most common CP grade are ASTM Grades 1, 2, 3 and 4. They differ by the varying degrees of oxygen and iron content; greater amounts of these elements increase tensile strength and lower ductility. Grade 2 is the most widely used, notably in corrosion resist applications. CP Grades have good ductility, good elevated temperature strengths to 572°F and excellent weldability. They cost less than alloyed grades, but have a relatively low tensile strength, such as 70,000 to 90,000 psi for Grade 2.
Grade 5 (Ti-6Al-4V), an alpha-beta, is the most widely used of any grade of titanium (50 to 70 percent of all uses, according various sources). The addition of aluminum and vanadium increases tensile strength to 120,000 psi and service temperature up to 752°F, but it also makes Grade 5 less formable and slightly harder to weld than Grade 2. It is used for a range of applications in the aerospace, marine, power generation and offshore industries.
Grade 23 is similar to Grade 5, but features reduced of oxygen content that improves ductility and fracture toughness with a just a slight loss of strength. Grade 9 strengths fall between Grade 4 and Grade 5, so it is sometimes referred to as a “half 6-4”. Grade 9 can be used at higher temperatures than Grade 4, offers 20 to 50 percent higher strength than commercially pure grades and is more formable and weldable than Grade 5.