Welding grade argon is generally employed for torch sheilding because arc stability is better than with helium. Argon-helium gas mixtures, however, do provide a higher voltage, hotter arc and greater penetration. No other gases (including argon-oxygen mixtures, niyrogen, and CO2) can be used. Therefore, you may use pure argon for fixture sheilding and the mix for your torch sheilding, if you wish to enjoy a cost savings, but it may just be more hassle supplying two separate bottles/gages/manifolds.
I can fax to you some very informative titanium welding data from Titanium Industries Inc, if you will e-mail or post your fax#.
Tim
My first question would be to ask; why the helium in the first place? Argon is suitable for shield and cover gas for manual or automated GTA joining of Titanium in or out of the glove box. As far as embrittlement goes; contamination will be the only cause. Both argon and helium are inert and with 100% coverage of weld puddle and trailing zone you will not experience embrittlement (assuming the work is clean.) As a secondary thought, too quick cooling is thought to be related to increased crystal size, some welders are alarmed by the visible grain boundaries, However this phenomena is not even addressed in AWS D 17.1 2001 Specification for fusion welding for Aerospace applications.
I guess if you have a laser laying around with nothing to do, it's a good move. But if not, it seems like a big investment for something your doing now adequately but could be enhanced by improved process control.
Here is a link to a consultant with whom I am well acquainted, an expert mettelurgist with experience in automation, process control and materials that I bet would have some specific Ideas for your application
http://www.pwt-online.com
Keep us posted on your very interesting dilemma
Lawrence.