Titanium and Titanium Alloys

Sommario:

Since the introduction of titanium and titanium alloys in the early 1950s, these materials have in a relatively short time become backbone materials for the aerospace, energy, and chemical industries.

The combination of high strength-to-weight ratio, excellent mechanical properties, and corrosion resistance makes titanium the best material choice for many critical applications. Today, titanium alloys are used for demanding applications such as static and rotating gas turbine engine components. Some of the most critical and highly-stressed civilian and military airframe parts are made of these alloys...

Since the introduction of titanium and titanium alloys in the early 1950s, these materials have in a relatively short time become backbone materials for the aerospace, energy, and chemical industries.

The combination of high strength-to-weight ratio, excellent mechanical properties, and corrosion resistance makes titanium the best material choice for many critical applications. Today, titanium alloys are used for demanding applications such as static and rotating gas turbine engine components. Some of the most critical and highly-stressed civilian and military airframe parts are made of these alloys.

The use of titanium has expanded in recent years to include applications in nuclear power plants, food processing plants, oil refinery heat exchangers, marine components and medical protheses.

The high cost of titanium alloy components may limit their use to applications for which lower-cost alloys, such as aluminium and stainless steels. The relatively high cost is often the result of the intristic raw material cost of metal, fabricating costs and the metal removal costs incurred in obtaining the desired final shape.

These titanium net shape technologies include powder metallurgy (P/M), superplastic forming (SPF), precision forging, and precision casting. Precision casting is by far the most fully developed and the most widely used titanium net shape technology. The annual shipment of titanium castings in the United States increased by 260% between 1979 and 1989.

As aircraft engine manufactures seek to use cast titanium at higher operating temperatures, Ti-6Al-2Sn-4Zr-2Mo and
Ti-6Al-2Sn-4Zr-6Mo are being specified more frequently. Other advanced high-temperature titanium alloys for service up to 595oC, such as Ti-1100 and IMI-834 are being developed as castings. The alloys mentioned above exhibit the same degree of elevated-temperature superiority, as do their wrought counterparts over the more commonly
used Ti-6Al-4V.

The wrought product forms of titanium and titanium-base alloys, which include forgings and typical mill products, constitute more than 70% of the market in titanium and titanium alloy production. The wrought products are the most readily available product form of titanium-base materials, although cast and powder metallurgy (P/M) products are also available for applications that require complex shapes or the use of P/M techniques to obtain microstructures not achievable by conventional ingot metallurgy.

Powder metallurgy of titanium has not gained wide acceptance and is restricted to space and missile applications. The primary reasons for using titanium-base products are its outstanding corrosion resistance of titanium and its useful combination of low density (4.5 g/cm3) and high strength. The strengths vary from 480 MPa for some grades of commercial titanium to about 1100 MPa for structural titanium alloy products and over 1725 MPa for special forms such as wires and springs.

Another important characteristic of titanium- base materials is the reversible transformation of the crystal structure from alpha (a, hexagonal close-packed) structure to beta (b, body-centered cubic) structure when the temperatures exceed certain level. This allotropic behavior, which depends on the type and amount of alloy contents, allows complex variations in microstructure and more diverse strengthening opportunities than those of other nonferrous alloys such as copper or aluminum.

Pure titanium wrought products, which have minimum titanium contents ranging from about 98,635 to 99,5 wt%, are used primarily for corrosion resistance. Titanium products are also useful for fabrication but have relatively low strength in service.

Titanium has the following advantages:

  1. Good strength
  2. Resistance to erosion and erosion-corrosion
  3. Very thin, conductive oxide surface film
  4. Hard, smooth surface that limits adhesion of foreign materials
  5. Surface promotes dropwise condensation
Commercially pure titanium with minor alloy contents include various titanium-palladium grades and alloy Ti-0,3Mo-0,8Ni (ASTM grade 12 or UNS R533400). The alloy contents allow improvements in corrosion resistance and/or strength.

Titanium-palladium alloys with nominal palladium contents of about 0,2% Pd are used in applications requiring excellent corrosion resistance in chemical processing or storage applications where the environment is mildly reducing or fluctuates between oxidizing and reducing.

Alloy Ti-0,3Mo-0,8Ni (UNS R533400, or ASTM grade 12) has applications similar to those for unalloyed titanium but has better strength and corrosion resistance. However, the corrosion resistance of this alloy is not as good as the titanium-palladium alloys. The ASTM grade 12 alloy is particularly resistant to crevice corrosion in hot brines.

Titanium alloy compositions of various titanium alloys. Because the allotropic behavior of titanium allows diverse changes in microstructures by variations in thermomechanical processing, a broad range of properties and applications can be served with a minimum number of grades. This is especially true of the alloys with a two-phase, a+b, crystal structure.

The most widely used titanium alloy is the Ti-6Al-4V alpha-beta alloy. This alloy is well understood and is also very tolerant on variations in fabrication operations, despite its relatively poor room-temperature shaping and forming characteristics compared to steel and aluminium. Alloy Ti-6Al-4V, which has limited section size hardenability, is most commonly used in the annealed condition.

Other titanium alloys are designed for particular application areas. For example:

  1. Alloys Ti-5Al-2Sn-2Zr-4Mo-4Cr (commonly called Ti-17) and Ti-6Al-2Sn-4Zr-6Mo for high strength in heavy sections at elevated temperatures.
  2. Alloys Ti-6242S, IMI 829, and Ti-6242 (Ti-6Al-2Sn-4Zr-2Mo) for creep resistance
  3. Alloys Ti-6Al-2Nb-ITa-Imo and Ti-6Al-4V-ELI are designed both to resist stress corrosion in aqueous salt solutions and for high fracture toughness
  4. Alloy Ti-5Al-2,5Sn is designed for weldability, and the ELI grade is used extensively for cryogenic applications
  5. Alloys Ti-6Al-6V-2Sn, Ti-6Al-4V and Ti-10V-2Fe-3Al for high strength at low-to-moderate temperatures.

Welding has the greatest potential for affecting material properties. In all types of welds, contamination by interstitial impurities such as oxygen and nitrogen must be minimized to maintain useful ductility in the weldment. Alloy composition, welding procedure, and subsequent heat treatment are highly important in determining the final properties of welded joints.

Some general principles can be summarized as follows:

  1. Welding generally increases strength and hardness
  2. Welding generally decreases tensile and bend ductility
  3. Welds in unalloyed titanium grades 1, 2 and 3 do not require post-weld treatment unless the material will be highly stressed in a strongly reducing atmosphere
  4. Welds in more beta-rich alpha-beta alloys such as Ti-6Al-6V-2Sn have a high likelihood of fracturing with little or no plastic straining.
Titanium and titanium alloys are heat treated for the following purposes:
  1. To reduce residual stresses developed during fabrication
  2. To produce an optimal combination of ductility, machinability, and dimensional and structural stability (annealing)
  3. To increase strength (solution treating and aging)
  4. To optimise special properties such as fracture toughness, fatigue strength, and high-temperature creep strength.

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