IT will come as no surprise to fabricators with experience in handling titanium that special consideration needs to be taken during preparation and welding.
Titanium and its alloys are reactive and can become contaminated by atmosphere gases. Adequate shielding using inert gas is essential if repeatable, high- quality joints are to be made.
For many years it was believed that welding-titanium could only be performed satisfactorily in sealed chambers equipped with glove ports and viewing ports and filled with high-purity, inert gas such as argon.
A better understanding of the mechanism of metal/gas interaction has shown that for some applications, as long as adequate local shielding is maintained during welding, the use of sealed chambers with or without vacuum pumping systems may not be absolutely necessary.
This understanding has afforded possibilities for welding large titanium fabrications and significantly extended the application areas for the metal and its alloys into major structural forms, especially in the aerospace industry, the motor sport industry and latterly in the petrochemical industry. The primary requirement then is to provide a protective gas atmosphere to the joint area during welding and subsequent cooling.
The weld purge gas must be used correctly. It should be 99.999% pure, even 99.995% gas can lead to some yellow discoloration.
If the colour of the welded metal begins to mottle, or if it exhibits any hint of blue, the gas isn’t pure enough, or it isn’t being applied correctly. The gas flow should be started several seconds before starting to weld. If there is enough shielding and the argon is being dispersed evenly over the part, a uniform colour will be seen.
Protective gas flow is still needed after welding until the metal has cooled below about 250°C to prevent discolouration.
High-purity gas must be used. All work areas must be thoroughly cleaned. All work components should be considered for cleaning, depending upon the welding method.
Any titanium filler rod material should be thoroughly checked for cleanliness.
The welding tungsten should be clean and new and not be contaminated by the welding of other materials on previous work. Welding torches and their associated hoses will occasionally have leaks that will draw in atmospheric gases to the shielding gas, so they should be leak tested prior to use. Cotton gloves should be worn for handling the parts after they have been cleaned.
Accessories for titanium welding
Speciality products are now available to provide quality gas coverage, including flexible enclosures for smaller components, weld purge monitors and trailing shields.
Additionally there are “clean” expandable stoppers available for use in sealing tubular components during gas purging.
The principle advantage of flexible welding enclosures, over the more traditional solid metal versions is a significantly lower cost. Additional benefits include minimal weight, easy movement and storage when not in use and the relatively fast times to achieve a satisfactory weld purge.
Flexible enclosures might be regarded as insufficiently robust for welding operations, but they are made of tough PVC plastic and the cold purging gas helps to quickly dissipate the heat that is generated by welding. Early flexible enclosures were circular, but they are now also produced in square and rectangular versions. Enclosures are available up to 2m long and 1 x 1m in section.
Low-cost weld purge monitors are available for measuring oxygen levels in any purge gas during the welding of titanium and its alloys. They show oxygen levels down to 10ppm using an alphanumeric display and incorporate automatic fault finding diagnostics, data acquisition software and push-button operated menu.
Devices called trailing shields will fit any make of GTAW (Tig) or PAW (Plasma) welding torch. They are available in flat format for sheet metal welding and in radiused format to match any diameter of pipe or vessel.
The radiused versions are manufactured for welding on inside and outside radii. Smaller versions are manufactured to suit manual welding.
For more information contact:
Huntingdon Fusion Technologies
E – firstname.lastname@example.org or email@example.com
W – www.huntingdonfusion.com
E – firstname.lastname@example.org
W – www.smenco.com.au