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, 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.

Reference Lists

 

 

 

CDM Technical documents