Titanium is a metal with very active chemical properties. It has great affinity for gases such as oxygen, hydrogen and nitrogen at high temperatures. Especially in the titanium welding process, this ability is more intense with the increase of welding temperature. Practice has proved that if the absorption and dissolution of titanium and gases such as oxygen, hydrogen and nitrogen are not controlled during welding, it will undoubtedly bring great difficulties to the welding process of titanium welded joints.
- Introduction
In recent years, with the development of the economy, especially with the deepening of reform and opening up, my country's economic construction has made great progress. At the same time, my country has also made great progress in the welding of pipelines and other projects. Titanium welding is a common type of welding for us. In the process of titanium welding, how to do a good job in the quality control of its welding has a very important influence on the color of titanium welding welds. Due to the intuitiveness of the color of titanium welds, it is of great significance to study the relationship between the color of titanium welding welds and the welding quality. In this paper, the author combines his own research on quality control and process of titanium welding for many years and actual work experience to explore the relationship between the welding quality of titanium welding and the color of titanium welds, hoping to play a certain role in the research in this field.
- The influence of titanium characteristics on titanium welding
1. Effects of oxygen and nitrogen
Oxygen and nitrogen are interstitially dissolved in titanium, causing the titanium lattice to distort, increase deformation resistance, increase strength and hardness, but reduce plasticity and toughness. Oxygen and nitrogen in the weld are not good and should be avoided.
2. Effect of hydrogen
The increase of hydrogen will cause the impact toughness of titanium weld metal to drop sharply, while the plasticity will drop a little. Hydride will cause the brittleness of the joint.
3. Effect of carbon
At room temperature, carbon is interstitially dissolved in titanium, increasing strength and reducing plasticity, but not as obvious as oxygen and nitrogen. When the amount of carbon exceeds the solubility, hard and brittle TiC is generated, which is distributed in a network and is prone to cracks. The national standard stipulates that the carbon content in titanium and titanium alloys shall not exceed 0.1%. During welding, the oil stains on the workpiece and welding wire can increase the carbon content, so they need to be cleaned during welding.
- Analysis of the weldability of titanium
Titanium has good weldability. Due to its low thermal conductivity (0.041Cal/℃·cm·s), titanium metal only melts within the arc burning range and has good fluidity; moreover, it has a small thermal expansion coefficient (8.6×10-6/℃, much smaller than carbon steel), which greatly improves the weldability of titanium metal.
- The relationship between the color of the weld and the welding quality of titanium welding
1. The color change of the weld of titanium and titanium alloy titanium tube and its defect generation mechanism
The defects and generation mechanism of the weld of titanium and titanium alloy titanium tube are as follows. When welding titanium tubes, the argon gas protection layer formed by the argon arc welding gun can only protect the welding pool from the harmful effects of the air, but has no protective effect on the weld and its surrounding areas that have been solidified and are in a high temperature state. The weld of the titanium tube 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 from 400℃ and nitrogen from 600℃, and the air contains a lot of nitrogen and oxygen.
As the oxidation level gradually increases, the color of the titanium tube weld changes and the plasticity of the weld decreases. Silvery white (no oxidation) Golden yellow (TiO, titanium begins to absorb hydrogen at about 250°C. Slight oxidation) Blue (Ti2O3 oxidation is slightly serious) Gray (TiO2 oxidation is serious).
2. The quality of titanium welding can be judged by the color of the titanium weld surface
The test of different colors and hardness of titanium welds is shown in the figure below
(I) Experiments have proved that as the color of the weld deepens, that is, the degree of oxidation of the weld increases, the hardness of the weld increases accordingly. Through the test of peer experiments, the hardness of titanium metal increases, and harmful substances such as oxygen and nitrogen in the weld increase, which greatly reduces the quality of welding.
(II) The weldability of titanium is closely related to its chemical and physical properties, but the key is that under high temperature, the high activity of titanium is easily polluted by air. When heated, its grains expand, and when the welded joint cools, 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 chemically active and reacts much more easily with O and H than steel. It reacts rapidly above 600℃. It absorbs a large amount of H and O at 100℃, and its H dissolving capacity is tens of thousands times greater than that of steel, thereby generating titanium hydride, which sharply reduces toughness. Gas impurities increase the tendency of cold cracks and delayed cracks, and increase notch sensitivity. Therefore, the purity of argon gas used for welding should not be less than 99.99%, the humidity should not be higher than 0.039%, and the hydrogen content of welding wire should be less than 0.002%. The heat transfer coefficient of titanium is 1/2 of that of steel. At 882℃, the transition from α to β occurs. At higher temperatures, the β grains grow rapidly and jump, and the performance deteriorates significantly. Therefore, the temperature must be strictly controlled, especially the high temperature residence time in the welding thermal cycle. There are no problems of thermal cracks and intergranular cracks when welding titanium, but there are problems of pores, especially when welding α+β alloys.
- Precautions for titanium welding
Based on the above research, the following issues should be noted when welding titanium metal:
1. During titanium welding, the welding area and the high-temperature area after welding should be strictly protected to prevent the entry of air into the welding and high-temperature areas and cause serious impact on the quality of the weld. Therefore, 99.99% pure argon and a rear drag shield are necessary.
2. The weld groove requires mechanical processing (grinding is not allowed);
3. Spot welding should be avoided and high-frequency arcing 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℃.
- Conclusion
The quality control of titanium welding has a very important influence on the color of the titanium welding weld. At the same time, the quality of titanium welding can also be judged according to the color of the titanium welding weld. There is a very important relationship between the two.
Contact us for more information. Thank you
Nicole
Company: Baoji Jimiyun Dynamic Co., Ltd
Country:China
Add:Baoti road,Jintai,Baoji city,Shaanxi,China
Cel:+86 13369210920
Gmail:nicole@jmyunti.com
Website:www.jm-titanium.com
