Titanium weld contamination
The absorption of interstitially dissolved gases like oxygen, nitrogen, or hydrogen into the weld pool, leading to a drastic loss of toughness and embrittlement of the titanium system.
Titanium is extremely sensitive to oxygen, nitrogen, and hydrogen from the surrounding air during welding. Once these gases get into the weld pool, the material becomes hard and brittle – the part will break under load. You often notice this by a bluish or gray discoloration of the weld, which shows that the shielding gas coverage wasn't sufficient. That's why you must completely cover the weld and the heat-affected zone with shielding gas, also on the back side. The surface must be free of grease and oxides before welding, and you must keep the heat input as low as possible so that the weld pool stays short. If the weld is discolored after welding, it's usually unusable in that area and must be redone.