The relationship between the color of titanium weld seams and the welding quality

1. Color changes of welds in titanium and titanium alloy tubes and the mechanism of defect generation
The welding defects and generation mechanism of titanium and titanium alloy tubes are as follows. When performing welding on titanium tubes, the argon gas shielding layer formed by the argon arc welding gun can only protect the welding molten pool from the harmful effects of the air, but has no protective effect on the weld and the adjacent area that has already solidified and is in a high-temperature state. In this state, the welds of the titanium tubes and the adjacent areas still have a strong ability to absorb nitrogen and oxygen from the air. Oxygen begins to be absorbed from 400°C, and nitrogen begins to be absorbed from 600°C, and the air contains a large amount of nitrogen and oxygen.

As the oxidation level gradually increases, the color of the titanium pipe weld changes and the weld plasticity decreases in a certain pattern. Silver-white (no oxidation) - Golden yellow (TiO, approximately at 250℃ titanium begins to absorb hydrogen. Slight oxidation) - Blue (slightly severe oxidation of Ti2O3) - Gray (severe oxidation of TiO2). 2. The color of the titanium weld seam surface can be used to determine the quality of the titanium welding.

The tests for the different colors and hardness of the titanium weld seams are shown in the following figure. (1) Through experiments, it has been proven that as the color of the weld becomes darker, that is, as the degree of oxidation of the weld increases, the hardness of the weld also increases. Through tests conducted by peers, the hardness of titanium metal increases, harmful substances such as oxygen and nitrogen in the weld increase, and the quality of the welding is greatly reduced.
(2) The weldability of titanium is closely related to its chemical and physical properties. However, the key point is that under high-temperature conditions, the high reactivity of titanium is prone to be affected by air pollution. During heating, its grains tend to expand. When the welded joint cools down, it will form brittle phases. Titanium has a very high melting point, reaching 1668 ± 10℃, which is more than the energy required for welding steel. At the same time, titanium's chemical reactivity is relatively active, reacting with O and H much more easily than steel. It undergoes rapid chemical combination above 600℃. At 100℃, it absorbs H and O in large quantities, and its ability to dissolve H is tens of thousands times greater than that of steel, thereby generating hydrogenated titanium, causing a sharp decline in toughness. The increase in gas impurities increases the tendency for cold cracks and delayed cracks, as well as the notch sensitivity. Therefore, the purity of the welding argon gas should be no less than 99.99%, the humidity should not exceed 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. At 882℃, it undergoes the transformation from α to β. At higher temperatures, the β grains rapidly and discontinuously grow, resulting in a significant deterioration of performance. Therefore, the temperature must be strictly controlled, especially the duration of high-temperature stay in the welding heat cycle. When welding titanium, there are no problems of hot cracks or intergranular cracks, but there is a problem of pores, especially in welding α + β alloys.