The major problem with welding titanium is not having it catch on fire. I have never heard of this happening but imagine it might be possible. The problem is weld contamination. The metal and welding rod need to be clean and the weld needs to be in a clean and inert atmosphere; typically with the parts flooded with argon gas. I have made some welds that looked beautiful to my eye and yet had them fail catastrophically. I recall having lunch with a professional welder quite some time ago and we got around to discussing titanium and he told me of one job he had where they were welding titanium and they did it in a room that was filled with argon wearing suits and hoods with air supplied and pumped into and out of the suits, beyond the welding room. I guess it was a type of clean room in which one could not survive due to lack of breathable air. He said they dressed up like space men before entering and getting to the job at hand. Apparently when titanium is elevated way up there in temperature it gladly reacts with other elements and can be contaminated with its physical properties significantly altered.