How It Works: Why is the Welding of Titanium Difficult?

Titanium is a very unique material, being as strong as some steels but much less dense and exhibiting excellent corrosion resistance. 

When Titanium was developed in the 1950’s and 60’s it was thought of as a new wonder metal because of its alloyed strength being similar to steel but, itself being very light. This means that the alloys of Titanium have a high strength to weight ratio. Strength to weight relates the density of the material to its ability to withstand permanent deformation or fracture under pressure. The values for Titanium indicate that the material is light-weight but can bear significant load. 

However, there was a fabrication difficulty with this material. Titanium and its alloys were were found to be reactive; titanium itself burning in pure oxygen at 600°C and in nitrogen at around 800°C. Oxygen and nitrogen also diffuse into titanium at temperatures above 400°C, causing severe embrittlement. These facts mean that it is a challenge to weld and not all of the current arc welding processes are suitable. The basic problem is atmospheric contamination such that the weld zone can become very crack sensitive. Oxygen and nitrogen, picked up from the air and entrained in the gas shield, impure shielding gas and hydrogen derived from moisture or from surface contaminates can be a real problem. The maximum tolerance for these elements is very low and so when welding material cleanliness is absolutely essential.

Because of its affinity for embrittling elements, fluxed welding processes such as FCAW and SMAW are not recommended. Arc welding is thus restricted to the gas shielded processes, Gas Tungsten Arc (GTAW) Gas Metal Arc (GMAW) and Plasma Arc (PAW) with argon and argon/helium gas shielding.  Other more esoteric welding processes have also been used such as Electron Beam Welding (EBW) and Laser Beam Welding (LBW).

Returning to the cleanliness issue, degreasing and wire brushing of weld preparations (using stainless steel brushes) is mandatory. Degreasing of GTAW filler wires is also mandatory and the cleaned wire should be handled with clean cotton gloves so that contamination from grease and perspiration from the fingers is eliminated. GMAW consumables should be stored in clean dry conditions and not left unprotected in the fabrication atmosphere.

During welding, those parts of the weldment exposed to temperatures above about 520°C will absorb oxygen and nitrogen and must therefore be protected until they have cooled below this critical temperature. The molten weld pool will be protected by the normal gas shroud but the cooling weld and its HAZ will need additional protection by the use of so called “trailing shields” with its own protective gas supply following along behind the welding torch. Other surfaces that may see these temperatures such as the back face of the weld root also needs similar protection by the provision of a gas purge in that location. Figure 1 shows a Titanium alloy overlay using pulsed GTAW that employed a “trailing gas shroud” to protect the weld metal.

 Titanium Alloy Overlay using Pulsed GTAW with an Additional Trailing Gas Shroud

Figure 1. Titanium Alloy Overlay using Pulsed GTAW with an Additional Trailing Gas Shroud

There are several elements that can be added to Titanium to produce alloys which essentially produce three groups in addition to the commercially pure form. The groups are defined as: 

  1. Commercially pure, unalloyed 

  2. The alpha and near alpha alloys

  3. The alpha-beta alloys

  4. The beta alloys

It’s not the purpose of this article to delve into the metallurgy of these particular groups. However, the first two, commercially pure and the alpha/near alpha are readily weldable whereas some of the higher strength alloys (in the alpha beta group) are more complex with those with higher beta levels are impractical to weld.

Filler metals, being all solid wires and with matching compositions to the commoner of the alloys, are available. The relevant specifications recognised in Canada being the AWS A5.16/A5.16M Specification for Titanium and Titanium-alloy welding electrodes and rods 

As mentioned above, weldability is in general very good with the exception of the high beta alpha-beta alloys. The fundamental problem in welding titanium alloys is the elimination of atmospheric contamination (weld pool, HAZ and adjacent hot areas) to ensure embrittlement of the weld zone does not take place. To achieve this, welding procedures need to be very carefully planned and be executed with the greatest of care. 

Mick J Pates IWE

President, PPC and Associates

 

 


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