Titanium is a reactive metal; it will burn in pure oxygen at 600°C and in nitrogen at around 800°C. Oxygen and nitrogen will also diffuse into titanium at temperatures above 400°C raising the tensile strength but embrittling the metal. In the form of a powder or metal shavings titanium also constitutes a fire hazard.
Despite this reactivity titanium is used extensively in chemical processing, offshore and aerospace applications. This is due to:
- The tenacious protective oxide film that forms, giving the alloys very good corrosion resistance, particularly in chloride containing environments.
- No loss of toughness at temperatures down to -196°C
- Good creep and oxidation resistance at temperatures up to almost 600°C.
- Similar strength to steel but at approximately half the weight.
Because of the affinity of titanium and its alloys for oxygen, nitrogen and hydrogen and the subsequent embrittlement, fluxed welding processes are not recommended although they have been used, primarily in the former USSR. Arc welding is therefore restricted to the gas shielded processes (TIG, MIG and plasma-TIG) although power beams, the solid phase processes and resistance welding are also used.
Titanium is allotropic; it has two different crystallographic forms depending on the temperature and chemical composition. Below 880°C it forms the hexagonal close packed alpha phase, above 880°C it exists as body centred cubic beta phase.
A range of elements may be used to improve the mechanical properties, some stabilise the alpha phase and others promote the formation of beta. Oxygen, carbon, nitrogen and aluminium promote the formation of the alpha phase; chromium, molybdenum, niobium, tin and vanadium promote the formation of beta. By suitable additions of these elements it is possible to produce four families of titanium alloys, divided on the basis of microstructure, into commercially pure titanium, alpha or near alpha alloys, alpha-beta alloys and beta alloys. ASTM designations, a simple numbering system, are a commonly used shorthand way of identifying the various alloys and both these and the alloy composition eg Ti-6Al-4V, will be used within this article.
Commercially pure, unalloyed ASTM 1 - 4 and 7 grades contain small amounts of contaminants such as oxygen, nitrogen and carbon, typically less than 0.2%, and have mechanical properties matching those of a good quality low carbon steel. The fewer contaminants, the lower is the tensile strength. The majority of these alloys are used for their corrosion resistance. Welding is straightforward and has little effect on the mechanical properties in the HAZ and they are generally welded in the annealed condition.
The alpha and near alpha alloys, typified by the Ti-5Al-2.5Sn alloy, have ultimate tensile strengths (UTSs) of 500-900MPa, 0.2% proof (PS) of 600-800MPa and elongations (El%) of around 18%. As with the commercially pure alloys the mechanical properties of this group are insensitive to heat treatment. Weldability is good, the alloys being welded in the annealed condition.
The alpha-beta alloys are sensitive to heat treatment, solution treatment and ageing, increasing the strength by 50% compared with the annealed condition. The very high strength alpha-beta alloys such as Ti-5Al-2Sn-2Zr-4Mo-4Cr may have a UTS of 1200MPa, PS of 1150MPa and an El% of 10. Weldability of the alloys within this group, however, is dependent on the amount of beta present, the most strongly beta stabilised alloys being embrittled during welding and, although it is possible to restore some of the ductility by a post-weld heat treatment, this is often impractical. These very high strength, high beta content alloys are therefore rarely welded. Contrast this with possibly the most frequently used alpha-beta alloy, Ti-6Al-4V (ASTM Grade 5) with a UTS of 950MPa, a PS of 850MPa and El% of 15. This alloy has good formability, is readily workable, has good castability, excellent weldability and could be regarded as the alloy against which to benchmark all others.
The fully beta alloys, eg Ti-13V-11Cr-3Al, have similar strengths but with slightly improved ductility, typically around 15% elongation. The beta phase is termed metastable - cold work or heating to elevated temperatures may cause partial transformation to alpha. The alloys have high hardenability, very good forgeability and are very ductile. Weldability is good, taking place with the alloy in the annealed or solution treated condition although to obtain the full strength it is generally necessary to weld in the annealed condition, cold work, solution treat and then carry out an ageing treatment.
Filler metals, all solid wires and matching the composition of the commoner of the alloys, are available, the relevant specifications being AWS A5.16/A5.16M:2007 Specification for titanium and titanium-alloy welding electrodes and rods and BS EN ISO 24034.2010 Welding consumables, solid wires and rods for fusion welding of titanium and titanium alloys. Although readily available, the range of consumables is somewhat restricted with only fourteen or fifteen compositions being produced in accordance with these specifications.
Weldability, as mentioned above, is in general very good. The exception is the high beta alpha-beta alloys. The fundamental problem in welding titanium alloys is the elimination of atmospheric contamination. Contamination of the weld metal and the adjacent HAZs will increase tensile strength and hardness but may reduce ductility to an unacceptably low value such that cracks may occur even in conditions of only moderate restraint. The most likely contaminants are oxygen and nitrogen, picked up due to air entrained in the gas shield or from impure shield gas, and hydrogen from moisture or surface contamination.
The maximum tolerable limits in weld metal have been estimated as 0.3% oxygen, 0.15% nitrogen and 150ppm hydrogen so scrupulous cleanliness is essential for both parent metals and filler wires. Degreasing the weld preparation followed by stainless steel wire brushing and a further degrease is generally sufficient. Heavily oxidised components may need to be pickled in a nitric/hydrofluoric acid mixture to remove the oxide layer. Degreasing of the filler wire for TIG welding should be done as a matter of course and the cleaned wire handled with clean cotton gloves; grease and perspiration from the fingers can cause local contamination and/or porosity. MIG wire should be ordered in a degreased condition, stored in clean dry conditions and not left unprotected on the shop floor.
During welding those parts of the weldment exposed to temperatures above 520°C will absorb oxygen and nitrogen and must therefore be protected until they have cooled below this critical temperature. Fortunately heat conduction in titanium is low so the area affected is limited in size and chill blocks can be used to reduce this heated zone even further. The molten weld pool is protected by the normal gas shroud but the cooling weld and its HAZ will need additional protection by the use of a trailing shield with its own protective gas supply following along behind the welding torch. The back face of the weld also needs similar protection by the provision of an efficient gas purge.
Surface discolouration will give a good indication of the degree of atmospheric contamination as shown in the the colour chart. Under perfect shielding conditions the weld will be bright and silvery in appearance. Discolouration at the outer edges of the HAZ is not generally significant and may be ignored. As contamination increases the colour changes from silver to a light straw colour, then dark straw, dark blue, light blue, grey and finally a powdery white.
The light and dark straw colours indicate light contamination that is normally acceptable. Dark blue indicates heavier contamination that may be acceptable depending on the service conditions. Light blue, grey and white indicate such a high level of contamination that they are regarded as unacceptable. In multi-pass welds the contamination will obviously affect any subsequent weld runs so that surface appearance alone is not a reliable guide to whether or not unacceptable contamination has occurred. A simple bend test is a reliable but destructive method of checking if the weld is unacceptably embrittled but note that the bend radius varies depending on the particular alloy. For example, a 3t bend radius is used for testing a Grade 2 weld but a 10t bend radius is used when testing Ti-6Al-4V. Portable hardness checks may also be carried out on production items; this requires knowledge of the hardness expected in the specific alloy weld metal.
Part 2 of this article will consider some of the other welding problems and provide guidance on TIG and MIG welding of titanium.
This article was written by Gene Mathers.