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Mechanism and physical and chemical properties of titanium alloy welding

The welding of titanium and titanium alloy requires very strict protection during welding. When the carbon content of the welding seam is 0.55%, the plasticity of the welding seam almost disappears and becomes a very brittle material, and the thermal treatment after welding cannot eliminate this brittleness.According to China's technical conditions, the carbon content of titanium alloy base material is no more than 0.1%.There are a lot of elements in titanium alloy, they have an impact on the physical properties of titanium, carbon is titanium and titanium alloy in the common impurities, when the carbon content is below 0.13%, carbon deep in alpha titanium, weld strength limit some increase, some plastic decline, but not as strong as the role of oxygen nitrogen.However, when the carbon content in the weld was further increased, the number of reticular TiC in the weld increased with the increase of carbon content, which caused a sharp decline in the weld plasticity and led to cracks under the action of welding stress.

Titanium pipe weld defect is due to titanium pipe welding, argon gas maintenance layer formed by the argon arc welding gun can only maintain good welding molten pool from the harmful effects of air, and to have frozen in a state of high temperature near the weld and its nearby area, no protection, and is in this state of titanium pipe near the weld and its area still has a strong ability to absorb nitrogen and oxygen in the air.Oxygen is absorbed at 400℃, nitrogen is absorbed at 600℃, and air contains a lot of nitrogen and oxygen.With the increase of oxidation level, the weld color of titanium tube changed and the weld plasticity decreased.Silver (no oxidation) gold (TiO, titanium begins to absorb hydrogen around 250℃.Slight oxidation) blue (Ti2O3 oxidation is slightly serious) gray (TiO2 oxidation is serious).

1.The influence of the carbon

Titanium and titanium alloys are relatively stable but in the welding process, at room temperature.Liquid droplet and pool metals strongly absorb hydrogen, oxygen, and nitrogen, and in the solid state, these gases interact with them.With the increase of temperature, the ability of titanium and titanium alloy to absorb hydrogen, oxygen and nitrogen also increased significantly. Titanium began to absorb hydrogen around 250℃, oxygen from 400℃ and nitrogen from 600℃. After these gases were absorbed, they would directly cause embrittlement of welding joints, which was an extremely important factor affecting welding quality.

2.The effect of hydrogen

The main reason is that with the increase of hydrogen bomb content in the gap, hydrogen is the most serious factor affecting the mechanical properties of titanium.The change of hydrogen content in weld has the most significant influence on the impact property of weld.The number of flake or acicular TiH2 precipitated from weld increased.The strength of TiH2 is very low, so the notch of HiH2 in sheet or needle shape can significantly reduce the impact performance.The effect of hydrogen content change on strength and plasticity is not very obvious.

3.The influence of oxygen

The hardness and tensile strength of the weld increased obviously, and the oxygen content of the weld basically increased linearly with the increase of oxygen content in argon.The plasticity is significantly reduced.In order to ensure the performance of the welded joint, it is necessary to prevent oxidation of welding seam and welding heat affected zone.

4.The effect of nitrogen

Nitrogen and titanium plate play an important role in the high temperature above 700℃.The formation of brittle and hard titanium nitridation (TiN and the degree of lattice deflection caused by the formation of gap solid solution between nitrogen and titanium is more serious than the result caused by the same amount of oxygen), therefore, nitrogen is more significant than oxygen in improving the tensile strength and hardness of industrial pure titanium weld and reducing the plasticity of weld.When the nitrogen content of the weld is above 0.13%, the weld will crack due to over-brittleness.

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