Titanium is a very active metal with chemical properties.
It has great affinity for oxygen, hydrogen, nitrogen and other gases at high temperatures.
Especially in the titanium welding process, this ability is stronger with the increase of welding temperature.
The practice has proved that if the absorption and dissolution of titanium and oxygen, hydrogen, nitrogen and other gases are not controlled during welding, it will undoubtedly bring great difficulties to the welding process of titanium welded joints.
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Titanium welding is a common kind of welding.
In the process of titanium welding, how to control the welding quality has a very important influence on the weld color of titanium welding.
Because of the intuitiveness of titanium weld color, it is of great significance to study the relationship between titanium weld color and welding quality.
In this article, we discuss the relationship between the welding quality of titanium welding and the color of titanium welding seam based on his years of research on titanium welding quality control and technology as well as practical work experience, hoping to play a role in the research in this field.
2. Effect of titanium characteristics on titanium welding
1. Effect of oxygen and nitrogen
The interstitial solid melting of oxygen and nitrogen in titanium will distort the titanium lattice, increase the deformation resistance, increase the strength and hardness, and reduce the plasticity and toughness.
The welding oxygen and nitrogen contained in the weld is unfavorable and should be avoided.
2. Effect of hydrogen
The increase of hydrogen will make the impact toughness of titanium weld metal decrease sharply, while the plasticity decreases slightly, and hydride will cause the brittleness of the joint.
3. Impact of carbon
At room temperature, carbon is dissolved in titanium in the form of gaps, which increases the strength and decreases the plasticity, but is not as obvious as oxygen and nitrogen.
When the carbon content exceeds the solubility, hard and brittle TiC is generated, which is distributed in a network, and is easy to crack.
According to the national standard, the carbon content in titanium alloy shall not exceed 0.1%.
During welding, the oil contamination of the workpiece and welding wire can increase the carbon content, so it is necessary to clean up during welding.
3. Analysis of titanium weldability
Titanium has good weldability.
Due to its small thermal conductivity (0.041Cal/℃ · cm · s), titanium metal only melts in the arc burning range and has good fluidity;
Furthermore, it has a small coefficient of thermal expansion (8.6 × 10-6/℃, much less than carbon steel), which greatly improves the weldability of titanium metal.
4. Relationship between weld color and welding quality of titanium welding
1. Color change and defect generation mechanism of titanium and titanium alloy titanium pipe welds
The weld defects of titanium and titanium alloy titanium pipes and their generation mechanisms are as follows.
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During titanium pipe welding, the argon gas maintenance layer formed by the argon arc welding gun can only protect the welding pool from the harmful effects of air, while it has no protective effect on the solidified weld and its surrounding areas near the high-temperature state.
However, the titanium pipe weld and its surrounding areas in this state still have a strong ability to absorb nitrogen and oxygen in the air.
It starts to absorb oxygen at 400 ℃ and nitrogen at 600 ℃, while air species contain a lot of nitrogen and oxygen.
With the increasing of oxidation level, the weld color of titanium tube changes and the weld plasticity decreases.
- Silver white (no oxidation);
- Golden yellow (TiO, titanium begins to absorb hydrogen at about 250 ℃. It is slightly oxidized);
- Blue (Ti2O3 oxidation is slightly serious);
- Gray (TiO2 oxidation is serious).
2. The quality of titanium welding can be judged by the color of titanium weld surface
Tests for different colors and hardness of titanium welds are shown in the figure below
（1） It is proved by experiments that the hardness of the weld increases with the deepening of the weld color, that is, the oxidation degree of the weld increases.
Through the test of the same trade, the hardness of titanium metal increases, and harmful substances such as oxygen and nitrogen in the weld increase, which greatly reduces the welding quality.
（2） The weldability of titanium has a very important relationship with its chemical and physical properties, but the key is that, at high temperatures, titanium’s high activity is vulnerable to air pollution.
When heated, its grains swell, and when the welded joint is cooled, it will form a brittle phase.
The melting point of titanium is very high, reaching 1668 ± 10 ℃, which is more energy than that required for welding steel.
At the same time, titanium is more active in chemistry, and its interaction with O and H is much easier than that of steel. It will be rapidly combined above 600 ℃.
At 100 ℃, a large amount of H and O are absorbed, and the ability to dissolve H is tens of thousands of times greater than that of steel, thus generating titanium hydride, which sharply reduces the toughness.
Gas impurities increase the tendency of cold cracking and delayed cracking, and increase notch sensitivity.
Therefore, the purity of argon for welding shall not be lower than 99.99%, the humidity shall not be higher than 0.039%, and the hydrogen content of welding wire shall be below 0.002%.
The heat transfer coefficient of titanium is 1/2 of that of steel.
The transformation from α to β occurs at 882 ℃.
The higher the temperature is, the faster the b grain will grow, and the performance will deteriorate significantly.
Therefore, the temperature should be strictly controlled, especially the high temperature residence time in the welding thermal cycle.
There are no hot cracks and intergranular cracks when welding titanium, but there are porosity problems, especially when welding α+β alloy.
5. Precautions for titanium welding
Based on the above research, the following issues should be paid attention to when welding titanium metal:
1. During titanium welding, the welding area and high temperature area after welding shall be strictly protected to prevent the air from entering the welding area and the high temperature area from seriously affecting the weld quality, so 99.99% pure argon and trailing protective cover are necessary.
2. The weld groove shall be machined (no grinding method is allowed);
3. Spot welding shall be avoided and high-frequency arc starting shall be adopted.
4. Avoid post weld heat treatment;
If post weld heat treatment is necessary, the heat treatment temperature shall be less than 650 ℃.
The quality control of titanium welding has a very important influence on the weld color of titanium welding.
At the same time, the quality of titanium welding can also be judged according to the weld color of titanium welding.
There is a very important relationship between the two.