Titanium is a metal that is chemically very active. It has a great affinity for gases such as oxygen, hydrogen, and nitrogen at high temperatures, especially during titanium welding.
In practice, if the absorption and dissolution of titanium with gases such as oxygen, hydrogen, and nitrogen are not controlled during welding, it will undoubtedly bring great difficulties to the welding process of titanium joints.
In recent years, with the development of the economy, especially with the continuous deepening of reform and opening up, China’s economic construction has made great progress.
At the same time, China has also made significant progress in welding engineering, such as pipeline welding. Titanium welding is a common type of welding in our country.
How to ensure good quality control in titanium welding, and its impact on the color of welds is of great significance.
Due to the intuitiveness of the color of titanium welds, the study of the relationship between the color of titanium welds and welding quality is important.
In this article, we combine our research on the quality control and process of titanium welding over the years, as well as practical work experience, to explore the relationship between the welding quality of titanium welding and the color of titanium welds.
We hope that this study plays a certain role in this field.
2. Impact of Titanium Characteristics on Titanium Welding
1. The Influence of Oxygen and Nitrogen
Oxygen and nitrogen solidify in gaps in titanium, causing the distortion of the titanium lattice. This leads to an increase in deformation resistance, strength, and hardness.
However, plasticity and toughness decrease.
The presence of oxygen and nitrogen in the weld seam is detrimental, and efforts should be made to avoid it.
2. The Influence of Hydrogen
Increasing hydrogen can cause a sharp drop in the impact toughness of the weld metal in titanium, while decreasing its ductility.
Hydrogen can also cause joint brittleness due to the formation of hydrogen compounds.
3. The Influence of Carbon
At room temperature, carbon is dissolved in the form of gaps in titanium, leading to an increase in strength and a decrease in ductility.
However, the effect of carbon is not as pronounced as that of oxygen and nitrogen. When the carbon content exceeds its solubility limit, it creates a hard and brittle substance called TiC, which is distributed in a mesh-like pattern and can easily lead to cracking.
The national standard stipulates that the carbon content in titanium and titanium alloys cannot exceed 0.1%.
During welding, oil contamination on the workpiece and wire can increase carbon content, so it is important to clean them thoroughly.
3. Analysis of Titanium Weldability
Titanium has good weldability due to its low thermal conductivity (0.041Cal/℃·cm·s). As a result, titanium metal only melts within the range of the arc burning and has good fluidity.
Additionally, it has a small coefficient of thermal expansion (8.6×10-6/℃, far smaller than that of carbon steel), which greatly improves the weldability of titanium metal.
4. The Relationship between the Color of Titanium Welds and Welding Quality
1. Color Changes in Titanium Tube Welds and Defects Produced
During the welding of titanium tubes, the argon gas used for welding only forms a protective layer that prevents harmful air from affecting the welding pool.
However, it does not protect the already solidified welds and their nearby areas that are still close to high temperatures. The welds and areas nearby that are in this state still have a strong ability to absorb nitrogen and oxygen from surrounding air.
Titanium can start absorbing oxygen from 400ºC, and nitrogen from 600ºC. Since the air contains a large amount of nitrogen and oxygen, the oxide level gradually increases, leading to changes in the color of titanium welds and a decrease in their ductility.
The sequence of colors as the oxide level increases is as follows: silver-white (no oxidation), gold (TiO, slight oxidation at around 250ºC), blue (Ti2O3, slight oxidation), gray (TiO2, severe oxidation).
2. The Quality of Titanium Welding Can Be Judged by the Color on the Surface of the Titanium Welds
(1) Through practical tests, it has been proven that as the color of the weld deepens, i.e., the degree of oxidation of the weld increases, the hardness of the weld also increases.
Peer tests have shown that an increase in harmful substances such as oxygen and nitrogen in the weld, greatly reduces the welding quality and increases the hardness of the titanium metal.
(2) The weldability of titanium is closely related to its chemical and physical properties.
The key issue is that the high reactivity of titanium makes it vulnerable to air contamination at high temperatures. During heating, the grains of titanium swell, and when the joint cools, the weld becomes brittle due to the formation of a brittle phase.
Titanium has a high melting point, reaching 1668±10℃, requiring more energy than welding steel. At the same time, titanium is more reactive than steel, and its reaction with O and H is much easier, quickly combining above 600℃.
At 100℃, it absorbs a large amount of H and O, with H solubility being tens of thousands of times larger than that of steel. This leads to the formation of titanium hydride, causing a rapid decrease in toughness. The increase in gas impurities increases the tendency for cold cracks and delayed cracks, and increases notch sensitivity.
Therefore, the purity of argon gas used for welding should not be lower than 99.99%, with humidity not higher than 0.039%, and the hydrogen content of the welding wire should not exceed 0.002%.
The heat transfer coefficient of titanium is half that of steel. At 882℃, α to β transformation occurs, and at higher temperatures, β grains grow abruptly, leading to a significant decline in performance.
Therefore, temperature control must be strictly enforced, especially to control the high-temperature holding time in the welding heat cycle.
Although there are no hot cracks or intergranular cracks during welding of titanium, there is still a problem of gas pores, especially when welding α+β alloys.
5. Precautions for Titanium Welding
Based on the above research, the following issues should be noted when welding titanium:
1. Strict protection should be provided for the welding area and the high-temperature area after welding, to prevent air from entering and affecting the quality of the weld. Therefore, 99.99% pure argon and a trailing shield are necessary.
2. The weld groove should be machined mechanically (grinding should not be used).
3. Spot welding should be avoided and high-frequency arc ignition should be used.
4. Post-weld heat treatment should be avoided. If post-weld heat treatment is necessary, the temperature should be less than 650℃.
The quality control of titanium welding has a significant impact on the color of titanium welds.
Additionally, the quality of the titanium welding can be judged based on the color of the titanium welds. The relationship between the two is very important.