Alan,
Very few aerospace manufacturers or repair vendors employ large enclosures for welding titanium and as you say *other aerospace materials* What "other" alloys could possibly demand a purge enclosure? I have seen miles of titanium duct fabricated and repaired a good share of it, never have I seen an enclosure employed for either fab or fix. Does your engineering doccumentation actually call out a requirement for this chamber?
Excellent results are obtained in GTA titanium welds (manual and semi/automaitc) with well engineered argon backups, trailing shields and purging fixtures.
My advice is for you to get a tour of other duct manufacturers (Boieng/Douglas, Lockheed etc.) and take a peek at how they do their work. Also if you are using automatic or semi-automatic equipment to weld your tubes and ducts, the manufactures of this equipment (Liburdi/Dimetrics comes to mind) most likely have testamonial and data sheets that prove the benefit of good trailing shields.
Numerous considerations need to be taken to produce consistantly sound Titanium welds, but a 100% inert atmosphere enclosing the componant is rare indeed, not unheard of just rare.
The Navy Joining center has published extensively on this subject as has EWI and TWI.
Persuasive salesmen have for years been selling chambers-bubbles-boxes (we have our share) that end up collecting dust in shop corners while production continues to increase due to careful planning and quality process control.
Lastly, if you are selling your ducts to Boeing, Pratt Whitney, Lear or any other Major, perhaps you can get your hands on a copy of there "Standard Practice" manuals for welding. These will tell you what is viable and as well as the latest cutting edge in process control.
This is a great question! I'm really looking forward to other responces
Lawrence