Titanium is a highly reactive metal with excellent chemical properties. It has a great affinity for gases such as oxygen, hydrogen, and nitrogen at high temperatures, especially during the titanium welding process. This ability becomes stronger with increasing welding temperatures. Practice has proven that if the absorption and dissolution of gases such as oxygen, hydrogen, and nitrogen during welding are not controlled, it will undoubtedly bring great difficulties to the welding process of titanium welded joints.
1、 Preface
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 tremendous progress. At the same time, China has also made significant progress in the welding of pipelines and other engineering projects. Titanium welding is a common type of welding, and the quality control of the welding process has a significant impact on the color of the weld seam. Due to the intuitive nature of titanium weld color, the study of the relationship between titanium weld color and welding quality is of great significance. Based on years of research and practical work experience in quality control and process of titanium welding, the author of this article explores the relationship between the welding quality of titanium welding and the color of titanium welds, hoping to contribute to the research in this field.
2、 The Influence of Titanium Characteristics on Titanium Welding
1. Effects of oxygen and nitrogen
Oxygen and nitrogen interstitial fusion in titanium causes lattice distortion, increases deformation resistance, increases strength and hardness, but reduces plasticity and toughness. The presence of welding oxygen and nitrogen in the weld seam is unfavorable and should be avoided.
2. The impact of hydrogen
The increase of hydrogen will cause a sharp decrease in the impact toughness of titanium weld metal, while the plasticity will decrease slightly, and hydrides will cause brittleness of the joint.
3. The impact of carbon
At room temperature, carbon is solid dissolved in titanium in the form of gaps, increasing strength and decreasing plasticity, but 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 prone to cracking. The national standard stipulates that the carbon content in titanium alloys should not exceed 0.1%. During welding, oil stains on the workpiece and welding wire can increase the carbon content, so it needs to be cleaned thoroughly during welding.
3、 Analysis of Weldability of Titanium
Titanium has good weldability. Due to its low thermal conductivity (0.041Cal/℃· cm · s), titanium metal only melts within the arc combustion range and has good fluidity; Moreover, it has a small coefficient of thermal expansion (8.6 × 10-6/℃, much smaller than carbon steel), which greatly improves the weldability of titanium metal.
4、 The relationship between weld color and welding quality in titanium welding
1. Color change and defect generation mechanism of titanium and titanium alloy titanium tube welds
The defects and mechanisms of titanium and titanium alloy titanium tube welds are as follows. When welding titanium tubes, the argon gas maintenance layer formed by the argon arc welding gun can only maintain the welding pool from the harmful effects of air, and has no protective effect on the weld and its surrounding areas that have already solidified and are in a high temperature state. However, the titanium tube weld and its surrounding areas in this state still have a strong ability to absorb nitrogen and oxygen from the air. Oxygen absorption starts at 400 ℃ and nitrogen absorption starts at 600 ℃, while air contains a large amount of nitrogen and oxygen.
As the oxidation level gradually increases, the color of titanium pipe welds changes and the plasticity of welds decreases. Silver white (non oxidized) golden yellow (TiO), titanium begins to absorb hydrogen at around 250 ℃. Slight oxidation) blue (slightly severe oxidation of Ti2O3) gray (severe oxidation of TiO2).
2. The quality of titanium welding can be judged by the color of the surface of the titanium weld seam
The testing of different colors and hardness of titanium welds is shown in the following figure
（1） Through experiments, it has been proven that as the color of the weld seam deepens, that is, the degree of oxidation of the weld seam increases, the hardness of the weld seam also increases. Through peer experiments, the hardness of titanium metal increases, and harmful substances such as oxygen and nitrogen in the weld seam increase, greatly reducing the quality of welding.
（2） The weldability of titanium is closely related to its chemical and physical properties, but the key is that at high temperatures, the high reactivity of titanium is easily contaminated by air. When heated, its grains swell, and when the welded joint cools, it forms a brittle phase. The melting point of titanium is very high, reaching 1668 ± 10 ℃, which requires more energy than welding steel. At the same time, the chemical properties of titanium are relatively active, and it is much easier to react with O and H than steel. It rapidly combines above 600 ℃. At 100 ℃, it absorbs a large amount of H and O, and its ability to dissolve H is tens of thousands of times greater than that of steel, resulting in the formation of titanium hydride and a sharp decrease in toughness. Gas impurities increase the tendency for cold cracking and delayed cracking, and increase the sensitivity of notches. Therefore, the purity of argon gas 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 below 0.002%. The heat transfer coefficient of titanium is half of that of steel, and it undergoes a transition from alpha to beta at 882 ℃. At higher temperatures, the beta grains grow rapidly and their performance deteriorates significantly. Therefore, it is necessary to strictly control the temperature, especially the high-temperature residence time in the welding thermal cycle. There are no hot cracks or intergranular cracks when welding titanium, but there are porosity issues, especially when welding alpha+beta alloys.
5、 Precautions for Titanium Welding
Based on the above research, the following issues should be noted when welding titanium metal:
1. During the titanium welding process, strict protection should be provided for the welding area and the high-temperature area after welding to avoid the serious impact of air entering the welding and high-temperature areas on the quality of the weld seam. Therefore, 99.99% pure argon and a trailing protective cover are necessary.
2. The weld groove requires mechanical processing (grinding method is not allowed);
3. Spot welding should be avoided and high-frequency arc starting should be used.
4. Avoid post weld heat treatment; If post weld heat treatment is necessary, the heat treatment temperature should be less than 650 ℃.
6、 Conclusion
The quality control of titanium welding has a significant impact on the color of the weld seam, and the quality of titanium welding can also be judged based on the color of the weld seam. There is a very important relationship between the two.