(1) Welding porosity During the welding of titanium materials, porosity is prone to occur.
(2) Embrittlement and overheating Hydrogen is one of the most harmful elements in titanium. It can reduce the plasticity and toughness of titanium and cause embrittlement. If the base metal or welding material contains a large amount of hydrogen, it should be dehydrogenated in advance. Titanium will rapidly combine with oxygen and nitriding at a temperature above 600°C to produce titanium dioxide and titanium nitride (with extremely high hardness). When heated to above 800°C, titanium dioxide dissolves in titanium and diffuses deep into the internal structure of metallic titanium, forming an intermediate brittle layer of 0.01 to 0.08 mm. The higher the temperature, the longer the time, the more serious the oxidation and nitriding, and the plasticity of the welded joint will decrease sharply. In addition, titanium is also easy to form brittle carbides with carbon, reducing plasticity and weldability.
(3) Welding thermal wave cracks Common metal welding cracks include thermal cracks, cold cracks, reheat cracks and laminar tearing cracks. Welding thermal wave cracks are another new type of crack discovered in recent years. That is, a kind of crack formed after repeated heating and cooling at the weld, and its development trend is eventually fracture. It mostly occurs in the multi-layer and multi-pass weld area (mainly near the fusion zone) of a thick-walled pipe fitting or the weld repair place. It is characterized by changes in the material properties of the crack area (especially reduced plasticity and toughness), relaxation of the crystal grains and distortion of the crystal lattice. There is local hardening, and sometimes there are a few finer cracks beside the cracks. It has a strong hysteresis, and its concealment is more harmful than cold cracks.
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