Titanium is a highly reactive metal with unique chemical properties. Exhibits a strong affinity for oxygen, hydrogen, nitrogen and other gases, especially at high temperatures.
This property is particularly pronounced during the titanium welding process, where the strength of this ability increases with temperature.
Experience has shown that if the absorption and dissolution of gases such as oxygen, hydrogen, nitrogen and others are not controlled during welding, this can pose significant challenges to the titanium joint welding process.
Related Reading: 78 Welding Basics
1. Preface
Titanium welding is a frequently used welding method. Welding quality control in the titanium welding process has a significant impact on the color of the resulting weld.
Due to the visual nature of titanium weld color, the study of the correlation between titanium welding seam color and welding quality is highly significant.
In this article, we aim to explore the relationship between titanium welding quality and titanium welding seam color based on our extensive research on quality control and titanium welding technology, as well as practical experience in the field. We hope that this article can contribute to future research in this area.
2. Effect of titanium characteristics on titanium welding
1. Effect of oxygen and nitrogen
The interstitial fusion of solid oxygen and nitrogen within titanium can distort its lattice structure, resulting in increased creep resistance, strength and hardness, while decreasing its plasticity and toughness.
It is recommended to avoid the presence of oxygen and nitrogen during welding, as they can have unfavorable effects.
2. Effect of hydrogen
Adding hydrogen to titanium weld metal will significantly decrease its impact resistance while also slightly reducing its plasticity. Furthermore, the presence of hydride can cause brittleness in the joint.
3. Carbon impact
At room temperature, carbon dissolves in titanium in the form of vacancies, which increases strength and decreases plasticity, but its effect is not as pronounced as that of oxygen and nitrogen.
If the carbon content exceeds its solubility, hard and brittle TiC forms and is distributed in a network, which makes it prone to cracking.
The national standard specifies that the carbon content in titanium alloy should not exceed 0.1%.
During welding, oil contamination on the workpiece and welding wire can increase the carbon content. Therefore, it is essential to clean surfaces before welding.
3. Titanium weldability analysis
Titanium is known for its excellent weldability. Its low thermal conductivity (0.041 Cal/℃ · cm · s) allows it to melt only within the arc's burning range and present good fluidity.
Furthermore, with a small coefficient of thermal expansion (8.6 × 10-6/℃, much lower than that of carbon steel), the weldability of titanium metal is greatly improved.
4. Relationship between weld color and titanium welding quality
1. Mechanisms of color change and defect generation in welded titanium and titanium alloy tubes.
The welding defects of titanium tubes and titanium alloys and their generation mechanisms are as follows.
Related Reading: Types of Welding Defects
When welding titanium tubes, the argon arc welding gun creates a holding layer of argon gas that only protects the welding puddle from the harmful effects of air. However, this layer does not provide any protection to the solidified weld or adjacent areas that are close to the high temperature state.
However, the solder on the titanium tube and surrounding areas in this state still has a strong ability to absorb nitrogen and oxygen from the air. This absorption process begins at 400℃ for oxygen and 600℃ for nitrogen, as air mainly consists of nitrogen and oxygen.
As the oxidation level increases, the color of the titanium tube weld changes and the plasticity of the weld decreases.
- Silver white (no oxidation);
- Golden yellow (TiO, titanium starts to absorb hydrogen at about 250 ℃. It is slightly oxidized);
- Blue (Ti2O 3 oxidation is slightly severe);
- Gray (TiO 2 oxidation is serious).
2. The quality of titanium welding can be judged by the surface color of the titanium weld
The figure below shows tests for different colors and hardnesses of titanium welds.
(1) Experiments have shown that the hardness of the weld increases as the color of the weld deepens, indicating an increase in the oxidation degree of the weld. In the case of the same trade, the hardness of titanium metal also increases, but harmful substances such as oxygen and nitrogen in welding tend to increase as well, leading to a significant reduction in welding quality.
(2) The weldability of titanium is closely related to its chemical and physical properties, the key factor being its vulnerability to atmospheric pollution at high temperatures due to its high activity. Titanium grains swell when heated, and after cooling, the welded joint may form a brittle phase.
Titanium has a very high melting point of 1668±10℃, which requires more energy than steel for welding. Furthermore, titanium is more chemically active and interacts more easily with oxygen and hydrogen than steel, which results in rapid combination at temperatures above 600℃.
At a temperature of 100 ℃, a large amount of hydrogen (H) and oxygen (O) is absorbed, and titanium has the ability to dissolve H at a rate tens of thousands of times that of steel. This generates titanium hydride, which significantly reduces toughness.
Gaseous impurities increase the tendency for cold cracking and delayed cracking, as well as notch sensitivity. Therefore, the purity of argon used for welding should not be less than 99.99%, the humidity should not exceed 0.039%, and the hydrogen content of the welding wire should be less than 0.002%.
The heat transfer coefficient of titanium is half that of steel. The transformation from α to β occurs at 882 ℃. The higher the temperature, the faster the β grain will grow, leading to a significant deterioration in performance. Therefore, the temperature must be strictly controlled, especially the high temperature residence time in the welding thermal cycle.
When welding titanium, there are no hot or intergranular cracks, but there may be porosity problems, especially when welding α+β alloys.
5. Precautions for titanium welding
Based on the above research, the following issues should be observed when welding titanium metal:
1) During titanium welding, it is essential to strictly protect the welding area and high temperature area after welding to prevent air from entering and seriously affecting the welding quality. Therefore, it is necessary to use 99.99% pure argon and a protective cover on the right.
2) The weld groove must be machined and grinding methods are not allowed.
3) Spot welding should be avoided and high frequency arc starting should be used instead.
4) It is recommended to avoid post-welding heat treatment. However, if necessary, the heat treatment temperature should not exceed 650 ℃.
6. Conclusion
Titanium welding quality control has a significant impact on the color of the resulting weld. Additionally, the color of titanium weld can also serve as an indicator of the overall welding quality.
Therefore, there is a crucial relationship between the two.
