In recent years, with the development of the economy, especially with the continuous deepening of the reform and opening-up, China's economic construction has made tremendous progress. At the same time, China has also made significant progress in welding engineering such as pipelines. Titanium welding is a relatively common type of welding. In the process of titanium welding, how to properly control the welding quality has a very important impact on the color of the titanium weld. Due to the intuitiveness of titanium weld color, research on the relationship between titanium weld color and welding quality is of great significance. This article, combining the author's years of research and practical experience in the quality control and process of titanium welding, explores the relationship between welding quality and titanium weld color, hoping to contribute to research in this field.
1. Color changes of titanium and titanium alloy tube welds and the mechanism of defect formation
The defects and mechanisms of titanium and titanium alloy tube welds are as follows: During titanium tube welding, the argon gas shielding layer formed by the argon arc welding torch can only protect the weld pool from the harmful effects of air, but it provides no protection to the already solidified welds and nearby areas that are at high temperatures. In this state, the titanium tube welds and surrounding areas still have a strong ability to absorb nitrogen and oxygen from the air. Oxygen absorption begins at 400°C, and nitrogen absorption begins at 600°C, while air contains large amounts of nitrogen and oxygen.
As oxidation gradually increases, the color of the titanium tube weld changes and the weld's plasticity decreases according to the following pattern: silver-white (no oxidation), golden yellow (TiO, slight oxidation, occurs around 250°C when titanium starts to absorb hydrogen), blue (Ti2O3, oxidation slightly more severe), gray (TiO2, severe oxidation).
2. The quality of titanium welding can be judged by the color of the titanium weld surface.
As the color of the weld deepens, that is, as the degree of oxidation of the weld increases, the hardness of the weld also increases. Tests conducted in peer experiments show that as the hardness of titanium increases, harmful substances such as oxygen and nitrogen in the weld also increase, greatly reducing the quality of the weld.
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 contamination from the air. During heating, its grains expand, and when the welded joint cools, it can form brittle phases. Titanium has a very high melting point, reaching 1668±10℃, which requires more energy than welding steel. At the same time, titanium is chemically more reactive; it reacts with oxygen and hydrogen more easily than steel, and above 600℃ reacts rapidly. Even at 100℃, it absorbs large amounts of hydrogen and oxygen, with a hydrogen solubility tens of thousands of times higher than steel, forming titanium hydride and sharply reducing toughness. Gas impurities increase the tendency for cold cracking and delayed cracking, and increase notch sensitivity. Therefore, the purity of argon used for welding should not be lower than 99.99%, humidity should not exceed 0.039%, and the hydrogen content in the welding wire should be below 0.002%.
Titanium's thermal conductivity is half that of steel. At 882℃, it undergoes an alpha to beta phase transformation. At higher temperatures, beta grains grow abruptly, and performance deteriorates significantly. Therefore, it is necessary to strictly control the temperature, particularly the high-temperature dwell time during the welding thermal cycle. When welding titanium, there are no issues with hot cracks or intergranular cracks, but porosity is a problem, especially when welding alpha-beta alloys.
