Welding requirements for titanium equipment such as titanium heat exchanger and titanium coil

The welding performance of titanium coils and heat exchangers has many remarkable characteristics, which are determined by the physical and chemical properties of titanium and titanium alloys.

1, the impact of gas and impurity pollution on welding performance
At room temperature, titanium and titanium alloys are relatively stable. But the test results show that in the welding process, the liquid droplet and molten pool metal have strong absorption of hydrogen, oxygen and nitrogen, and in the solid state, these gases have acted with them. With the increase of temperature, titanium and titanium alloy absorption hydrogen, oxygen, nitrogen ability also increased significantly, about 250℃ began to absorb hydrogen, from 400℃ began to absorb oxygen, from 600℃ began to absorb nitrogen, these gases are absorbed, will directly cause the welding joint embrittlement, is an extremely important factor affecting the quality of welding.

2, welding joint crack problem
When titanium and titanium alloy are welded, the possibility of hot cracking in the welded joint is very small, because the impurity content of S, P, C and other impurities in titanium and titanium alloy is very small, the low melting point eutectic formed by S and P is not easy to appear on the grain boundary, and the effective crystallization temperature range is narrow, the shrinkage amount of titanium and titanium alloy is small, and the weld metal will not produce hot cracking. When titanium and titanium alloys are welded, cold cracks can occur in the heat-affected zone, which is characterized by cracks occurring several hours or even longer after welding, so it is also called delayed cracks. The research shows that the crack is related to the diffusion of hydrogen in the welding process. During the welding process, hydrogen diffused from the high-temperature deep pool to the lower-temperature HAZ, and the increase of hydrogen content increased the amount of TiH2 precipitated in the area, which increased the brittleness of the HAZ. In addition, the volume expansion of hydride precipitated caused greater structural stress, and hydrogen atoms diffused and accumulated to the high-stress parts of the area, resulting in the formation of cracks. The main way to prevent this delayed crack is to reduce the source of hydrogen in the welded joint.

3, the porosity in the weld
When welding titanium and titanium alloys, porosity is a common problem. The root cause of the formation of stomata is the result of hydrogen influence. The porosity of weld metal mainly affects the fatigue strength of joint. The main technological measures to prevent the formation of pores are:

1) The protection gas should be pure, and the purity should not be less than 99.99%

2) Thoroughly remove the surface of the weldment, the surface of the welding wire oxide oil and other organic matter.

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

4) Correct selection of welding process parameters to increase the residence time of the molten pool to facilitate the escape of bubbles.