The relationship between the color of titanium welds and welding quality

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Titanium is a metal with highly reactive chemical properties; at high temperatures, it has a strong affinity for gases such as oxygen, hydrogen, and nitrogen. This tendency becomes even more pronounced during titanium welding, as the welding temperature increases. Practice has shown that if the absorption and dissolution of titanium by gases such as oxygen, hydrogen, and nitrogen is not controlled during welding, it undoubtedly poses significant difficulties to the welding process of titanium joints.  I. Preface In recent years, with the development of the economy and particularly as reform and opening up have progressed further, significant progress has been made in China’s economic construction. Meanwhile, our country has also made great progress in welding for projects such as pipelines. Titanium welding is a commonly used type of welding. During the titanium welding process, ensuring proper quality control has a significant impact on the color of the welds produced. Due to the intuitiveness of titanium weld color, research on the relationship between the color of titanium welds and welding quality is of great significance. Based on years of research in titanium welding quality control and processing techniques, as well as practical work experience, the author of this article explores the relationship between the welding quality of titanium and the color of titanium welds, in the hope of contributing to research in this field. II. The Influence of Titanium’s Properties on Titanium Welding 1. The Effect of Oxygen and Nitrogen Oxygen and nitrogen get dissolved in titanium as interstitial elements, causing distortion of the titanium lattice, an increase in resistance to deformation, as well as an increase in strength and hardness; however, plasticity and toughness decrease. The presence of oxygen and nitrogen in the weld is undesirable, and it should be avoided. 2. Effect of hydrogen: An increase in hydrogen causes a sharp decline in the impact toughness of titanium weld metal, with only a slight decrease in plasticity; hydrides contribute to the brittleness of the joint. 3. Effect of carbon: At room temperature, carbon is dissolved in titanium in an interstitial form, which increases strength but reduces ductility; however, this effect is not as significant as that of oxygen and nitrogen. When the carbon content exceeds the solubility limit, hard and brittle TiC is formed, which distributes in a network-like pattern and can easily lead to cracks. National standards specify that the carbon content in titanium and its alloys must not exceed 0.1%. During welding, oil residues on the workpiece and welding wire can increase the carbon content, so it is necessary to clean them thoroughly before welding. 3. Analysis of titanium’s weldability: Titanium possesses good weldability; due to its low thermal conductivity (0.041 Cal/℃·cm·s), it melts only within the area affected by the arc, and it exhibits good fluidity ； Moreover, it has a low coefficient of thermal expansion (8.6×10-6/℃, much lower than that of carbon steel), which improves the weldability of titanium metal. 4. The relationship between the color of titanium welds and welding quality 1. Changes in the color of welds on titanium and titanium alloy tubes, as well as the mechanisms behind defects. The defects that occur in the welds of titanium and titanium alloy tubes, along with the mechanisms underlying their formation, are as follows: when welding titanium tubes, the argon gas shield created by the TIG welding torch can only protect the welding pool from the harmful effects of air; it does not provide protection for the welds that have already solidified and are in a high-temperature state, nor for the areas surrounding them. In such conditions, the welds and adjacent areas still have a strong ability to absorb nitrogen and oxygen from the air. It begins to absorb oxygen at 400°C and nitrogen at 600°C, whereas air contains large amounts of both nitrogen and oxygen. The pattern of changes in the color of titanium tube welds and the decline in weld plasticity as the oxidation level increases. Silver-white (without oxidation); golden-yellow (TiO – titanium begins to absorb hydrogen at around 250°C). Slight oxidation) Blue (slightly more severe oxidation of Ti2O3) Gray (severe oxidation of TiO2). 2. The quality of titanium welding can be assessed by the color of the surface of the titanium weld. Tests on the different colors and hardnesses of titanium welds are shown in Figure (1) below. Experiments have shown that as the color of the weld deepens, that is, as the degree of oxidation of the weld increases, its hardness also increases. Further experiments have demonstrated that an increase in the hardness of titanium metal is accompanied by an increase in harmful substances such as oxygen and nitrogen within the weld, which in turn reduces the quality of the welding. (II) The weldability of titanium is closely related to its chemical and physical properties; however, the key issue is that at high temperatures, titanium’s high reactivity makes it susceptible to contamination by air. During heating, its grains expand, and when the welded joint cools down, brittle phases are formed. Titanium has a very high melting point, reaching 1668±10°C, which requires more energy than welding steel. Additionally, titanium is quite chemically reactive; it reacts with O and H much more easily than steel, and begins to combine rapidly at temperatures above 600°C. At 100°C, it absorbs large amounts of H and O; its ability to dissolve hydrogen is tens of thousands of times greater than that of steel, thereby forming titanium hydride and causing a sharp decrease in toughness. Gas impurities increase the tendency for cold cracking and delayed cracking, as well as notch sensitivity. Therefore, the purity of argon gas used for welding should be no less than 99.99%, the humidity should be no higher than 0.039%, and the hydrogen content in the welding wire should be below 0.002%. The heat transfer coefficient of titanium is half that of steel; it undergoes an α-to-β transformation at 882°C. At higher temperatures, the β grains grow rapidly in a discontinuous manner, resulting in a significant deterioration of its properties. Therefore, it is necessary to strictly control the temperature, especially the duration of exposure to high temperatures during the welding heat cycle. When welding titanium, there are no issues with thermal cracks or intergranular cracks, but porosity is a problem, especially when welding α+β alloys. 5. Precautions for titanium welding: Based on the above research, the following points should be taken into account when welding titanium: 1. During titanium welding, it is necessary to provide strict protection for the welding area as well as the high-temperature areas resulting from welding, in order to prevent air from entering these areas and affecting the quality of the weld. Therefore, 99.99% pure argon along with a trailing shielding gas shield is essential. 2. The weld groove shall be prepared by mechanical processing (grinding 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 below 650°C. VI. Conclusion The quality control of titanium welding has a significant impact on the color of the welds produced; meanwhile, the quality of titanium welding can also be assessed based on the color of these welds. There is a very important relationship between the two.