Color Changes and Defect Generation Mechanism of Weld Seams in Titanium and Titanium Alloy Titanium Tubes

The defects and their generation mechanisms of titanium and titanium alloy titanium tube welds are as follows: During the welding of titanium tubes, the argon gas shielding layer formed by the argon arc welding torch can only protect the welding pool from the harmful effects of air, but has no protective effect on the already solidified weld and the nearby area that is still at a high temperature. The weld and the nearby area of the titanium tube in this state still have a strong ability to absorb nitrogen and oxygen in the air. It starts to absorb oxygen from 400℃ and starts to absorb nitrogen from 600℃, while the air contains a large amount 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. Silver white (no oxidation), golden yellow (TiO, titanium begins to absorb hydrogen at approximately 250°C. Slight oxidation), blue (Ti2O3, more severe oxidation), gray (TiO2, severe oxidation).

The quality of titanium welding can be judged by the color of the titanium weld seam surface.

The tests for different colors and hardness of titanium welds are as shown in the following figure.

Experimental evidence shows that as the color of the weld seam darkens, indicating an increase in the degree of oxidation of the weld seam, the hardness of the weld seam also increases. Through tests by peers, it is found that the hardness of titanium metal increases, and harmful substances such as oxygen and nitrogen in the weld seam also increase, significantly reducing the quality of the welding.

(2) 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 prone to air contamination. During heating, its grains expand, and when the welded joint cools, it forms brittle phases. Titanium has a high melting point, reaching 1668 ± 10°C, which requires more energy than welding steel. Moreover, titanium is chemically very active and reacts with O and H much more easily than steel, especially above 600°C when the reaction intensifies. At 100°C, it absorbs large amounts of H and O, with its hydrogen absorption capacity being tens of thousands of times greater than that of steel, thereby forming titanium hydride and sharply reducing its toughness. Gas impurities increase the tendency of cold cracking and delayed cracking, as well as notch sensitivity. Therefore, the purity of argon used for welding should be no less than 99.99%, and the humidity should not exceed 0.039%. The hydrogen content of the welding wire should be below 0.002%. Titanium's thermal conductivity is half that of steel. At 882°C, it undergoes an α to β transformation, and at higher temperatures, the β grains grow rapidly in a leapfrog manner, significantly deteriorating its performance. Therefore, temperature control must be strict, especially the high-temperature dwell time in the welding thermal cycle. When welding titanium, there are no issues of hot cracking or intergranular cracking, but there are problems with porosity, especially when welding α+β alloys.