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Welding and detection of titanium alloy
Jun 08, 2016

2016-06-03 titanium micro media

At room temperature, titanium and titanium alloy is relatively stable, titanium and titanium alloy as structural material has many advantages, such as small proportion, tensile strength and yield strength high, in 300 to 500 DEG C still has enough high strength, has excellent corrosion resistance in seawater and most of the acid, alkali and salt medium, in aviation, chemical industry, the nuclear industry has gained a wide range of applications. However, with the increase of temperature, the ability of titanium and its alloys to absorb hydrogen, oxygen and nitrogen increased gradually. Titanium from 250 degrees to start hydrogen absorption, starting at 400 degrees C, starting from 600 C.

Due to the high affinity of titanium alloy with O2, N2 and H2, the joint will become brittle and the impact property, ductility and toughness of the welded joint of titanium alloy can be reduced. A delayed crack occurs in the heat affected zone when hydrogen is contained in the titanium alloy. When the content of oxygen and nitrogen in the weld is high, the weld or heat affected zone will be crack under the action of large welding stress. So it is very necessary to protect the inert gas (or vacuum chamber). Due to the high cost of using the vacuum chamber, the general use of inert gas protection methods. Protection gas helium and argon, due to argon helium prices higher than, generally speaking, not to the special requirements of the titanium alloy welding joint and heat affected zone, using high pure argon gas protection can prevent oxidation.

Most titanium alloys can be welded using oxygen acetylene welding methods, and all titanium alloys can be welded using solid state welding methods (e.g., TIG, MIG, plasma arc welding, laser and electron beam welding). In fact, the titanium alloy welding joint crack tendency than ferrous metals (such as iron alloy, little Nie Hejin). In spite of the titanium alloy has such good properties and some other excellent welding characteristics, some engineers are still thought of titanium alloy welding is quite difficult, mainly in titanium alloy welding requirements for protective gas is particularly high, general only very professional staff can assurance and meets the requirements of gas protection. In fact, a lot of welding tools can be used to weld titanium alloy. Due to the introduction of the air in the welding process of O2, N2 and carbon containing material makes the titanium alloy melting welding head become brittle, so the welding area must be cleaned and the use of inert gas protection. The welding material is basically based on the characteristics of the material being welded. The welding of titanium alloy is generally based on the ductility and strength of the welded joint. The welding property of common titanium alloy is shown in the table.

Laser welding of titanium alloy is more and more widely used in laser welding, laser welding of small deformation, high production efficiency, and the degree of automation is higher than the electron beam and TIG. Compared with electron beam welding, laser welding does not require complex equipment such as vacuum chamber, so the laser welding is more practical, and laser welding can be directly welded with different welding conditions. CO2 laser due to large power, the use of 25kW/h can be a one-time welding of 20mm thick titanium plate. Nd:YAG laser welding is more flexible due to the energy transfer by using the optical fiber, but the penetration depth is limited due to the low power of the YAG. Laser welding is prone to splash, so that the surface is not clean, can not be carried out after the treatment must be particularly careful.

After welding inspection: the welding parts of titanium alloy visual inspection, mainly in order to assess the quality of gas protection. When the surface is silver white, said protective gas is very good; and when the surface is pale yellow or deep yellow, said titanium alloy subject to little pollution, but it is still acceptable; surface is dark blue, said pollution is relatively serious, but due to the different conditions used, and some can be used to; surface for shallow blue, pollution is serious, almost impossible to use; surface is gray blue or gray, pollution is very serious, do not use; the same surface is white, pollution is very serious, is not available.

Coloration detection approach: hardness testing and eddy current testing can be used to joint detection, whether or not containing impurities, due to containing impurities and joint properties will be changed, mainly containing impurities in the joint of the hardness and resistivity will be higher. Portable hand-held hardness tester can test the hardness of welded parts in situ. The application of this technology can easily detect the quality of welding. Under normal circumstances, the welding crack of titanium alloy is relatively few. However, sometimes it will crack in the weld or in the presence of impurities. At this point, the defect can be found through the color inspection, and this method also has a certain effect on the loose. Must pay attention to is that before continuing to carry out welding must be clean and clean.