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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 provided and 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 percent pure - even 99.995 percent gas can lead to some yellow-straw 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 until the metal has cooled below about 250°C to prevent discolouration.
As mentioned above, high-purity gas must be used. All work areas must be thoroughly cleaned according to recommended procedures that are widely available. All work components should be considered for cleaning, depending upon the welding method. For example, 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 are 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. The welded joint must be allowed to cool before gas coverage is removed.
Accessories for Titanium Welding
Specialty products are now available to provide quality gas coverage. Particularly low cost flexible enclosures for smaller components, weld purge monitors and trailing shields. Additionally there are ‘clean’ expandable stoppers available, made from clinically clean materials for use in sealing tubular components during gas purging.
Flexible Welding Enclosures
The principle advantage of flexible welding enclosures over the more traditional solid metal versions is significantly lower cost. Additional benefits also include the flexibility of minimal weight, the ability to move and store 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, however 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 in section but developments in manufacturing technology have now afforded opportunities to produce square and rectangular enclosures as well. Rectangular cross sections and the opportunity to produce to a length to suit the customer’s requirements have made the new additions attractive to a wider range of end-users. The rectilinear concept offers users the opportunity to match the shape to meet specific requirements. Such enclosures are available up to 2m long and 1m x 1m in section.
Weld Purge Monitors
There are now low cost instruments designed specifically for Weld Purging, measuring oxygen levels in any purge gas during the welding of titanium and its alloys. They show oxygen levels down to 10 ppm using an alphanumeric display and incorporate an impressive array of features, which include automatic fault finding diagnostics, data acquisition software, and push-button operated menu.
Devices called ‘Trailing Shields’ are available and they 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 radii as well as outside. Smaller versions are manufactured to suit manual welding where their size and lightweight do not restrict the manual welding operation.
For mechanized welding machines, where weight and size considerations are less constraining, the Trailing Shields are larger giving greater gas coverage and allowing the machines to weld much faster than they would be able to without trailing shields.
Tube, Pipe and Tank end blocks
In recognition of the cleanliness required for reactive metal welding, clinically prepared Expandable plugs are also available and widely used to seal pipes so that good gas coverage is maintained. Such plugs are available from 12 to 160 mm (0.5 to 6 in) diameter.