Similar to aluminium oxide, titanium oxide is hydroscopic and absorbs moisture from the atmosphere. Welding of titanium components with hydrated layers on the joint surfaces results in increased levels of gaseous hydrogen dissolved in the weld metal and subsequent pore formation upon solidification. The main features to note in order to minimize the incidence of porosity in Ti welds are:
- Dry machining is often best, rather than using lubricants, for final preparation of the joint surfaces. Machining with a rotary burring rod, a lathe or milling machine should always be employed, i.e. not guillotined surfaces.
- Welding within 48hours of final joint preparation is highly recommended
- Degreasing the joint faces is essential.
- Pickling can be used as a final joint preparation stage, but welding within 48hours becomes even more important.
Not all the welding processes exhibit the same vulnerability to weld metal porosity formation. Although TIG, MIG, and laser welding exhibit weld metal porosity, keyhole plasma appears to result in a minimal amount of weld metal porosity, indicating the greater tolerance of this process to joint surface condition. For laser welds, there is also the possibility of entrapped plasma gases. Solid state welding processes are also known to result in pore-free joints.
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Welding titanium - a designer's and user's handbook