I've built two frames in grade 9. Never again.
* It's brutal on tooling - it also burns, literally, if you get it too hot in the presents of o2.
* Ti demands spotlessly clean grinding & machining equipment. Any foreign metal particles embed when welding, will compromise joint integrity - often serious.
* Welding must be TIG and back purging is required.
* AT 2-3 bucks per/inch, any screw-ups will be costly.
* And FWIW, 6Al-4V (Grade 5) is much more difficult to weld than Grade 9 - the man on the torch better know what he's doing.