Nitrogen Affinity of Titanium

Titanium reacts strongly with nitrogen and oxygen from the air at high temperatures. If the shielding gas isn't right, the material picks up nitrogen and becomes hard and brittle. You'll notice it as cracks in the weld metal. So when welding titanium, you need really good gas coverage. Argon is the usual choice, and you also need a trailing shield so the still-hot weld doesn't come into contact with air. If the shielding isn't right, the weld discolors and the structure becomes brittle – then you might as well redo the weld.