The protection requirements of titanium and titanium alloy welding are very strict. When the carbon content of the welding seam is 0.55%, the plasticity of the welding seam will almost disappear and become a very brittle material. Moreover, the thermal treatment after welding cannot eliminate this brittleness.

There are many elements in titanium alloy, which have an impact on the physical properties of titanium. Among them, carbon is a common impurity in titanium and titanium alloy. When the carbon content is below 0.13%, the strength limit of welding seam in titanium is somewhat increased and the plasticity is somewhat decreased, but the effect of inoxic nitrogen is strong. However, when the carbon content of the welding seam is further increased, the number of reticular TiC in the welding seam increases with the increase of carbon content, resulting in a sharp decline in the plasticity of the welding seam and the occurrence of cracks under the action of welding stress.

The weld defect of titanium pipe is due to the fact that the argon gas maintenance layer formed by argon arc welding gun can only keep the welding pool safe from harmful effects of air during the welding of titanium pipe.

However, there is no protective effect on the solidified weld and its adjacent areas under high temperature, and the weld and its adjacent areas under such a state still have a strong ability to absorb nitrogen and oxygen from the air.

There's a lot of nitrogen and oxygen in the air when it starts to absorb oxygen at 400 degrees and nitrogen at 600 degrees. With the increase of oxidation level, the color of titanium tube weld changed and the plasticity of weld decreased.

It shows silver-white with no oxidation, while in golden yellow when TiO titanium begins to absorb hydrogen around 250℃, which is slightly oxidized. And it is blue with slightly oxidized Ti2O3 and gray with seriously oxidized TiO2.

1. The impact of carbon

Titanium and titanium alloys are relatively stable but in the welding process under room temperature. Liquid droplet and pool metal have a strong absorption of hydrogen, oxygen and nitrogen. These gases have already interacted with titanium alloys in the solid state.

With the increase of temperature, the ability of titanium and titanium alloy to absorb hydrogen, oxygen and nitrogen also increases obviously. Titanium starts to absorb hydrogen around 250℃ , absorb oxygen from 400℃ and nitrogen from 600℃. After these gases are absorbed, they will directly cause embrittlement of welded joints, which is a very important factor affecting welding quality.

2. Hydrogen is the influence

The main reason is that with the increase of hydrogen content, hydrogen is the most serious factor affecting the mechanical properties of titanium in gaseous impurities. The change of hydrogen content in welding has the most significant effect on the impact performance of weld.

The precipitation of lamellar or acicular TiH2 in the welds increased. The strength of TiH2 is very low, so the impact performance of laminar or acicular HiH2 is significantly reduced The influence of hydrogen content on strength and plasticity is not obvious.

3. The effect of oxygen

The hardness and tensile strength of welding seams increased significantly, and the oxygen content of welding seams basically increased linearly with the increase of oxygen content in argon.

And plasticity is significantly reduced. In order to ensure the performance of welding joints, oxidation of welding seam and welding heat should be strictly prevented in the welding process.

4. Effects of nitrogen

Nitrogen and titanium plates interact dramatically at temperatures above 700℃. The lattice deflection caused by the formation of brittle TiN and the formation of interstitial solid solution between nitrogen and titanium is more serious than that caused by the same amount of oxygen.

Therefore, nitrogen is more significant than oxygen to improve the tensile strength and hardness of industrial pure titanium weld and reduce the plastic property of weld. When the nitrogen content of the welding seam is above 0.13%, the welding seam will crack due to too brittle.