Material characteristics of titanium alloys and welding

Material characteristics ( a ) titanium : density ( about 4.5g/cm3), high tensile strength (441 ~ 1470MPa), that is the great strength ratio ( strength / density ) large . In sea water and most acids, alkalis, salts medium all have excellent corrosion sunrui Titanium EB Slab resistance compared .

Weldability (b ) a titanium alloy , having a number of significant features, which are due to the physical and chemical characteristics of welding titanium is determined .

1 impurity contamination caused by gas embrittlement of welded joints

At room temperature , titanium and titanium alloys are relatively stable. However, in the welding process , the liquid metal droplet and the pool has a strong absorption effect of hydrogen, oxygen, nitrogen , and in the solid state , with these gases have a role. With increasing temperature , titanium and titanium alloys absorb hydrogen , oxygen, nitrogen capacity also will rise significantly , about 250 ℃ titanium began to absorb hydrogen , began to absorb oxygen from 400 ℃ , 600 ℃ began to absorb nitrogen from these after the gas is absorbed directly will cause embrittlement of welded joint is extremely important factor affecting the quality of welding .

( A ) Effect of hydrogen is

Impurities in the hydrogen gas to affect the mechanical properties of titanium most serious factor. The impact of changes in hydrogen content in the weld performance the most significant effect on the weld , mainly due to the increase of the hydrogen bomb with the seam with the amount deposited weld flake or needle TiH2 increased. TiH2 low intensity , the effect of the sheet-like or needle guard according to HiH2 notch impact sunrui Titanium Welded Tube properties significantly reduce aggregate ; weld hydrogen to reduce the amount of change effect of improving the strength and the ductility is not very significant .

( 2) The effect of oxygen

With the oxygen content of the weld is substantially increase the oxygen content in the argon gas increases linearly with the increase of the oxygen content of the weld , the weld hardness and tensile strength significantly increased , and the plasticity is significantly reduced. In order to ensure the performance of welded joints , the welding process should prevent the weld and HAZ fat oxidation.

( 3 ) the impact of nitrogen

At temperatures above 700 ℃ , plays the role of nitrogen and the titanium , titanium nitride is formed a hard brittle (TiN) is formed of titanium and nitrogen and the slip degree of lattice distortion caused by solid solution of the gap , the consequences caused by the same amount of oxygen than more seriously , so nitrogen to improve industrial pure titanium weld tensile strength , hardness, ductility properties of the weld reduced more significant than that of oxygen . When the nitrogen content in the weld than 0.13% , the weld due to the brittle and cracks .

( 4 ) the impact of carbon

Carbon is also common in titanium and titanium impurities, experiments showed that when the carbon content of 0.13 % or less, because deep in the α carbon titanium weld strength limit somewhat improved plasticity some decline, but not as strong role of oxygen and nitrogen . But when further increase the carbon content of the weld , the weld has appeared mesh TiC, their number increased with the carbon content and increase the weld plastic sharp decline in the welding prone to stress cracking . When the carbon content of 0.55% weld , the weld disappearance of almost all the plastic and the material becomes very brittle . sunrui titanium sponge Weld heat treatment can not eliminate this brittleness. Of technical conditions , the carbon content of the titanium alloy of the base material is not more than 0.1% , the carbon content of the weld does not exceed the base material carbon content.

2 titanium weld defects and Mechanism .

( 1 ) Color oxide

Around about 250 ℃ titanium begins to absorb hydrogen and oxygen to be absorbed from 400 ℃ , 600 ℃ starts from the absorption of nitrogen , and the air contains large amounts of nitrogen and oxygen species . Titanium during welding , argon gas protection layer is formed by welding guns can better protect the weld pool from the harmful effects of air , which has been solidified in the vicinity of the weld and no high-temperature state near protection, while the area around the weld titanium tube and in this state is still strong absorption of nitrogen and oxygen in the air capability . Gradually increased with the degree of oxidation , and changes in the laws of titanium weld seam plastic drop colors occur. Silver-white ( no oxidation ) , golden yellow (TiO, slightly oxidized ) , blue (Ti2O3, oxidation slightly serious ) , Sunrui titanium coils gray (TiO2, oxidation severe ) .