How to prevent titanium tube welding defects

- Jan 15, 2019-

How to prevent titanium tube welding defects

Titanium tube weld defects are caused by the argon gas maintenance layer formed by the argon arc welding torch, which can only maintain the weld pool without the harmful effects of air, and welds that have been solidified and are in a high temperature state. The vicinity of the area has no protective effect, and the titanium tube weld in this state and its vicinity still have a strong ability to absorb nitrogen and oxygen in the air.

Porosity is a major problem often encountered when welding titanium tubes. The root cause of the formation of pores is the result of hydrogen influence. The main measures to prevent the generation of pores are:

1. Select the appropriate process parameters and welding specifications for welding, increase the residence time of the deep pool to facilitate the escape of bubbles, and effectively reduce the pores.

2. Welding under high purity argon protection, the purity of argon should not be lower than 99.99%

3. Thoroughly remove organic matter such as scale oil on the surface of titanium tube, titanium plate and titanium plate tube. It can be cleaned chemically and mechanically.

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

When welding titanium alloy, the argon gas protective layer formed by the argon arc torch can only protect the weld pool from the harmful effects of air, but not on the welded joint near the high temperature state and its vicinity. The protective effect, while the titanium tube weld in this state and its vicinity still has a strong ability to absorb nitrogen and oxygen in the air. As the degree of oxidation gradually increases, the color of the weld of titanium alloy changes and the plasticity of the weld decreases. Silver white (no oxidation), golden yellow (TiO, slightly oxidized), blue (Ti2O3, slightly oxidized), gray (TiO2, severe oxidation).

When titanium alloy is welded, there is little possibility of hot cracking in the welded joint. This is because the content of S, P, C and other impurities in titanium and titanium alloy is very small, and the low-melting eutectic formed by S and P is rarely in the grain boundary. The formation and the effective crystallization temperature range are narrow, and the shrinkage amount of the titanium and the titanium alloy when solidified is small, and the weld metal does not generate hot cracks.

When titanium pipe and titanium alloy pipe are welded, when the weld contains oxygen and nitrogen, the ability of titanium and its alloy to absorb hydrogen, oxygen and nitrogen gradually increases with the increase of temperature. Titanium absorbs hydrogen from 250 ° C, absorbs oxygen from 400 ° C, and starts to absorb nitrogen from 600 ° C. Due to the high affinity of titanium alloys with O2, N2 and H2, the inclusion of these gases in the joints makes the joints brittle and reduces the impact properties, ductility and toughness of titanium alloy welded joints.

When titanium is contained in the titanium alloy, delayed cracks are generated in the heat affected zone. When the oxygen content and the nitrogen content in the weld are high, the weld or heat affected zone may also crack under the action of large welding stress, and the crack is also a delayed crack. Therefore, inert gas (or vacuum chamber) protection is very necessary.

Since the use of a vacuum chamber is expensive, an inert gas protection method is generally used. The shielding gas mainly has helium gas and argon gas. Since the price of helium gas is higher than that of argon gas, in general, the welding joint and the heat-affected zone of the titanium alloy which are not particularly required can be prevented by using high-purity argon gas for