Titanium and its alloys have a strong chemical activity. When the temperature exceeds 400 ℃, it begins to react with oxygen, nitrogen, hydrogen and carbon. Above 600 ℃, the reaction is violent. Analysis of the increase in oxygen, nitrogen, hydrogen and carbon content will lead to the embrittlement of titanium and its alloy weld metal, so the gas protection of TA2 titanium tube welding is a key issue while controlling the temperature of the weld and heat-affected zone, Avoid generating coarse grains and overheated structures due to overheating, resulting in a reduction in the mechanical properties of the metal. Welding defects that are prone to welding :
1. Crack problem. When welding titanium-based materials, due to the small amount of sulfur and phosphorus impurities in the material, hot cracks rarely occur; however, cold cracks and delays are likely to occur when welding titanium materials. Mainly due to the poor thermal conductivity of titanium, the heat loss is slow, and the grains of the weld are prone to appear; when the gas impurity content is high, the plasticity of the welded joint decreases, especially when more hydrogen is dissolved in the weld Hydrogen embrittlement.
2. Gas protection. Due to the high gas purity requirements when welding titanium, we choose 99.999% high-purity argon; the range of argon to be protected is the base metal of the molten pool, the heat affected area, and the 10mm area outside the fusion line on both sides. For this purpose, special gas protection devices need to be developed; the inside of the pipeline is protected by an argon gas chamber device.
3. Stomata problem. When welding titanium and its alloys, it is often found that after the welding seam RT, there are aggregated pores near the fusion line. The pores are mainly hydrogen pores; as the solubility of hydrogen in titanium decreases with increasing temperature, the high temperature near the fusion line during welding will cause hydrogen to dissolve out. If the hydrogen partial pressure in the atmosphere around the welding zone is high, the hydrogen in the molten metal is not easily precipitated, so it gathers to form hydrogen gas pores.
After welding is completed, the quality of the weld seam is mainly judged by the appearance color of the weld seam. The color of the weld seam is directly related to the protection of argon gas and the cleanliness of the breach. If the titanium pipe appears blue during the welding process Color or bluish-purple should stop welding immediately, find the reason and improve the welding measures in time; if the surface of the welding seam appears dark gray, it should immediately stop welding for repair, remove all dark gray pants, and re-weld.
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