Research on the Relationship between Titanium Welding Seam Color and Welding Quality

In recent years, with the development of the economy, China has made tremendous progress in the welding of pipelines and other engineering projects. Titanium welding is a common welding method, and how to control the welding quality during the titanium welding process has a significant impact on the color of the weld seam. Due to the intuitive nature of titanium weld color, studying the relationship between titanium weld color and welding quality is of great significance.
一、 The Influence of Titanium Characteristics on Titanium Welding
1. Effects of oxygen and nitrogen: Oxygen and nitrogen interstitial solution in titanium causes lattice distortion, increased deformation resistance, increased strength and hardness, while plasticity and toughness decrease. The presence of oxygen and nitrogen in the weld seam is unfavorable and should be avoided.
2. The effect of hydrogen: The increase of hydrogen will cause a sharp decrease in the impact toughness of titanium weld metal, a slight decrease in plasticity, and hydrogen compounds will cause brittleness of the joint.
3. The influence of carbon: At room temperature, carbon is dissolved in titanium in the form of gaps, increasing strength and decreasing plasticity, but not as significant as oxygen and nitrogen. When the carbon content exceeds the solubility, hard and brittle TiC will be generated, which is distributed in a network and prone to cracking. The national standard stipulates that the carbon content in titanium and its alloys shall not exceed 0.1%. During welding, the oil stains on the workpiece and welding wire can increase the carbon content, so it is necessary to clean them thoroughly during welding.
二、 Analysis of Weldability of Titanium
Titanium has good weldability, and due to its low thermal conductivity (0.041Cal/℃ · cm · s), titanium metal only melts within the arc combustion range and has good fluidity. Moreover, titanium has a relatively small coefficient of thermal expansion (8.6 × 10-6/℃), much smaller than carbon steel, which greatly improves the weldability of titanium metal.

三、 The relationship between weld color and welding quality in titanium welding
1. Color change and defect generation mechanism of titanium welds: When welding titanium pipes, the argon protective layer formed by the argon arc welding gun can only maintain the welding pool from harmful effects of air, and has no protective effect on the solidified but high-temperature weld and its surrounding areas. In this state, titanium welds and their surrounding areas still have a strong ability to absorb nitrogen and oxygen from the air. Start absorbing oxygen from 400 ℃ and nitrogen from 600 ℃. As the degree of oxidation gradually increases, the color of the titanium weld will change, and the plasticity of the weld will decrease. The color changes are as follows: silver white (no oxidation), golden yellow (slight oxidation, TiO, titanium begins to absorb hydrogen at around 250 ℃), blue (Ti2O3, slightly oxidized), gray (TiO2, severely oxidized).
2. Judging welding quality through the surface color of titanium welds: Experiments have shown that as the color of the weld increases, that is, the degree of oxidation of the weld increases, the hardness of the weld also increases. The increase in hardness of titanium metal means an increase in harmful substances such as oxygen and nitrogen in the weld seam, which greatly reduces the quality of welding.
四、 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 to the welding area and high-temperature areas after welding to avoid air entering the welding and high-temperature areas and causing serious impact on the quality of the weld seam. Therefore, it is necessary to use 99.99% pure argon and a trailing protective cover. 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 ℃.
In summary, the quality control of titanium welding has a significant impact on the color of the weld seam in titanium welding. At the same time, the quality of titanium welding can also be judged based on the color of the weld seam. There is a close relationship between these two.