Welding titanium and titanium alloys

Titanium 's great affinity for the atmospheric gases oxygen, nitrogen and hydrogen rules out all welding processes in which the molten metal can come into contact with any of these elements and result in embrittlement.

Oxyacetylene welding is therefore not possible.

The primary fusion welding methods used are those carried out in an inert gas atmosphere (TIG and MIG welding). Other processes that can be used taking into account titanium 's specific requirements are plasma, resistance, electron beam, ultrasonic, diffusion, laser beam, friction and explosive welding.

While the TIG process is the most common method for titanium materials in pressure vessel and process equipment construction, plasma welding is mainly used for joining thick plates. In aerospace applications electron beam and diffusion welding are predominantly used.

The mechanical properties of the base metal are also largely achieved in the weld. However a slight ductility loss may occur, due primarily to the coarser microstructure that forms in the fusion zone.

It must however be pointed out that improper welding can seriously impair both the toughness and the corrosion resistance of the weld.

Under passivating conditions titanium welds show the same corrosion resistance as the base metal. Only in media in which the base metal also undergoes corrosion, e.g. in reducing agents, can the weld also suffer increased corrosion.

Post-weld heat treatment is usually not necessary with the commercially pure titanium grades. Stress relieving or recrystallization annealing to remove welding stresses is only recommended for very complex shaped parts or multiple pass welds and for titanium alloys.

1. Argon arc welding (TIG/MIG processes)

Air must be excluded from the weld, the heat-affected zone and the adjacent areas of the base metal when titanium is welded. In addition, the root side must also be protected from air contact. This protection must be maintained until the weld has cooled down to below approximately 300°C.

Depending on the geometry and size of the parts this is achieved by use of inert gas shielding, trailing shields, porous backup bars or by welding in evacuated inert gas-filled chambers.

As filler metal, tested bare wire of the same titanium group should be used. Testing should be based on specification VdTÜV-Merkblatt 1153. In practice it is not always possible to prevent slight hardening of the weld due to pickup of small amounts of atmospheric gas components, and so it may be advisable to use bare wire from a lower titanium group than that of the base metal.

If welds have to be made between titanium grades of different strength, the filler metal should be chosen based on the main demands on the weld. If strength is the priority, bare wire of the harder titanium group is used, while wire of the softer group is chosen if ductility requirements are important. In selecting the filler metal consideration should also be given to service temperature.

Careful weld preparation is essential to achieve good quality welds. To avoid weld defects such as pores, inclusions and local hardening, the weld area should be freed of all surface contaminants directly before welding by sanding, brushing or by degreasing followed by pickling in an aqueous hydrofluoric/nitric acid solution. As even hand sweat can lead to hardening, the weld area should be cleaned again directly prior to welding using a solvent that leaves behind no residue.

Sheets 2 ­ 2.5 mm thick can be welded in a single pass. With greater thicknesses, two or multiple pass welding is necessary. After each pass the weld area must be cleaned thoroughly to remove any discoloration. This also applies to tack welds, the number of which should be kept to a minimum by careful design of the weld fixture. To avoid local oxygen buildup the oxidation products should be removed before every restart. The same applies to the tip of the filler wire.

The weld puddle, the heated adjacent zones and the root side must be protected with argon. Under very favorable conditions the root side can also be protected by clamping the workpiece snugly on a flat, heat-dissipating backup plate.

The welding speed and the welding current depend on the quality of the inert gas shielding. Welding is carried out with a direct current power supply and a negatively poled electrode. The main inert gas used is argon. Good results are achieved with argon flow rates around 6 to 8 l/min. Higher rates do not improve the protection and often cause turbulence. Argon should be used with a purity of at least 99.99%.

Moisture content, which is frequently not specified in the analysis, is also important. Only argon with a very low moisture content should be used. The dew point should be below -50 °C.

In argon arc welding the parameters should be selected such that the finished weld has a bright silvery appearance. To a certain extent the quality of the weld can be judged by its coloring. Yellowish to bluish colors indicate slight hardening of the weld, which is however acceptable. By contrast, dark blue colors or a gray oxide layer point to inadequate protection of the weld and to embrittlement through oxygen and/or nitrogen pickup. The hardness of a good weld should not exceed that of the fully recrystallized base metal by more than 50 points. If hardness testing of the lightly ground weld surface delivers higher values, then the weld must be considered embrittled and must be completely removed.

2. Plasma welding

In addition to micro-plasma welding, plasma welding is particularly suitable for joining titanium plates with thicknesses between 3 and 20 mm. The advantages over TIG welding include greater penetration depth, higher welding speed, smaller welds and more uniform surfaces (cover and root). In this respect plasma welding is bettered only by electron beam welding.

3. Resistance welding

Titanium can be spot welded under similar conditions as for stainless steels. Inert gas shielding is not needed owing to the very short duration of the weld cycle and titanium 's relatively low electrical and thermal conductivity.

Using standard commercial copper-base electrodes (e.g. Cu-Cr alloys) with a flat head (radius approx. 75 mm) high shear strength levels can be achieved together with small electrode impressions, low distortion, reduced spraying and extensive freedom from porosity.

Hardening of the fusion zone by up to 50 Vickers points compared with the base metal can be viewed as normal and does not significantly impair the mechanical properties of the joint.

Seam welding and butt welding equipment should provide high electrical power for short heating times. Seam and butt welding can only be performed with argon shielding.

