The relationship between the color of titanium welds and welding quality

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Titanium is a metal with very reactive chemical properties; at high temperatures, it exhibits a strong affinity for gases such as oxygen, hydrogen, and nitrogen. This tendency becomes even more pronounced during titanium welding, as the welding temperature rises. 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, great advancements have been made in China’s economic construction. At the same time, 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, studying 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. Influence of oxygen and nitrogen Oxygen and nitrogen are 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 the titanium weld metal, while the plasticity decreases slightly; hydrides contribute to the brittleness of the joint. 3. Effect of carbon: At room temperature, carbon is dissolved in titanium in the form of interstitial atoms, 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 should 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 range of the arc flame, and exhibits good fluidity ； Moreover, it has a low coefficient of thermal expansion (8.6×10^-6/°C, much lower than that of carbon steel), which **improves the weldability of titanium. IV. 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 defect formation. The defects that occur in the welds of titanium and titanium alloy tubes and the mechanisms underlying their formation are as follows: when welding titanium tubes, the argon gas shield created by the TIG welding gun 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 (unoxidized); golden-yellow (TiO – titanium begins to absorb hydrogen at around 250°C). Slightly oxidized) 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 the figure below. Experimental results show that as the color of the weld deepens, that is, as the degree of oxidation of the weld increases, its hardness also increases. According to tests conducted, 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 **reduces the quality of the welding. (II) The weldability of titanium is closely related to its chemical and physical properties. However, the key point is that at high temperatures, titanium’s high reactivity makes it susceptible to air pollution. During heating, its grains expand; when the welded joint cools down, this leads to the formation of brittle phases. Titanium has a very high melting point, reaching 1668±10°C. This requires more energy than is needed for welding steel. Additionally, titanium is quite chemically reactive; it reacts with oxygen and hydrogen much more readily than steel does. At temperatures above 600°C, it undergoes rapid chemical reactions. 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 decline 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 backshield 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; simultaneously, 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.