When welding titanium pipes and titanium alloys, when the weld contains oxygen and the nitrogen content is relatively high, the weld or heat-affected zone will become brittle. At about 250℃，titanium atarts to absorb hydrogen, and from 400℃，it starts to absorb oxygen, and from 600℃， it starts to absorb nitrogen. The air contains larger nitrogen and oxygen, and the cracks will appear under larger welding stress. Its characteristic is that cracks occur several hours or even longer after welding and we call this delayed cracks. Studied have shown that the diffusion of hydrogen during welding is the main cause of this kind of cracks. During the welding process, hydrogen diffuses from the high temperature deep pool to the lower temperature heat affected zone. With the increase of hydrogen content, the amount of TiH2 precipitated in the zone increases, which increases the brittleness of the heat affected zone. In addition, due to the larger structural stress caused by the volume expansion of hydride precipitation, and the diffusion and accumulation of hydrogen atoms to the high stress part of the zone, cracks are formed. The main way to prevent this kind of delayed crack is to reduce the source of hydrogen in welded joint.

When titanium pipe is welded, air hole is a main problem. The fundamental reason for the formation of pores is due to the effect of hydrogen. The technological measures to prevent blowholes are as follows:

1) When welding under the protection of high purity argon, the purity of argon shall not be less than 99.99%

2) thoroughly remove organic matters such as oxide skin and oil stains on the surface of titanium tube, titanium plate and titanium plate tube eye. It can be cleaned by chemical and mechanical methods.

3) apply good gas protection to the molten pool, control the flow rate and flow rate of argon to prevent turbulence and affect the protection effect.

4) choosing suitable welding process parameters and welding specifications, increasing the residence time of deep pool to facilitate the escape of bubbles can effectively reduce the porosity.

In the welding of titanium alloy, the argon gas protective layer formed by argon arc welding gun can only protect the welding pool from the harmful effect of air, but has no protective effect on the solidified weld and its adjacent area at high temperature, and the weld and its adjacent area at this state still have strong ability to absorb nitrogen and oxygen in the air. With the increase of oxidation degree, the color of weld metal changes and the plasticity of weld metal decreases. Silver white (no oxidation), golden yellow (TiO, slight oxidation), blue (Ti2O3, slightly serious oxidation), gray (TiO2, serious oxidation). When titanium alloy is welded, the possibility of hot crack is very small. This is because there are few impurities such as s, P and C in titanium and titanium alloy. The eutectic with low melting point formed by s and P is seldom formed at the grain boundary. In addition, the effective crystallization temperature range is narrow, and the shrinkage of titanium and titanium alloy is small during solidification, so the weld metal will not have hot crack.