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PRODUCT DATA of 04: Titanium and its Alloys
Material04: Titanium and its Alloys
General InformationTitanium and Ti-alloys are generally chosen for their mechanical properties, temperature resistance or chemical resistance. The specific points of special interest for the spacecraft designer are considered here, since the basic aspects of titanium alloy assemblies are similar to those for aeronautic design.
Use in SpacecraftConventional Ti-alloys are used for primary and secondary structures; fasteners; in plumbing systems (standard tube alloy grades and commercially pure CP-grades) and in areas where operating temperatures preclude the use of aluminium alloys. "Memory alloys" based on titanium can find specialized uses as actuators.
Titanium alloys are preferred for contact with CFRP due to their low CTE and matched galvanic corrosion properties.
Main CategoriesThe characteristics of titanium alloys are generally grouped according to their metallurgical structure which is, in turn, controlled by the chemical composition and heat-treatment history.
Commercially pure (CP Ti) products are normally selected for chemical resistance. Impurities in CP Titanium can increase strength but with a loss in corrosion resistance.
Titanium alloys are normally selected for their strength properties, which depend on a number of specific heat-treatments (age hardening, quench and temper). The most commonly used titanium alloy is Ti6Al4V for which extensive mechanical and corrosion property data are available.
Processing and AssemblyAll classical methods of shaping and forming processes can be used, with wrought products being produced by rolling, extrusion, forging; cast products. Owing to titanium’s high-affinity for oxygen and other gases,melting and casting processes are carried out under vacuum to prevent contamination and subsequent property degradation. Titanium alloys can generally be joined by welding, brazing, riveting, bolting and adhesive bonding, although only certain alloys can be brazed. Not all alloys are weldable and a protective atmosphere is required (inert-gas or vacuum) to avoid pick-up of O, N and H which degrade properties. The filler material also needs careful selection to avoid potential hydrogen embrittlement problems: the use of CP filler wire to join CP alloys parts is possible, but CP filler for alloy parent parts shall not be used.
Some metals and processing chemicals can degrade the properties of titanium alloys by inducing stress corrosion or hydrogen embrittlement or by reducing fracture toughness.
PrecautionsThe properties of titanium alloys are strongly dependent on their previous thermal or mechanical history.
Some alloys have a limit on the section dimensions that can be successfully hardened by heat-treatment.
The fatigue life of titanium alloys is reduced by fretting at interfaces (either between Ti-alloy parts or Ti-alloy and other metals). Structural designs should avoid fretting.
The corrosion and chemical resistance of titanium alloys relies on the adherent, protective oxide layer which is stable below 535 ºC. Above this temperature, the oxide film breaks down and small atoms (e.g. C, O, N and H) embrittle the metal. Consequently high-temperature processing methods are done under vacuum or in an inert-gas atmosphere.
During production, the selection of appropriate processes and avoidance of surface contamination are vital to avoid property degradation. Contamination zones formed during processing can be removed by subsequent machining or by chemicalmilling of the surfaces of titanium parts.
  1. Corrosion shall be considered during the whole manufacture and prelaunch phase (see data on galvanic corrosion); electrolytic couples should be avoided and all metals should be suitably protected against external damage by the use of plating, conversion coatings, paints and strippable coatings. This is particularly important in special operating environments (fuel tanks for example).
  2. The metallic components proposed for use inmost spacecraft shall be screened to prevent failures resulting from SCC. Three ratings of alloys were chosen: high-resistance, moderate-resistance and low-resistance to SCC (these are listed in Tables 1, 2 and 3 respectively of ECSS-Q-ST-70-36). The alloys listed in Table 1 should be used for space applications. For alloys listed in Table 2 or 3 a detailed justification for space use shall be provided, demonstrating that SCC testing according to the standard method detailed in ECSS-Q-ST-70-37 was done. (Method incorporates constant load and alternate immersion in 3,5 % NaCl solution)
  3. Titanium alloys shall not be used with LOX or GOX at any pressure or with air at oxygen partial pressures above 35 kPa
  4. Titanium alloys shall not be machined inside spacecraft modules during ground processing or in flight, because machining operations can ignite titanium turnings and cause fire.
Hazardous and PrecludedTitanium alloys can be susceptible to hydrogen-embrittlement and are generally unsuitable for hydrogen-containing atmospheres.
Care shall be exercised to ensure that cleaning fluids or other chemicals used on titanium are not detrimental to performance. Surface contaminants which can induce stress corrosion, hydrogen embrittlement, or reduce fracture toughness include: hydrochloric acid, cadmium, silver, chlorinated cutting oils and solvents, methyl alcohol, fluorinated hydrocarbons, mercury and compounds containing mercury.
Effects of Space environmentVacuum poses no special problems. All metals in contact under vacuum conditions or in inert gas have a tendency to cold weld. This phenomenon is enhanced by mechanical rubbing or any other process which can remove or disrupt oxide layers. Fretting is a particular concern for titanium alloys.
Radiation at the level existing in space does not modify the properties of metals.
Temperature problems are similar to those encountered in technologies other than space, but are complicated by the difficulty of achieving good thermal contact in vacuum and the absence of any convective cooling.
Atomic oxygen in low Earth orbit has no effect on titanium.
Some Representative ProductsThere are several European sources of conventional titanium alloys, e.g. Timet (previously Imperialetal Industries - IMI (UK); Tital (D),Ugine Kuhlmann (F).
Sources of high-reliability fasteners include: Blanc Aero (F); Fairchild Fasteners Europe Hildesheim (D); Linread (UK).
Procurement to internationally recognized specifications is recommended, e.g. ISO,MIL Specs, B.S., SAE., DIN or AFNOR specifications. The materials listed in Table A-4 (from ECSS-Q-ST-70-36), can be considered.