Titanium and Titanium Alloys

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Introduction to Titanium and Titanium Alloys

Since their commercial introduction in the early 1950s, titanium and titanium alloys have quickly become backbone materials for the aerospace, energy, and chemical industries. Their remarkable combination of high strength-to-weight ratio, excellent mechanical properties, and superior corrosion resistance makes titanium the optimal material choice for numerous critical applications.

Today, titanium alloys are employed in demanding scenarios such as static and rotating gas turbine engine components. Some of the most critical and highly-stressed civilian and military airframe parts rely on these versatile alloys. The application scope has expanded significantly in recent years to include nuclear power plants, food processing facilities, oil refinery heat exchangers, marine components, and medical prostheses.

Despite their exceptional properties, the relatively high cost of titanium alloy components may limit their use to applications where lower-cost alternatives such as aluminum and stainless steels are insufficient. This elevated cost typically stems from the intrinsic raw material expenses, fabrication processes, and the substantial metal removal required to achieve desired final shapes.

Titanium Net Shape Technologies

To address cost challenges and improve manufacturing efficiency, several titanium net shape technologies have been developed. These include powder metallurgy (P/M), superplastic forming (SPF), precision forging, and precision casting. Among these, precision casting has emerged as the most fully developed and widely utilized titanium net shape technology. The United States witnessed a remarkable 260% increase in annual titanium casting shipments between 1979 and 1989, demonstrating the growing industry adoption of this approach.

As aircraft engine manufacturers seek materials capable of withstanding higher operating temperatures, alloys such as Ti-6Al-2Sn-4Zr-2Mo and Ti-6Al-2Sn-4Zr-6Mo are being specified with increasing frequency. Furthermore, advanced high-temperature titanium alloys designed for service up to 595°C, including Ti-1100 and IMI-834, are being developed as castings. These alloys exhibit the same degree of elevated-temperature superiority as their wrought counterparts when compared to the more commonly used Ti-6Al-4V.

Wrought Titanium Products and Applications

Wrought product forms of titanium and titanium-base alloys—including forgings and typical mill products—constitute more than 70% of the market in titanium and titanium alloy production. These wrought products represent the most readily available form of titanium-base materials, although cast and powder metallurgy (P/M) products offer advantages for applications requiring complex shapes or microstructures unachievable through conventional ingot metallurgy.

Powder metallurgy of titanium has not gained widespread acceptance and remains primarily restricted to space and missile applications. The principal reasons for utilizing titanium-base products are their outstanding corrosion resistance and advantageous combination of low density (4.5 g/cm³) and high strength. Strength values range from 480 MPa for some grades of commercial titanium to approximately 1100 MPa for structural titanium alloy products, with specialized forms such as wires and springs exceeding 1725 MPa.

A crucial characteristic of titanium-base materials is their reversible transformation from alpha (α, hexagonal close-packed) crystal structure to beta (β, body-centered cubic) structure when temperatures exceed certain thresholds. This allotropic behavior, which depends on alloy composition, enables complex microstructural variations and more diverse strengthening opportunities than found in other nonferrous alloys such as copper or aluminum.

Commercial Titanium Grades and Their Properties

Pure titanium wrought products, with minimum titanium contents ranging from approximately 98.635% to 99.5% by weight, are primarily utilized for their corrosion resistance. These titanium products also offer good fabrication characteristics, though they exhibit relatively lower strength in service applications.

• Good overall strength properties
• Superior resistance to erosion and erosion-corrosion
• Very thin, conductive oxide surface film
• Hard, smooth surface that minimizes adhesion of foreign materials
• Surface characteristics that promote dropwise condensation

Commercially pure titanium with minor alloy additions includes various titanium-palladium grades and alloy Ti-0.3Mo-0.8Ni (ASTM grade 12 or UNS R533400). These alloy additions enable improvements in corrosion resistance and/or strength characteristics. Titanium-palladium alloys, containing nominal palladium contents of approximately 0.2% Pd, are utilized in applications requiring excellent corrosion resistance in chemical processing or storage where environments may be mildly reducing or fluctuate between oxidizing and reducing conditions.

Alloy Ti-0.3Mo-0.8Ni (UNS R533400, or ASTM grade 12) serves applications similar to those for unalloyed titanium but provides enhanced strength and corrosion resistance. However, its corrosion resistance does not equal that of titanium-palladium alloys. The ASTM grade 12 alloy demonstrates particular resistance to crevice corrosion in hot brine environments.

Common Titanium Alloys and Their Applications

The allotropic behavior of titanium enables diverse microstructural changes through variations in thermomechanical processing, allowing a broad range of properties and applications to be served with a minimal number of grades. This versatility is especially evident in alloys with a two-phase, α+β, crystal structure.

Ti-6Al-4V alpha-beta alloy stands as the most widely used titanium alloy. This well-understood alloy demonstrates excellent tolerance to variations in fabrication operations, despite its relatively poor room-temperature shaping and forming characteristics compared to steel and aluminum. With limited section size hardenability, Ti-6Al-4V is most commonly employed in the annealed condition.

Other titanium alloys are specifically designed for particular application areas:

• Alloys Ti-5Al-2Sn-2Zr-4Mo-4Cr (commonly called Ti-17) and Ti-6Al-2Sn-4Zr-6Mo provide high strength in heavy sections at elevated temperatures
• Alloys Ti-6242S, IMI 829, and Ti-6242 (Ti-6Al-2Sn-4Zr-2Mo) offer superior creep resistance
• Alloys Ti-6Al-2Nb-1Ta-1Mo and Ti-6Al-4V-ELI are engineered to resist stress corrosion in aqueous salt solutions while maintaining high fracture toughness
• Alloy Ti-5Al-2.5Sn is designed for excellent weldability, with its ELI grade extensively used in cryogenic applications
• Alloys Ti-6Al-6V-2Sn, Ti-6Al-4V, and Ti-10V-2Fe-3Al deliver high strength at low-to-moderate temperatures

Welding Considerations for Titanium Alloys

Welding has the greatest potential for affecting material properties in titanium and its alloys. 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 play critically important roles in determining the final properties of welded joints.

Several general principles regarding titanium welding can be summarized as follows:

• Welding generally increases strength and hardness
• Welding generally decreases tensile and bend ductility
• 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
• 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

Heat Treatment of Titanium and Titanium Alloys

Titanium and titanium alloys are heat treated for several specific purposes:

• To reduce residual stresses developed during fabrication processes
• To produce an optimal combination of ductility, machinability, and dimensional and structural stability (annealing)
• To increase strength through solution treating and aging
• To optimize special properties such as fracture toughness, fatigue strength, and high-temperature creep resistance

The selection of appropriate heat treatment protocols depends on the specific titanium grade and intended application. Heat treatments must be carefully controlled to avoid undesirable microstructural changes or property degradation, particularly in complex alloy systems.