4. Electron beam welding

A major advantage of electron beam (EB) welding is the lower depth of heat penetration, which keeps welding stresses and distortion low. EB welding is particularly suitable for titanium and can be used for I-welds, i.e. without filler metal, at thicknesses up to around 100 mm.

Vacuum EB welding offers a number of additional advantages as the intensity of the electron beam produces extremely narrow welds with small heat affected zones free of weld colors. The process is suitable for thick sections and high welding speeds and provides exact reproducibility of even complicated welds and the guarantee of consistent quality.

With thicknesses over 10 mm the surfaces to be welded should have a roughness of Ra < 3.2 μm and join without a gap. Welds should generally be ground to allow exact testing and obtain a notch-free surface.

Guide values for the EB welding of alloy TiAl6V4 components in thicknesses between 4 and 22 mm at welding speeds of 9 - 70 mm/s are accelerating voltages of 110 - 150 kV and current levels of 20 - 55 mA.

5. Diffusion welding

In diffusion welding two metallic surfaces are joined without local fusion by the application of pressure and elevated temperature in a vacuum or controlled atmosphere.

As a result, the microstructure in the joint zone corresponds to that of the base metal. In an ideal case when the same materials are being welded the joint zone is indistinguishable from the base metal and has the same mechanical properties and corrosion resistance.

Diffusion welding is of particular interest for titanium because it is easier to achieve a homogeneous solid-state joint with titanium than with other metals. In addition, diffusion welding, which can be used for both linear and large-area welds, saves material whenever parts have to be milled from the solid or formed in a complicated forging operation.

In most cases diffusion welding is combined with superplastic forming (SPF/DW).

6. Laser beam welding

As with EB welding, a major advantage of laser beam welding is low welding stresses and consequent low risk of distortion. This is achieved by the high energy density of the laser beam producing a small pool and by the high welding speed. Advantageous for titanium materials is welding without filler metals and the use of shielding gas to avoid hardening.

The depth of the weld and the thickness of section that can be welded are primarily determined via the laser power. Laser beam welding can produce welds with a width to depth ratio of up to 1:5, welding right through a part. Complicated welds can be produced in hard-to-access areas as the beam can be redirected and focused by means of lenses, mirrors and optical fibers.

Several welding tests on titanium and titanium alloys up to thicknesses of 12 mm have been carried out using both Co2 and Nd:YAG lasers. The thicknesses welded and the welding speeds used are dependant on the laser power level. For more details on welding parameters, refer to the relevant publications.

7. Friction welding

Friction welding avoids the formation of a liquid phase during the welding process. The surfaces are joined in a dough-like condition at hot forming temperatures. The typical defects caused by melting and solidification such as pores, pinholes, shrinkage cracks, segregation, grain coarsening and cast structure are therefore avoided and the risk of gas pickup is low due to the short welding cycles.

Friction welding is also being used in the aerospace industry to attach individual blades to compressor discs. Results are available for the titanium alloys TiAl6V4, TiAl6Sn2Zr4Mo2 and TiAl5.8Sn4Zr3.5Nb0.7Mo0.5Si0.35C0.06.

8. Welding titanium with other metals

Welding titanium with other metals presents great difficulties due to embrittlement caused by the formation of intermetallic phases. Numerous tests to join titanium with molybdenum, tantalum, silver and vanadium by the TIG process without filler metal have resulted, with the exception of vanadium and silver and with niobium and hafnium, in welds of limited ductility.

Welding steel with titanium is very difficult due to the low solubility of iron in alpha titanium at room temperature. When titanium is welded with steel the intermetallic phases TiFe and TiFe2 form, which are very hard and brittle and prevent the production of technically useable welds.

One way to achieve ductile welds of steel and titanium is to use intermediate layers of materials capable of being welded with both titanium and steel, without brittle phases occurring. One such material is vanadium. Titanium/vanadium/steel joints have been produced successfully by resistance spot, electron beam and diffusion welding.

In the same way, initial plasma overlay welds using titanium grade 2 and grade 12 on HII boiler plate have been carried out successfully, with intermediate layers of copper.

Niobium is suitable for intermediate passes to produce serviceable copper-titanium welds, while silver has proved successful in the production of aluminum/titanium joints.

Another way of making welds between titanium and other metals such as stainless steel or aluminum is friction welding. In most cases however a loss of strength in the weld must be expected.

These methods of welding titanium with steel and other metals are special processes that for reasons of cost (intermediate passes) or shape (friction welding) are restricted to specific applications and are not yet widely used in volume production.

To produce flat, large-area joints between titanium and other metals explosive cladding has proved successful and is already being used on a routine basis.

9. References

• Merkblatt DVS 2713 "Schweißen von Titanwerkstoffen" (can be ordered from: Deutscher Verlag für Schweißtechnik (DVS) GmbH, Postfach 2725, 4000 Düsseldorf 1)
• VdTÜV-Werkstoffblatt 230 (printed and sold by: Deutscher Verlag für Schweißtechnik (DVS) GmbH, Postfach 101750, 5000 Köln 1)
• Fachbuch "Schweißen von Sondermetallen", (1971) (published by H. Schultz, Deutscher Verlag für Schweißtechnik (DVS) GmbH, Postfach 2725, 4000 Düsseldorf 1)

Deutsche Titan, Nov. 2000

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