At room temperature, titanium and titanium alloys are relatively stable. Titanium and titanium alloys have many advantages as structural materials, such as small specific gravity, high tensile strength and high yield strength. They still have high enough strength at 300 ~ 500 ℃. And most of the acid and alkali salt media have excellent corrosion resistance, and are increasingly widely used in aviation, chemical industry, and nuclear industry. But as the temperature rises, the ability of titanium and its alloys to absorb hydrogen, oxygen, and nitrogen gradually increases. Titanium starts absorbing hydrogen from 250 ℃, oxygen from 400 ℃, and nitrogen from 600 ℃. Due to the high affinity of titanium alloys with O2, N2 and H2, when these gases are contained in the joints, the joints will become brittle and reduce the impact performance, plasticity and toughness of the titanium alloy welded joints. Titanium alloys containing hydrogen can cause delayed cracking in the heat-affected zone. However, when the amount of oxygen and nitrogen in the weld is high, the weld or heat-affected zone will also crack under the action of a large welding stress, and this crack is also a delayed crack. Therefore, inert gas (or vacuum chamber) protection is very necessary. Due to the high cost of using the vacuum chamber, the method of inert gas protection is generally used. Shielding gas mainly includes helium and argon. Since helium is more expensive than argon, generally speaking, high-purity argon is used to protect the welding joints and heat affected areas of titanium alloys that are not specially required to prevent oxidation.
The generation of pores in titanium welded joints is mainly affected by the cleanliness before welding, the preparation of welded joints and the welding time, as well as other factors. Although hydrogen absorption caused by unclean surfaces is the main reason for the generation of pores in arc welding joints, other factors such as oxygen, nitrogen, carbon dioxide, and inert gas used for protection may cause the formation of pores. Like aluminum oxide, titanium oxide is hygroscopic and easily absorbs moisture from the ambient atmosphere. When a titanium part is welded to a joint with water (or water vapor), gaseous hydrogen will dissolve into the joint, and then a small hole will be formed during solidification. The measures to reduce the porosity in titanium welded joints are:

  • (1) Titanium parts and solder should be dried.
  • (2) It is recommended to use the prepared solder within 48h.
  • (3) The parts of the titanium parts to be soldered and the solder are greased and cleaned, and then pickled.
  • (4) Use high-purity argon or helium for protection. The selection of welding consumables generally requires that the composition of the welding consumables matches the titanium material to be welded, but in order to improve the performance of the joint, welding consumables with a yield strength lower than that of the base metal may sometimes be used. The composition and performance of industrial pure titanium welding wire are specified in AWS A5.16, and the composition and performance of welding wire specified in the national standard are consistent with AWS. In the case of ensuring strict cleaning of the area to be welded and good shielding gas, use the specifications in Table 3 for welding. The welding material is selected from the TA2 (AWS for ERTi-2) welding wire with the base metal and other components, and the interlayer temperature control is ≤100 ℃ , Clean the weld surface between layers. Judgment of welding seam protection effect: silver white is the best protection; light yellow is slight oxidation; blue means serious oxidation; gray is very poor, and it is treated as unqualified.
    The defect of titanium tube welding seam is due to the maintenance of the argon gas maintenance layer formed by the argon arc welding gun can only maintain the welding pool from the harmful effects of air when welding the titanium tube. The area around it has no protection, and the titanium tube weld in this state and the area around it still have a strong ability to absorb nitrogen and oxygen in the air. Oxygen absorption starts at 400 ° C, and nitrogen absorption starts at 600 ° C, and air species contain large amounts of nitrogen and oxygen. With the gradual increase of the oxidation level, the color of the weld of the titanium tube changes and the plasticity of the weld decreases. Silvery white (no oxidation) Golden yellow (TiO, titanium begins to absorb hydrogen at about 250 ° C. Slightly oxidized) Blue (Ti2O3 oxidation is slightly severe) Gray (TiO2) oxidation is severe.
    1. The influence of carbon. Titanium and titanium alloys are relatively stable, but in the welding process, at room temperature. Liquid droplets and molten pool metals have a strong effect of absorbing hydrogen, oxygen, and nitrogen, and in the solid state, these gases have interacted with them. As the temperature rises, the ability of titanium and titanium alloys to absorb hydrogen, oxygen, and nitrogen also increases significantly. Titanium begins to absorb hydrogen at about 250 ° C, absorb oxygen from 400 ° C, and absorb nitrogen from 600 ° C. These After the gas is absorbed, it will directly cause the embrittlement of the welded joint, which is an extremely important factor affecting the welding quality.
    2. The influence of nitrogen. Nitrogen and titanium plates have a dramatic effect at high temperatures above 700 ° C. The formation of brittle and hard titanium nitride (TiN and the formation of interstitial solid solution between nitrogen and titanium is more serious than the equivalent amount of oxygen. Therefore, nitrogen improves the resistance of industrial pure titanium welds Tensile strength, hardness, and plasticity of the weld are more significant than oxygen. When the nitrogen content of the weld is more than 0.13%, the weld is cracked due to excessive brittleness.
    3. Hydrogen is an influence. The main reason is that the amount of hydrogen-containing bombs increases with the slit. Hydrogen is the most serious factor affecting the mechanical properties of titanium among gas impurities. The change of hydrogen content in the weld seam has the most significant effect on the weld impact performance. The plate-like or needle-like TiH2 precipitated in the weld increased. The strength of TiH2 is very low, so the effect of sheet or needle-shaped HiH2 is notched, and the combined impact performance is significantly reduced; the effect of changes in the hydrogen content of the weld on strength and plasticity is not very obvious.
    4. The influence of oxygen. The hardness and tensile strength of the weld are significantly increased. The oxygen content of the weld basically increases linearly with the increase of the oxygen content in the argon gas and increases with the oxygen content of the weld. The plasticity is significantly reduced. In order to guarantee the performance of the welded joint, the welding seam and the welding heat affected zone should be strictly prevented from being oxidized during the welding process.