Gas shielding is not needed when friction welding titanium as the two hot surfaces in contact are shielded from the atmosphere by the presence of the other work piece. Some slight surface contamination is inevitable, but virtually all contaminated material is expelled as flash. Friction stir welding titanium does require a gas shield as the hot parts are normally open to the surrounding air, and oxygen and nitrogen are absorbed. The depth of contamination is only slight, and contaminated material would be removed by normal machining. The gas shield may be more important in protecting the friction stir tool rather than the titanium being welded. Titanium alloys are well suited to most friction and inertia welding processes, as they are to solid state bonding generally.